Geology of the Jemez Area, Chapter 6: The Birth of the Jemez

The table of contents be found here.

Composite simulation of early Jemez eruption

Digitally modified photograph of Craters of the Moon, showing what the young Jemez volcanic field might have looked like.

Fifteen million years ago, the continents had largely taken their present shape, and the climate was steadily cooling as carbon dioxide was absorbed into the great masses of silicate rock exposed in the growing Himalaya Mountains. The RIo Grande Rift was occupied by the fertile flood plain of the ancestral Rio Grande, where ancient relatives of elephants, rhinoceroses, camels, and horses grazed.

Deep below the surface, hot asthenosphere rising into the rift provided heat to generate magma from the fertile rocks of the Jemez Lineament. The faulting along the western side of the rift provided a path for this magma to rise towards the surface.

In this chapter, we will look at the birth of  the Jemez volcanic field during the late Tertiary Period.

  1. A Volcanic Field is Born
    1. A second phase of rifting
    2. Magma generation
  2. The Lobato Formation
    1. Basalt
      1. Pyroxenes
      2. Olivine
    2. Window Dike
    3. Lobato Mesa
    4. Clara Peak
      1. Aa and Pahoehoe
  3. The Paliza Canyon Formation
    1. Early basalts
    2. The Keres Highlands
      1. Andesite
    3. Late dacite volcanism
      1. Dacite
    4. Volcaniclastics
      1. San Miguel Mountains
    5. Cochiti Canyon and environs
    6. Los Alamos Canyon and points east
  4. Canovas Canyon Rhyolite
    1. Stagnation and bimodal volcanism
    2. Rhyolite
  5. The La Grulla Formation
  6. Paleomap of the Lower Keres Group

A Volcanic Field is Born

With the change from compressional to extensional tectonics twenty to thirty million years ago, the intersection of the Jemez Fault Zone, bounding the west side of the Rio Grande Rift, and the Jemez Lineament, marking an ancient suture zone, began to stir to life. Deep fracturing of the crust in an area of longstanding weakness and possibly "fertile" deep crustal rocks, with a lower than usual melting point, meant that large bodies of magma would repeatedly work their way to the surface here. 

It is hard to pin a specific date on the first eruptions of the Jemez volcanic field. There was volcanism associated with the opening of the Rio Grande Rift 25-30 million years ago, and some of this was close to the Jemez area. Small volumes of alkaline mafic magma were erupted in the southeaster Jemez from 25 to 18 million years ago, forming basanite flows that are interbedded with Santa Fe Group beds on the southeast flank of the San Miguel Mountains. Similar small volumes of basalt were erupted in the  northern Jemez around 19 to 20 million years ago, forming the Cerrito de la Ventana dike system we visited in the last chapter. This means that "first Jemez eruption" is partly a matter of definition. But most geologists place the beginnings of the Jemez Volcanic Field at about 14 million years ago.

A second phase of rifting

The first phase of rifting along the Rio Grande Rift lasted from about 30 to about 20 million years ago. It was during this first phase of rifting that the Cieneguilla Basanite was erupted. Hot aesthenosphere rising into the Rift supplied the necessary heat energy, and the decompression of the mantle rock as it approached the surface allowed the hot rock to partially melt. From the alkaline character of the magma that was erupted, we can deduce that only a small fraction of the rock melted and that it did so at a relatively great depth. Alkaline magma is relatively low in density, and this gave it the buoyancy necessary to penetrate the overlying crust along fissures opened by rifting. However, the supply of magma was soon exhausted and the fissures were sealed by their own effusions.

Rifting slowed for the next four million years. When rifting resumed 14 million years ago, the most active displacement  in the Espanola Basin became focused on the Jose fault zone. This is reflected in the displacements associated with the Cat Mesa Fault to its west. The Cat Mesa Fault, which runs along much of the eastern wall of Canon de San Diego, displaces Yeso Formation beds by about 240 meters (790 feet), but the earliest Jemez volcanic beds on the fault are displaced by less than 10 meters (30 feet) and the young Bandelier Tuff by less than two meters (six feet).  This tells us that the Cat Mesa Fault was an early rift fault, largely inactive by the time volcanism really got going in the Jemez. This narrowing of the active rift zone, and development of major fault zones bounding the rift, marked a shift from ductile extension to brittle extension as the crust cooled. Deep faulting along the western margin of the Rift intersected another, more mysterious feature, the Jemez Lineament. We discussed this feature in the second chapter of this book. The key part of that discussion is that the Jemez Lineament seems to correspond with the hydrous subduction scar where the Yavapai and Mazatzal Precambrian provinces were sutured together. The mantle here seems to be fertile for production of magma.

Magma generation

As the rift continued to open, hot aesthenosphere continued to rise into the rift. Where the rift crossed the Jemez Lineament, the heated mantle rock was particularly easily melted, allowing great volumes of magma to accumulate. Meanwhile, the overlying crust was still cooling in the aftermath of Ortiz volcanism. Colder crust is more dense, and this increases the buoyancy of any magma within the crust. The deep faulting accompanying rifting provided natural conduits for the rising magma.

The magma pooling in the upper mantle was likely basaltic and alkaline in composition. Small volumes of this magma periodically found a path to the surface, and the Rio Grande Rift has numerous small volcanic fields, most of which erupted just a few times and are unlike to erupt again. The basaltic magma was low in viscosity and formed small cinder cones and extensive flows rather than large volcanic mountains. We see all of these processes at work in the early stages of Jemez volcanism.

Similar small volcanic fields are found all along the Jemez Lineament. One wonders if the Lineament has become a zone of incipient rifting. If so, then it becomes easy to understand why volcanism is so highly focused in the Jemez. Here the crust is pulling apart not just in one, but in two directions. The result may be that the crust has thinned here more than at any other place in the Southwest, resulting both in greater magma generation and in easier access for the magma to the surface.

Geologists divide the volcanic rock erupted in the Jemez over the last fourteen million years into three groups. The Keres Group are formations of the southern Jemez dating from before the first known supervolcano eruption, 1.85 million years ago. The Polvadera Group are formations of the northeastern Jemez dating from roughly the same time period as the Keres Group. The Tewa Group are formations younger than about 1.85 million years old, which dominate the central Jemez. There is now debate over whether the Polvadera Group and Keres Group are really distinct, since they are about the same age and similar in composition, differing mostly in the accident of being on opposite sides of the Valles caldera. Some recent geologic works abandon the Polvadera Group, assigning all the older volcanic rocks of the Jemez to the Keres Group.

The Lobato Formation

The first phase of volcanism in the Jemez was eruption of highly fluid basaltic magma. This began slightly earlier in the northern Jemez than the southern Jemez.

Digital relief map of Lobato Formation exposures in the
        Jemez Mountains
Relief map of the Jemez with Lobato Formation outcroppings highlighted in red (basalt), yellow (andesite), and green (dacite)

Thirteen million years ago, a volcanic fissure opened in the broad river valley between the Sierra Nacimiento and the Sangre de Cristo Mountains. Highly fluid alkaline basaltic lava poured out of the fissure, accompanied by fountains of gas and molten rock. The lava spread quickly, drowning the surrounding county and exploding into cinder and steam whenever the lava entered one of the many small streams crossing the valley floor. However, the eruption soon ended, and the lava hardened into black rock like that of the malpais seen today near Grants and Carrizozo.

Time passed, and fresh sediments accumulated, burying the lava flow. The cycle repeated itself, with increasing frequency, as the Rio Grande Rift continued to open. The eruptions continued even as regional uplift caused the accumulation of sediment to slow to a halt and give way to erosion.

Geologists have assigned the series of mafic lava flows representing the earliest volcanism of the northern Jemez to the Lobato Formation. The Lobato Formation was erupted between 14.1 million and 7.6 million years ago, with the period of highest activity from 10.8 to 9.2 million years ago. It consists mostly of olivine basalt flows found throughout the northeastern Jemez. The formation takes its name from Lobato Mesa, where the hard basalt has protected the underlying poorly cemented Santa Fe Group sediments from erosion to produce a high plateau. Lobato Formation basalt overlies the Abiquiu Formation but is interbedded with the Santa Fe Group, showing that sediments were still accumulating in the Espanola Basin when the first Lobato flows were erupted.

The oldest Lobato Mesa flow is located in Santa Clara Canyon and has a radioisotope age of 13.2 million years. Santa Clara Canyon is on tribal lands of the Santa Clara Pueblo, and the pueblo holds this area particularly sacred. Permission to visit has rarely been granted even to professional geologists, so much of the canyon has never been mapped in detail.

There is evidence that the earliest eruptions of the Lobato Formation may have included lavas higher in silica that are now either eroded away or buried under younger flows. Geologists have identified a Lower Coarse White Ash Zone in the Chamita Formation which contains silica-rich volcanic ash with a radiometric age of around 13 million years. These beds are thickest east-northeast of the northern Jemez Mountains. There are also spotty gravel beds below the oldest Lobato basalt flows containing dacite clasts that are also about this age. This suggests that some of the earliest volcanic activity in the northeast Jemez came from a dacitic vent that is now buried under younger flows.

We will introduce ourselves to the Lobato Formation at a somewhat younger but easily accessible basalt flow found northwest of the village of Hernandez, where mesas of the Chamita Formation of the Santa Fe Group are topped by Lobato Formation basalt.

Lobato Mesa
        formation basalt atop a mesa
Lobato Mesa Formation basalt flows atop a mesa of Chamita Formation northwest of Hernandez. 36 05.111N 106 08.318W

These particular beds were probably nearly level when laid down, and have been tilted to the northeast by continuing rifting in the Rio Grande Rift. The bottom of the flow has irregularities showing that the Chamita Formation had already begun to erode due to regional uplift when it was drowned in lava.

The flow is exposed in a road cut.

Lobato flow
Early Lobato Basalt flow. 36 5.958N 106 8.391W

Notice the thin baked zone in the Chamita Formation beds underlying the flow. The flow has been dated as about 9.6 to 10 million years old.

Lobato
            flow sample
Early Lobato Basalt flow. 36 5.958N 106 8.391W

Under the loupe, the rock shows visible phenocrysts of (probably) augite and plagioclase, with oxidized spots that were probably olivine. The very dark color suggests that this is a low-silica olivine basalt.

Basalt

I introduced basalt in the first chapter of this book, but it's time now to make a closer acquaintance with this important character in our story.

Geologists define basalt as an extrusive igneous rock (a rock that solidifies from magma that reaches the surface) containing 45% to 52% silica and not more than 5% oxides of the alkali metals, sodium and potassium. This is a mafic composition, rich in magnesium and iron, and it is a composition that can be produced from partial melting of the upper mantle. The basalt of the Lobato Mesa Formation likely came from partial melting of the hot mantle rock that rose into the Rio Grande Rift, which produced magma that reached the surface relatively undifferentiated and uncontaminated by crustal rock.

Magma more enriched in silica forms basaltic andesite, while magma poorer in silica (which is rare) forms picrobasalt. Magma more enriched in alkali metals forms trachybasalt. Most basaltic magma is relatively "dry", with a water content between about 0.1% and 1.5%. The more alkaline basaltic magmas tend to have the higher water content.

Basalt consists mostly of plagioclase feldspar and pyroxenes. There are often a few small phenocrysts visible in a sample, and these are used to further classify basalts. Most of the basalt of the Lobato Mesa Formation is olivine basalt. The Lobato Formation also contains a few flows of quartz basalt and plagioclase basalt, the latter containing phenocrysts of plagioclase in addition to the plagioclase in the ground mass.

Because of its relatively low silica content, basalt lava is low in viscosity, having about the consistency of ketchup. It tends to flow away from the eruption center rather than piling up to form a steep mountain.

I described plagioclase in the second chapter of this book. Pyroxenes are distantly related to amphiboles, which I also described in Chapter Two.

Pyroxenes

Like amphiboles, pyroxenes are inosilicates (chain silicates) rich in iron and magnesium, but their backbone consists of a single rather than a double chain of silica tetrahedra:

Single chain inosilicate backbone

These chains are held together by metal atoms, typically iron or magnesium, though some varieties contain calcium, sodium, or aluminum. Aluminum is less likely to substitute for silicon than in amphiboles. Examples of pyroxenes are orthopyroxene, (Fe,Mg)SiO3;, sometimes called hypersthene in older publications; diopside, CaMgSi2O6; and augite, (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6. The arrangement of the silica chains resembles that of amphiboles, but the single chains produce narrower "I-beams" than the double chains of amphiboles.

Here is a single large crystal of augite.

Augite crystal
Single crystal of augite

Unlike amphiboles, pyroxenes contain no water in their structure, and crystallize from hotter and drier magmas. They can sometimes be distinguished under the loupe from amphiboles by their shorter crystals. Amphiboles tend to form long, needle-like crystals.

Olivine

Olivine varies in composition between Fe2SiO4 (fayalite) and Mg2SiO4 (forsterite), with magnesium freely substituting for iron in the structure. It is a nesosilicate, containing isolated silica tetrahedra that are not connected to each other. The negative charge on these isolated tetrahedra is balanced by the iron and magnesium. Olivine crystals are typically deep green to dark brown in color, but are unstable under surface conditions, slowly oxidizing to quartz, magnesium oxide, and hematite (a mixture which, while not a true mineral, is called iddingsite.) The basalts of the Lobato Mesa Formation are old enough that almost all the olivine in them has oxidized to iddingsite.

The presence of forsterite indicates that a volcanic rock is quite low in silica. In particular, quartz is almost never found in the same volcanic rock as forsterite. The excess silica required to form quartz would instead react with the olivine to form pyroxenes. In fact, one sometimes sees a reaction rim of pyroxene around olivine phenocrysts, where an increase in the chemical activity of silica in the magma converted the outer layer of the olivine crystals to pyroxene.

As a magma cools, the olivine that crystallizes from it becomes increasingly iron-rich. Unlike forsterite, fayalite is stable in the presence of excess silica, and one occasionally finds both quartz and fayalite phenocrysts in an igneous rock.

Window Dike

Here's a zoomed in map of the northwest Jemez showing major Lobato outcrops. This will help us stay oriented as we examine this formation more closely.

Lobato Formation
Major locations for Lobato Formation

The road cut we visited earlier, to introduce ourselves to the Lobato Formation, is the easternmost exposure on this map. The bulk of the formation lies to its west, and for the most part it is exposed on high ground, as small mountains or mesa tops, well away from the main highways. There has been a lot of erosion since this formation was erupted, and it has cut deeply into the relatively soft Tesuque Formation sediments on which the Lobato Formation sits.

The erosion has exposed some of the deeper plumbing of the Lobato Formation as a series of dikes, of which Window Dike is a popular destination for hikers.

Not far north of the little village of Chili is the trailhead for the Lemitas Trail.  In good weather, during the cooler part of the year, this is a very pleasant hike through badlands of the Ojo Caliente Formation. About four miles in, the trail reaches Window Dike.

Window Rock dike


Window Rock dike

Window Rock dike. \36.125621N 106.21471072W

The dike is composed of rather coarsely crystalline basalt.

Window Rock dike sample

Window Rock dike. \36.125621N 106.21471072Wp

Window Dike is the easternmost of a group of four or five dikes trending north. Another is visible west of Window Dike.

Window Rock dike
          sample

Next dike west of Window Rock. Looking west from near \36.125621N 106.21471072W

Radiometric dates from the southern portions of these dikes range from 9.7 to 10.7 million years, similar to most of the Lobato Formation. Here hot basaltic magma ascended through fissures in the Ojo Caliente beds towards the surface.

How much of the magma actually reached the surface? There are fields of basalt cobbles near the dikes that suggest that the dikes produced some surface flows, but they could not have been extensive.

Basalt cobble field near Window Rock

Basalt cobble field east of Window Rock. 36.1193876N 106.20521072W

Lobato Mesa

South and west of the dikes are the Santa Clara Mountains and Lobato Mesa itself. Seen from the east, Lobato Mesa is an impressive escarpment.

Lobato
          Mesa
Gallina Mesa. Looking west from near 36.116275,N 106.2019713W

Lobato Mesa is at right, with La Sotella forming the high point of Lobato Mesa just right of center. The distant peak with a blanket of late spring snow is Tschicoma Peak, the highest point in the Jemez and namesake of the Tschicoma Formation, which we will visit in due course. Clara Peak and Cerro Roman are left of center.

Lobato Mesa is the eastern margin of El Alto, a high plateau underlain by Lobato Formation basalt.  El Alto may be viewed from 31 Mile Road (Forest Road 565) where it crosses Gallina Mesa.

Gallina Mesa
El Alto Plateau. Looking north from Gallina Mesa near 36 01.495N 106 22.441W

The series of meadows and ridges in the middle distance is El Alto. The parallel ridges trending north are fault blocks, with the block furthest to the east forming Lobato Mesa itself. El Alto is mostly private land belonging to the merced or land grant community of Abiquiu and, while there is a good gravel road from Abiquiu to the mesa, visitors are restricted to the road and its branches until they reach National Forest land to the southwest, south, and east.

El Alto was originally a nearly flat plateau of Lobato Formation basalt. This erupted from fissures like those of Window Dike, but here the flows were voluminous enough to form a thick and extensive sheet that resisted erosion. It was broken into fault blocks much later, possibly in connection with Tshicoma volcanism, of which we'll learn much more later in the book.

Forest Road 31 south of Abiquiu ascends El Alto, and is a beautiful drive through the Abiquiu merced. The road branches to the east and passes onto National Forest land as one approaches the west side of Lobato Mesa. Here the road ascends the mesa and affords a wonderful view of El Alto to the west.

El Alto Mesa

El Alto Mesa seen from Lobato Mesa. 36 06.092N 106 18.039W

The Tschicoma Highlands forms the skyline in the left half of the panorama. Here younger volcanism of the Tschicoma Formation has deeply buried the Lobato Formation. In the middle distance, running almost the length of the panorama, is a series of forested hills. This is the western edge of a fault block, similar to the one from which photograph was taken, that is tilted to the east.

The cap of Lobato Mesa is a light gray basalt.

Lobato Mesa olivine basalt

Olivine basalt capping Lobato Mesa. 36 06.018N 106 17.841W

Olivine basalt
          from Lobato Mesa

The basalt has dark brown flecks of iddingsite formed from alteration of olivine phenocrysts. This makes this a fairly low-silica rock. Based on dates of nearby formations and field relations, this rock must be around 10 million years old. The relatively light color may be a consequence of weathering, or it may indicate a relatively low iron content consistent with this being an alkaline basalt (trachybasalt). While the older flows of the Lobato Formation are mostly olivine basalt, the upper flows tend to be trachybasalt.

The road fades out atop the mesa, but it is a short hike to the eastern escarpment.

Looking east from Lobato Mesa

Looking east from Lobato Mesa. 36 06.189N 106 17.499W

To the left, we look north along an escarpment, with the Tusas Mountains in the background. The escarpment itself is not a fault escarpment, or at least no evidence of a controlling fault has been found; the faults here tend to be down to the west anyway. This is an erosional scarp, formed because hard Lobato Mesa basalts rest on very soft sediments of the Ojo Caliente Member, Tesuque Formation, Santa Fe Group. In the distance just to the right of the escarpment is Sierra Negra.

To the northeast, one sees Chama-El Rio Member, Tesuque Formation, in the valley floor, with a prominent ash bed. Beyond are hills of Ojo Caliente Member, with a fault on their western side. This fault is thrown down to the east. We saw a closer view of this area in the last chapter. Further to the east are ridges cored with the basalt dikes of the Window Dike swarm.

The plateau to the right looks like a basalt plateau thrown down by a fault, but the geologic map has it underlain by Santa Fe sediments and shows no fault. It looks like an erosional surface, left over from a time when the Rio Grande had not cut down nearly as deeply as it now does. It must be quite an ancient surface given its height above the current river level; I won’t even venture a guess.

At far right is La Sotella, a remnant of an ancient shield volcano of the Lobato Mesa Formation. The lower plateau just beyond La Sotella to its left is a separate flow, thrown down by a fault.

La Sotella is made up of numerous thin flows, which unfortunately barely show up in this lighting.

La
          Sotella

The flows can be made out through binoculars. These flows range from 10.2 to 10.8 million years old and are composed of olivine basalt with sizable feldspar phenocrysts.

La Sotella and the high terrain immediately to its south are examples of a new phase in Lobato volcanism, in which activity focused on a few vents centered around Clara Peak.

Clara Peak

To the east of Gallina Mesa is the Clara Peak volcanic center of the Lobato Mesa Formation. Like La Sotella, Clara Peak is a remnant of a shield volcano, formed from successive eruptions of low-viscosity basalt, and the Santa Clara Fault has dropped the south flank of the volcano and exposed its interior.

Tshicoma Highlands and Santa Clara Mountains
Tschicoma Highlands and Santa Clara Mountains. Looking west from near 36.0060623N -106.1709557W

At left are the Tschicoma Highlands, which are underlain by younger rocks of the Tschicoma Formation. At right and closer to the camera are the Santa Clara Mountains. Clara Peak is the high point at right center, while Cerro Roman is the hill at far right. This area is heavily faulted, with faults generally trending towards the northeast. The Santa Clara Fault runs to the south of both peaks, while another fault runs between Cerro Roman and Clara Peak. Other faults cut across these main faults. As a result, the original form of the Santa Clara shield volcano or volcanoes has been severely deformed and the area deeply eroded.

31 Mile Road runs up the south flank of Clara Peak, where the interior of the volcano has been exposed by faulting and erosion. One can see numerous thin basalt flows separated by clinker in the road cut on the southwest flank of Los Cerros, at the western end of the volcanic center:

Clara Peak basalt
        flows
Los Cerros basalt flows. 36 01.356N 106 15.737W

A flow is an individual bed of volcanic rock produced by a single eruption of lava. It generally cools to solid rock as a single body, crystallizing from the base and top towards the middle.  A shield volcano is built up of numerous thin flows like these, produced by numerous eruptions over a long period of time from a long-lived vent.

Aa and Pahoehoe

Crack open any dictionary of geological terms, and the first entry will be for aa. This is one of the two forms that flows of basaltic lava commonly take on land, the other being pahoehoe. Aa is lava whose surface is rough, jagged, and crumbly, while pahoehoe is lava whose surface is smooth and ropy. Hot lava fresh from the vent tends to be pahoehoe, but it can quite abruptly change to aa, probably as a result of cooling and losing dissolved gases.

Few lava flows in the Jemez area are fresh enough to show the contrast clearly. One must visit a more recently active basaltic volcanic field to see the best examples. One such example is Craters of the Moon in Idaho, whose youngest flows are less than two thousand years old. Here one can find both pahoehoe flows

Pahoehoe lava at
          Craters of the Moon, Idaho
Pahoehoe flow at Craters of the Moon, Idaho. Near 43.457N 113.560W

and aa flows:

Aa lava at Craters of the
          Moon, Idaho
Aa flow at Craters of the Moon, Idaho. Near 43.458N 113.561W

Compare the aa in the second picture with the pahoehoe in the foreground.

The broken clasts of volcanic rock that cover aa flows are known as clinker. The solid surface shatters under the forces generated by the still-liquid magma flowing beneath, then the clasts are carried to the front of the flow, where they is buried by the advancing flow. In effect, the aa flow advances on a carpet of its own clinker. Returning to the photograph of the road cut at Los Cerros, we can see the carpet of clinker beneath each flow, suggesting that these were aa flows. The layer of clinker that forms on top of an aa flow is known as top breccia, while the carpet of clinker beneath is known as basal breccia. Clinker left to the sides of an advancing narrow aa flow form marginal levees. Thus clinker surrounds a massive center on all sides.

The photograph also shows numerous white patches of caliche on the road cut. Caliche is composed primarily of calcite and can be thought of as a kind of limestone, but it is not formed by the same processes as more conventional limestone. It forms in the subsurface in arid climates (such as the modern Jemez) from slow weathering of calcium-rich parent rock. Caliche typically includes a fair amount of sand, silt, and clay that is bound into the calcite matrix.

Just east of here, the flows are thicker and are separated by more distinct beds of clinker. For example:

Thicker flows on
          Los Cerros
Thick basalt flows on Los Cerros. 36 01.374N 106 15.667W

There is a quite massive flow on top and another just exposed at road level, with a thick bed of clinker between.

Further down the road, there is a massive bed of cinder.

Cinder beds on
          31 Mile Road
Cinder beds along 31 Mile Road, Looking north from 36.0257763N 106.2587937W

Cinder with
          dike on Los Cerros
Closer view of cinder beds with dike. 36 01.604N 106 15.400W

Where there are flows, there is cinder, and vice versa. Almost all magma contains dissolved gas, and when basaltic magma reaches the surface, the gas comes boiling out, creating a great fountain of lava and hot gas the breaks the lava up into small, foamy blobs. Some of these cool and solidify while still in the air, landing as bits of rock that are full of vesicles or frozen bubbles. Cinder tends to pile up around a vent that is erupting basalt magma, forming a structure called a cinder cone, while the remaining degassed magma pools as lava that flows under the sides of the cinder cone and away from the vent.

Cinder cones are very short-lived geological structures, because the loose cinder is easily eroded away.  The cinder cone that produced the beds shown is long gone, but the beds remain, most likely because they were rapidly buried by subsequent lava flows. Here the cinder is intruded by a younger basalt dike. There are no really well-preserved cinder cones in the Jemez area, though we'll see some recognizable examples later in the book. There are some excellent young cinder cones in other locations in the Southwest, such as Capulin Mountain.

Here is some scoria from the cinder beds.

Lobato Formation
        scoria
Scoria from the Lobato Formation, Clara Peak

Scoria is the mafic counterpart of pumice, having the same composition as basalt or andesite but rich in gases that have bubbled out to form numerous cavities in the rock. It is characteristically found near the volcanic vent, where the lava retains much of its gas content, and is most common towards the top of flows, where the lighter bubbly lava accumulates. Unlike cinder, scoria solidifies on the surface of the ground rather than as small blobs blown into the air. Lava found further from the vent has usually lost its gas content while still molten and shows fewer cavities.

As we continue descending the road to the east, we reach the heart of the shield volcano.

Gabbro intrusion in
        the heart of the volcano
Gabbro intrusion in the heart of Los Cerros. 36 01.604N 106 15.374W

The reddish rock to the left is well compacted beds of basaltic cinder. These extend for about thee hundred feel along the road to the west. The gray mass to the right is gabbro, which has the same composition as basalt, but cooled slowly enough to form visible crystals. This is likely the main vent within the volcano, which cooled very slowly after the final eruptions.

Gabbro intrusion
Other side of gabbro intrusion. 36 01.729N 106 14.924W

In the center of the intrusion, we see some particularly nicely crystallized gabbro.

Gabbro intrusion

Los Cerros gabbro
        intrusion, up close

Gabbro
Gabbro from the Lobato Formation, Los Cerros. 6.0278823N 106.2571757W

The gabbro becomes relatively fine grained further to the east, likely because it cooled more rapidly, and could be described as microgabbro. An older name still sometimes used is diabase. The rock consists largely of feldspar and pyroxenes with some olivine phenocrysts, which have mostly altered to iddingsite from exposure to moisture and oxygen. This is the same composition as an olivine basalt. The distinction is entirely in the much larger size of the grains in the rock.

The surrounding basalt is notable for the presence of some lenses of gabbro.


Gabbro
          pegmatite
Veins of gabbroic pegmatite, Los Cerros. 36 01.645N 106 15.152W


Gabbro
Gabbroic pegmatite from Los Cerros. 36 01.645N 106 15.152W

These veins were presumably injected into the surrounding rock from the magma body in the central vent.

Here's a sample of the surrounding basalt.

Lobatto Formation
        basalt from Clara Peak
Basalt from the Lobato Formation, Clara Peak

Note the large white patch of caliche.

Basalt outcrops on the southeastern flank of the mountain are more massive, with no obvious cinder beds.

Lobato Formation
          massive basalt below Clara Peak
Massive basalt of the Lobato Formation, 36.0273876N 106.2543054W

These more massive beds likely erupted as pahoehoe flows.

Some of the uppermost beds of the Lobato Formation are dacite, a rock considerably richer in silica than basalt is. However, the dacite is a minor component of the formation. This is not true of the Keres Group to the south or of the La Grulla Formation to the west, where early basaltic volcanism was followed by more voluminous intermediate volcanism. Keres Group volcanism  began almost the same time as Lobato volcanism, but lasted longer and progressed to more evolved magmas before ending around 6 million years ago.

The Paliza Canyon Formation


Digital relief map of Paliza Canyon Formation exposures in
      the Jemez Mountains
Relief map of the Jemez with Paliza Canyon Formation outcroppings highlighted in green (basalt and basaltic andesite), red (andesite), and yellow (dacite).

Within about a million years of the first Lobato Formation flows, volcanism had spread to what is now the southern Jemez. Curiously, as with the northeastern Jemez, some of the earliest flows took the form of rhyolite, a rock high in silica. However, these flows were quite small. The first really voluminous eruptions were of basalt, followed by a pulse of felsic volcanism, and thereafter the eruption products varied considerably in composition, with some tendency for the later eruptions to produce more silica-rich lava. There was also a definite tendency for activity to shift to the east over time.

The bulk of the older volcanic rock in the southern Jemez is mapped as the Paliza Canyon Formation, which has a diverse composition ranging from basalt through andesite to dacite. Ages for these rocks range from 13 million years to 7 million years, with a peak from 9 to 7.5 million years. These rocks are now exposed as peaks, ridges, and domes rising above the surrounding, much younger Bandelier Formation or found at the bottoms of canyons cutting through the Bandelier Formation.

Southern Jemez
          mountains
Southern Jemez Mountains. Looking north from 35 33.149N 106 17.585W\

The Paliza Canyon Formation is the largest in the Jemez and accounts for as much as half of the original volume of the Jemez volcanic field.

Early basalts

Early basalt flows have been discovered in wells drilled in the Pajarito Plateau. The GR-4 well, drilled at the mouth of Rendija Canyon where it joins Guaje Canyon, intersected five basalt flows. The youngest was dated at 11.55 million years and the oldest at 13.22 million years. These may represent some of the oldest flows that can plausibly be assigned to the Paliza Canyon Formation, though they might also plausibly be assigned to the Lobato Formation.

The lower flows of the Paliza Canyon Formation tend to be olivine basalt, similar to those of the Lobato Formation. These are most prominently exposed in the mesas east and southeast of the village of Ponderosa. The oldest of the Paliza Canyon flows in the Ponderosa area is Chamisa Mesa.

Chamisa Mesa
Chamisa Mesa. Looking south from near 35 39.958N 106 39.878W

Chamisa Mesa is capped with a basalt flow dating back 9.9 million years. Underneath are rift fill sediments of the Santa Fe Group. The basalt of Chamisa Mesa was long thought to be the oldest volcanic formation of the southern Jemez Mountains, and it has sometimes been mapped as a separate formation from the Paliza Canyon Formation. However,  the basalt of Chamisa Mesa is now assigned to the Paliza Canyon Formation.

North of Chamisa Mesa and east of the village of Ponderosa is Borrego Mesa, which is underlain by Zia Formation sediments capped with Paliza Canyon basalt flows.

Borrego Mesa
Borrego Mesa.  Looking east from near 35 39.958N 106 39.878W

The basalt cap of Borrego Mesa is dated at around 9.5 million years old.

Borrega Mesa
          basalt outcrop
Paliza Canyon basalt on Borrego Mesa. 35 42.373N 106 37.319W

Borrega
          Mesa basalt sample
Paliza Canyon basalt of Borrego Mesa. 35 42.373N 106 37.319W

Similar basalt flows were erupted along a line of vents west of Bear Springs Peak. These flows cover a substantial area, reaching from Church Canyon near Jemez Springs to Bodega Butte on Zia Pueblo.

Paliza Canyon basalt is exposed southwest of Cerro del Pino along Forest Road 10.

Paliza Canyon
        Basalt outcropping

Paliza Canyon
        Basalt
Paliza Canyon Basalt. 35 45.188N 106 36.771W

The Paliza Canyon Formation in this area sits on top of Tertiary Abiquiu Formation volcaniclastic sandstones, which sit on Triassic Chinle Formation sediment beds. This column is exposed in San Juan Canyon, just west of Forest Road 10, which is reached by a long but easy hike along the canyon or a much shorter but steeper hike from southwest of Cerro del Pino.

San Juan Canyon
San Juan Canyon. 35 45.290N 106 37.260W

The area to the left is landslide deposits through which there are some exposures of Abiquiu Formation, seen in the last chapter. To the right, the west face of San Juan Canyon is Bandelier Tuff overlying Paliza Canyon Formation andesite and basalt, which in turn overlies Abiquiu Formation. These beds are partially mantled by soil, but here is a good exposure of Abiquiu Formation just below the contact with the Paliza Canyon Formation:

Abiquiu
          formation in west wall of San Juan Canyon
Abiquiu formation in west wall of San Juan Canyon. 35 45.551N 106 37.440W

and, a short distance further up slope, exposures of Paliza Canyon Formation two-pyroxene andesite.

Paliza Canyon formation in west wall of San Juan Canyon
Paliza Canyon formation in west wall of San Juan Canyon. 35 45.552N 106 37.465W

It would not seem right to discuss the Paliza Canyon Formation without showing some of the outcropping of basalt in Paliza Canyon itself. This outcrop is found along an old logging road not far from Forest Road 10.

Basalt in Paliza Canyon

Basalt of the Paliza Formation in Paliza Canyon. 35 43.009N 106 37.146W

Further up Paliza Canyon, there are some strikingly flow-banded basalt outcrops.

Flow-banded basalt

Flow-banded basalt of Paliza Formation. 35 43.134N 106 37.115W

Such structure in basalt usually indicates that the basalt was very cool and nearly solidified when it reached this point.

Perhaps the most accessible outcropping of Paliza Canyon Formation for the casual visitor is the basalt flow at the base of Las Conchas, just across from the Las Conchas recreational area, which is cut by State Road Four.

Paliza Formation basalt at
          Las Conchas
Paliza Formation basalt at Las Conchas. 35 48.838N 106 31.533W

Unaltered Paliza
          Formation basalt at Las Conchas
Paliza Formation basalt at Las Conchas. 35 48.838N 106 31.533W

Note the dark patches of iddingstine, showing that this is an olivine basalt. This outcrop has a relatively young radiometric age of 8.05 million years, but this is significantly younger than some flows that appear to overlie this flow further south. Much of the basalt nearby shows hydrothermal alteration, and this has likely thrown off the radiometric dating.

Many of the flows of the Paliza Canyon Formation thicken abruptly across faults. This shows that the Rio Grande Rift was continuing to open even as the Paliza Canyon lavas were erupted, with the focus of both rifting and volcanic activity steadily moving east.

The Keres Highlands

Paliza Canyon Formation underlies much of the southern rim of the Valles caldera and the area to the south. This is a region of rugged north-south canyons and ridges, some still heavily forested, with only limited access by rough gravel road. Geologists have dubbed this the Keres Highlands, since it is the type area for most of the formations making up the Keres Group.

The Keres Highlands reflects a new phase in Keres volcanism. Not only had activity shifted to the northeast, but the erupted lava was increasingly silicic. The previous basalt eruptions had heatened and softened the crust, and this made it increasingly difficult for magma to find a path to the surface. Magma stagnated deep in the crust, slowly losing heat to the surrounding country rock. As it did so, silica-poor minerals crystallized out, as we discussed in the first chapter of the book. Some of the heated country rock melted as well, adding silicic material to the magma. However, chemical clues in the magma shows that not much crustal material was assimilated, and most of the evolution of the magma was due to differentiation through fractional crystallization of low-silica minerals that settled out of the magma.

When enough of the heavier minerals had crystallized out, the magma became buoyant again, and continued rifting towards the northeast gave the magma  a better chance to force its way to the surface. Large volumes of andesitic magma were erupted, and these magmas were considerably more viscous than the basalt that preceded them. They tended to form large volcanic cones, or stratovolcanoes, composed of alternating layers of andesite flows and volcanic debris, with occasional flows of lower-silica basalt or higher-silica dacite. The Keres Highlands is a mass of coalesced stratovolcanoes, which have since been deeply faulted and eroded.

Andesite

Andesite is an extrusive igneous rock with a silica content between 57% and 63% and not more than about 6% alkali metal oxides. This is described as an intermediate composition, with more mafic magmas forming basaltic andesite, more felsic magmas forming dacite, and more alkali-rich magmas forming trachyandesite. Much of the "andesite" of the Paliza Canyon Formation is sufficiently alkaline to qualify as trachyandesite. Because andesite contains more silica than basalt does, an andesitic magma is considerably more viscous, with about the consistency of smooth peanut butter.  It can flow away from an eruptive vent, but only with difficulty, so that andesitic eruptions form volcanic hills and mountains rather than wide-spreading flows.

Like basalt, andesite is composed primarily of plagioclase and pyroxenes, though with more plagioclase and less pyroxene than basalt and with some of the pyroxene replaced with hornblende. Another distinction is that andesite usually has abundant and sometimes sizable phenocrysts, whereas phenocrysts are small and scattered in most basalts.

Here's a sample of the andesite underlying the summit of St. Peter's Dome.

Porphyritic andesite at summit of St. Peter's Dome
Andesite from road cut near summit of St. Peter's Dome. 35 45.453N 106 22.239W

This is a classical porphyritic andesite, with large phenocrysts of plagioclase and much smaller phenocrysts of pyroxene.

The eastern side of the Keres Highlands is visible from State Road Four as it turns north along the east side of upper Frijoles Canyon.

Keres Highlands
          from east
Keres Highlands. Looking west from 35 50.149N 106 24.309W

The ridge on the skyline is Aspen Ridge, the easternmost ridge of the Keres Highlands, Peeking over Aspen Ridge in a few places is the next ridge to the west, Peralta Ridge. The peak at the southern end of Aspen Ridge (at left) is Bearhead Peak.

East of the Keres Highlands, the Paliza Canyon Formation is buried under younger Bandelier Tuff, with only occasional outcrops in canyons. The eastern half of the Keres Highlands is dominated by Aspen Ridge and Peralta Ridge. Here is a panorama from a knob on Aspen Ridge:

View from knoll on Aspen Ridge

Panorama from east side of Aspen Ridge. 35 47.888N 106 30.012W

The panorama begins to the west, and Aspen Ridge extends across the left side of the panorama. You can see the road I came in. Redondo Peak is prominent on the skyline, and Cerros del Abrigo and Cerro del Medio are visible to its right, with the north caldera wall behind. Rabbit Mountain dominates the south caldera wall. On the other side of the foreground trees, we see the San Miguel Mountains in the distance, with mesas of Bandelier Tuff in the middle distance. Towards the right side of the panomara, we look almost directly down Bland Canyon. At the right end of the panorama is the southern part of Aspen Ridge.

West of Aspen Ridge is Peralta Canyon, and then Peralta Ridge. Here's the view looking east from a convenient vantage point on Peralta Ridge.

Aspen Ridge and Peralta Canyon

Peralta Canyon and Aspen Ridge viewed from Peralta Ridge. 35 47.341N 106 31.700W

Aspen Ridge stretches across the entire panorama, with Peralta Canyon in the foreground. Cerro Pico and Cerro Balitas are visible in the distance near the center of the panorama. The prominent knoll at center right is Woodard Ridge. The high knolls on the right sided of the panorama make up Bearhead Ridge, which extends  to Bearhead Peak further south.

The topographic high point of the south rim is Los Griegos at 3085 meters (10,121 feet). Just to its south is Cerro Pelado, while the peak to its east is Las Conchas Peak at the north end of Peralta Ridge.

Caldera south rim
Los Griegos and Cerro Pelado. Looking east from near 35 48.670N 106 37.247W

Los Griegos is visible right of center, with the knobby peak of Las Conchas nearly centered behind and to the left of Los Griegos. The peak just to the right of Los Griegos is Cerro Pelado. All are underlain by Paliza Canyon andesite dating to between 8.78 and 9.44 million years old. Los Griegos also has an exposed basalt beds along its eastern flank, showing that occasional eruptions of basalt continued to take place, as fresh magma from the depths sometimes found a path to the surface along an active rift fault.

Los Griegos basalt flows

Basalt flow on east flank of Los Griegos. Looking west from 35 47.879N 106 31.644W

Similar basalt beds are found on the east side of Peralta Ridge.

Basalt beds on Peralta Ridge

Basalt flow on east flank of Peralta Ridge. Looking northeast from near 35 47.879N 106 31.644W

The cliffs here seem to be part of the same basalt flows that we saw on the east flank of Los Griegos. They’re at a lower elevation here, because the Paliza Canyon Fault has thrown down this area relative to Los Griegos. Most of the canyons of the southern Jemez appear to be structurally controlled; that is, they are aligned with major faults, which produce a zone of crushed rock that is susceptible to erosion. The most extreme example of this phenomenon is San Juan Canyon, which turns sharply twice to follow cross-cutting faults. These faults are aligned with the Canada de Cochiti fault zone, to which the focus of rifting shifted during the eruption of the Paliza Canyon andesites.

The heart of the Keres Highlands is relatively inaccessible. However, the western slopes of the highlands are skirted by Forest Road 10, the main forest road through the southern Jemez, and there are good outcroppings of Paliza Canyon andesite along the road north of Cerro del Pino.

Outcropping of
        Paliza Canyon andesite

Paliza Canyon
        andesite
Paliza Canyon Formation andesite. 35 46.791N 106 35.937W

Geological papers on the Paliza Canyon Formation describe this as a glassy two-pyroxene andesite. It's widespread throughout the southern Jemez. Two-pyroxene andesite contains both clinopyroxene, Ca(Mg,Fe)Si2O6, and orthopyroxene, (Mg,Fe)2Si2O6. As with many other minerals, magnesium and iron freely substitute for each other. The presence of both forms of pyroxene in the andesite is thought to point to a specific range of conditions under which the andesite differentiated underground, though there is disagreement about how reliable this "thermometer" is.

A little further down the road is more andesite on the hillside, showing flow banding.

Andesite with
        flow banding
Andesite with flow banding. 35 46.740N 106 36.060W

Forest Road 10 provides a spectacular view across the southwestern Jemez as it begins its descent towards Paliza Canyon.

Panorama of southwestern Jemez

Panorama of southwestern Jemez. 35 43.740N 106 37.222W

Andesite flows of the Paliza Canyon Formation make up much of the  the skyline to the east (left side of panorama.) To the south is Borrego Mesa, underlain mostly with Paliza Canyon basalt with a single small andesite flow forming the dome right of center. The canyon to the left of the rise in the right side of the panorama is Paliza Canyon and to the canyon to the right is San Juan Canyon.

The Keres Highlands formed a topographic barrier to the Bandelier Tuff pyroclastic flows, which flowed around them to the southeast (in the Cochiti area) and to the southwest (forming the prominent cliffs to the right in the panorama.)

Paliza Canyon volcanism likely extended north into what is now the Valles caldera, and included flows now found at the base of the north caldera rim. The rim is best viewed from the northern caldera moat, which is the area between the topographic rim of the caldera and the ring of domes along the caldera ring fracture. We'll visit these features again in later chapters, but, for now, our interest is in the rocks underlying the north rim.


Moat
          panorama
The caldera north moat. 35 57.479N 106 31.134W

The panorama is centered on the north caldera rim, with Cerro de la Garita as the summit. The top of this mountain is porphyritic dacite and andesite of the La Grulla Plateau that is about 7.5 million years old, while most of its slope is Paliza Canyon Formation andesite that is about 8 million years old. This crops out at the base of the rim.

Paliza Canyon andesite on north rim

Paliza Canyon andesite on north rim
Paliza Canyon andesite at base of north rim. 35 58.403N 106 32.102W

The Paliza Canyon andesite in the lower part of the north rim is extensively hydrothermally altered, as we'll see later in the book, and it is separated from the overlying La Grulla Formation by thin sedimentary beds interpreted as an erosional surface. The La Grulla Formation shows no signs of hydrothermal alteration.

Late dacite volcanism

The later Paliza Canyon magmas were increasingly silicic, progressing from andesite to dacite. Dacite is so rich in silica that it forms steep knobs and domes, with occasional short stubby flows. Dacite underlies much of the highest terrain in the southern Jemez.

Dacite

Dacite is an extrusive igneous rock formed from lava that is fairly rich in silica, about 63%-77%, with less than about 8% alkali oxides. Magma with less silica forms andesite and magma with more alkali oxides forms trachyte. If the magma is rich in silica (70% or more) and crystallizes with more alkali feldspar than plagioclase, it is classified as rhyolite rather than dacite.

Dacite contains enough silica that dacitic lava is highly viscous, with about the consistency of Silly Putty. Such viscous lava does not flow easily away from its eruptive centers. As a result, most dacite eruptions take the form of a large dome with a semisolid crust that grows from within, rather like bread rising (an endogenous dome.)

Endogenous dome
Diagram of endogenous dome

When such a dome forms over a vent in a crater floor or on a lava flow, it is sometimes known as a tholoid dome. Occasionally enough dacite erupts to form thick, stubby flows extending from the eruptive center, which solidify into high ridges. These are spoken of by geologists as high-aspect flows because of their steep faces.

Dacite is composed mostly of plagioclase feldspar, with small quantities of pyroxene, hornblende, or biotite. The more silica-rich dacites contain small quantities of quartz as well. If the quartz is abundant enough to make sizable phenocrysts, but plagioclase remains the most important component, the rock may be classified as a rhyodacite.

Cerro del Pino is a dome of Paliza Canyon Formation biotite-hornblende dacite with a radiometric age of 9.42 million years. This is one of the older dacite flows in the southern Jemez. The high-silica lava forms lumpy domes and high-relief flows, which in this case extend to the east along what was likely a paleocanyon.


 Cerro del
        Pino
Cerro del Pino. Looking northeast from 35 45.188N 106 36.771W

The dacite here is quite distinctive.

Cerro del
          Pino dacite
Cerro del Pino dacite. From near 35 45.188N 106 36.771W

The sample is rich with large plagioclase phenocrysts. The loupe reveals sparse needlelike crystals of hornblende, and sparser tiny flakes of biotite.

Both Los Griegos and Las Conchas have a cap of hornblende dacite 8.71 million years old.

Las Conchas
          dacite

Las Conchas dacite

Hornblende dacite of Las Conchas and Los Griegos. 35 47.844N 106 31.686W

The dacite is choked with white plagioclase phenocrysts. This thick mush of crystals in a viscous matrix would have had considerable difficulty just emerging from its vents, yet it is present both here and on neighboring Los Griegos.

On the eastern flank of Aspen Ridge, one encounters a very pretty Paliza Canyon hornblende dacite

Hornblende dacite

Hornblende dacite of Paliza Canyon Formation. 35 47.642N 106 29.122W

Hornblende dacite
          sample

At first glance, this looks little different than the andesite further up the ridge. On closer examination, however, the plagioclase phenocrysts (white patches) are seen to have a distinct lath shape (they are euhedral) and they form clumps. Furthermore, the clumps incorporate a few black crystals of hornblende at their centers.

Where State Road Four enters the Valles caldera from the east, it crosses the north slopes of Rabbit Mountain. Rabbit Mountain is a dome of the much younger Cerro Toledo Rhyolite, but basalt and andesite of the Paliza Canyon Formation is exposed on its northwest flank, at the base of the caldera rim.

Paliza Canyon
          Formation exposed on Rabbit Mountain
Paliza Canyon Formation. 35 49.886N 106 28.942W

South of Rabbit Mountain is an extensive area of biotite-hornblende dacite of the Paliza Canyon Formation. This relatively silica-rich flow is also one of the youngest in the Paliza Canyon Formation, at about 6.5 million  years old.

Paliza
          Canyon Formation dacite dome
Paliza Canyon Formation. Looking north from 35 48.5785N 106 27.897W

The knobby appearance of this flow is again typical of high-silica lava. Here's a closer look at a sample

Paliza Canyon Formation hornblende-biotite dacite sample
Paliza Canyon Formation

The plagioclase phenocrysts are obvious, but, under the loupe, fresh surfaces show needlelike crystals of black hornblende and a few small flakes of biotite.

North of Paliza Canyon is a small dacite dome that has not been  dated, but is likely one of the youngest dacite domes of the southern Jemez. Northeast of the dome is an impressive high-aspect flow.

Dacite flow in Paliza Canyon

High-aspect dacite flow of the Paliza Formation. 35 43.213N 106 37.227W

This dacite is chemically similar to the dacites of the Tshicoma Formation, which underlie the mountains just west of Los Alamos. The dacite shows signs of having formed from a mixture of partially differentiated magma from the upper mantle and melted crust. A sample seems to have a quite low specific gravity, and the general look and feel suggests a tuff. However, it is extremely tough rock (no pun intended). It does not much resemble dacites of the Tschicoma Formation or of Cerro los Pinos, which have prominent phenocrysts.

Dacite sample

Dacite of the Paliza Formation. 35 43.213N 106 37.227W

Across the road is a large boulder field, apparently part of the same flow.

Dacite flow

Dacite flow of the Paliza Formation. 35 43.254N 106 37.236W

This has the appearance of a pristine high-silica flow. Such flows tend to be covered with boulders like these, which conceal the liquid interior of the flow. Most such flows are quickly denuded of this rubbly coating, which is easily eroded off. Here it looks like the rubble has somehow been preserved.

Dacite sample

Sample of dacite boulder from flow. 35 43.254N 106 37.236W\

Volcaniclastics

As the stratovolcanoes built a great stack of increasingly silicic volcanic rock, volcanic debris began to form an vast apron around the volcanic centers. This apron is preserved as volcaniclastic beds.

Volcaniclastics are present in quantity in Paliza Canyon. This particular outcrop is prominent in the south wall of the canyon.

Volcaniclastic cliffs

Volcaniclastics in south wall of Paliza Canyon. Looking northeast from 35 43.179N 106 37.029W


Volcaniclastics in Paliza Canyon

Volcaniclastics in south wall of Paliza Canyon. Looking south from 35 43.293N 106 36.978W

Volcaniclastics are also present in the north wall of the canyon.

Volcaniclastics

Volcaniclastics in north wall of Paliza Canyon. Looking west from 35 43.352N 106 36.918W

The exposures here are at the southwestern limit of the volcaniclastic beds of the Paliza Canyon Formation.

To the northwest, in upper San Diego Canyon, thick Lobato Formation volcaniclastic beds are present in the canyon walls near Hummingbird Music Camp.

Debris avalanche
Paliza Canyon debris avalanche.  Looking north from near 35 52.299N 106 40.151W

The Paliza Canyon exposure is the very dark rock below the cap of lighter Bandelier Tuff. This exposure is particularly interesting because it is composed of a jumble of Paliza Canyon Formation andesite boulders that appears to fill a paleovalley in the underlying Abo Formation. This valley extends south some distance and parallels the modern canyon. The exposure is interpreted as a debris avalanche that came off the flank of a Paliza Canyon volcano to the north that later foundered into the Valles caldera. The debris transitions abruptly to solid andesite flows just south of La Cueva.

Paliza Canyon basalt and andesite that may have erupted from the same stratovolcano is exposed in the west and north rims of the Valles caldera, where it overlies Santa Fe Group rift fill sediments and is overlain by Tshicoma dacite (of which we'll have much more to say in Chapter 8.)  Exposures are readily accessible along SR 126.

Paliza Canyon
          flow in west caldera rim
Paliza Canyon outcrop in western caldera rim.  Looking north from 35 53.029N 106 39.780W

Exposures continue north along Forest Road 376 on the face of the caldera rim.

Such as:

Basalt in caldera west rim

Paliza Canyon Formation basalt in caldera west rim. 35 53.850N 106 39.755W

This looked like basalt, but the area is mapped as Tertiary sediments. However, the stratigraphic position corresponds to Paliza Canyon basalt, so that’s probably what this is.

Paliza Canyon Formation basalt in west caldera rim

Paliza Canyon Formation basalt in caldera west rim. 35 53.654N 106 39.767W

Also basalt, but this time the location is at the edge of a mapped Paliza Canyon basalt outcropping.

The thickest and most extensive beds of Paliza Canyon Formation volcaniclastics are found in the southwest Jemez, in the vicinity of the San Miguel Mountains.

San Miguel Mountains

The southeasternmost Paliza Canyon Formation exposures are in the San Miguel Mountains, which are prominent on the skyline west of Frijoles Canyon Overlook at Bandelier National Monument.

Panorama of San Miguel
          Mountains from the east
Panorama of Frijoles Canyon from Frijoles Overlook. 35 46.385N 106 15.714W

We saw this picture in the last chapter, where our attention was focused on the Pajarito Escarpment at the base of the mountains. It is probably no coincidence that the San Miguel volcanic center is located nearly on the fault, which has dropped the eastern side of the volcanic center to expose a cross section of the volcano. Alas, this is heavily eroded and it is not easy to pick out the various formations at this distance.

A few features do stand out. At left are pink cliffs of what looks like more Bandelier Tuff. However, the geological map identifies this as a a much older tuff, the Peralta Tuff, which is about 6.8 million years old. The Peralta Tuff overlies Paliza Canyon flows in much of the southern Jemez, and we'll have more to say about it later.

At the center of the panorama is St Peter's Dome itself. There is an old ranger station at the summit that is just visible in the full resolution image. The sharp peak to the right is Boundary Peak, so called because it is located almost exactly on the boundary between National Forest land to the west and Bandelier National Monument to the east. Both St Peters Dome and Boundary Peak are underlain by andesite of the Paliza Canyon Formation.

The small conical hill at right is Rabbit Hill, a dome underlain by Bearhead Rhyolite. The Bearhead Rhyolite is a high-silica Keres Group formation that was erupted through the lower-silica Paliza Canyon Formation about 6 to 7 million years ago. We'll come back to this in the next chapter.

For a closer look at the San Miguel Mouintains, one must take the Dome Road (Forest Road 298), which branches off State Road 4 and passes Graduation Flats, so called because it is a traditional site for various post-graduation bacchanalia for Los Alamos High School students. The road continues south from Graduation Flats across mesas of the Bandelier Formation, through which small domes and ridges of Paliza Canyon Formation protrude.

There is a nice view of the San Miguel Mountains from the ridge to the west.

San Miguel Mountains from the west
San Miguel Mountains from the west. 35 46.807N 106 25.042W

The foreground ridge at left is underlain by Paliza Canyon Formation hornblende andesite and extends east to Rabbit Hill. The Sangre de Cristo Mountains are visible on the distant skyline to its right. St. Peter's Dome is the summit of the San Miguel Mountains, with Cerro Picacho and Cerro Balitas to the right and with Cochiti Reservoir visible in the distance between the two. In the distance, left of Cerro Picacho, is Tetilla Peak in the Cerros del Rio south of White Rock. Sandia Crest is at right in the far distance, while Bearhead Peak is visible at far right peeking over the road cut.

Here's a sample of hornblende andesite from the foreground ridge.

Hornblende andesite
Hornblende andesite. 35 46.373N 106 25.290W

White plagioclase phenocrysts are visible in the photo. Under the loupe, numerous small needelike crystals of black hornblende are also visible.

Here's a close-up panorama of the San Miguel Mountains:

San Miguel Mountains from
          the west close up
San Miguel Mountains from the west. 35 46.807N 106 25.042W

From here it's on to St. Peter's Dome itself. Taking the turnoff onto Forest Road 289, one heads east. 

A warning to the adventurous: The main Dome Road is suitable for passenger vehicles in good weather as far as the Dome turnoff. Forest Road 289 is another matter; it is rocky just east of the turnoff, and downright lousy close to the summit of St. Peter's Dome. The first rough patch can be handled by passenger vehicles if you go slowly. There is a very large mudhole further on, but passable in dry weather. The rough spots on the peak ... well, we'll get to those presently.

Most of this road crosses a surface of Tshirege Member, Bandelier Tuff. As one ascends St. Peter's Dome, one encounters dark andesite of the Paliza Canyon Formation that is in striking contrast to the Tshirege Member.

Paliza Canyon andesite

Andesite
Andesite from the Paliza Canyon Formation, St. Peter's Dome. 35 45.799N 106 22.396W

Notice the chocolate brown color in contrast with the light gray color of the hornblende andesite sample. This demonstrates that color is not a completely reliable guide to classifying volcanic rocks.

And then the road gets really bad.

Image

If you take a passenger vehicle this far, I recommend hiking the rest of the way. It's only about 300 meters to the parking area, where you should stop anyway.

The road continues past a metal gate into an area underlain by very coarse, poorly consolidated gravel. Do not drive past the gate, which marks the boundary of the Bandelier Wilderness Area, which is off-limits to vehicles.

Volcaniclastics?
Paliza Canyon volcaniclastics. 35 45.620N 106 22.335W

The San Miguel Mountains are located near where the four corners of four quadrangles meet, and the geologic maps of this area disagree slightly on what to call these gravel beds. Three of the quadrangle maps map this as Tertiary volcaniclastics of the Paliza Canyon Formation. The fourth maps this as Quaternary sediments of the Cochiti Formation. It probably comes down to  a matter of semantics: Either way, it's rock eroded from the highlands of the Paliza Canyon Formation. We'll have more to say about both the Paliza Canyon volcaniclastics and the Cochiti Formation later on.

The abandoned fire lookout is visible on top of the summit. There are numerous such lookouts throughout the Jemez area, from a time when they were the main line of defense against wildfires. Nowadays the Forest Service relies on aerial spotters and citizens with cell phones, which is much more cost effective.

The knob east of the summit (left in the photograph) is a wonderful place to take a panorama, if you don't mind scrambling and aren't bothered too much by heights.

Panorama from St. Peter's Dome
Panorama from St. Peter's Dome. 35 45.441N 106 22.135W

The left edge of the panorama looks almost directly west, with Aspen Ridge on the skyline to the right of the ranger station. Redondo Peak peeks over the nearer terrain to the right of Aspen Ridge. To its right is Rabbit Mountain, then Sawyer Dome. Then comes the remaining peaks of the Sierra de los Valles. Clara Peak is visible on the skyline ito their right. As we look north into the Rio Grande valley, Boundary Peak dominates the foreground, with beds underneath dipping to the west. There are two flows beneath the summit that form particularly resistant beds; the upper is andesite and the lower is hornblende dacite. One gains the impression that the center of the volcano lay east of the present summit and has been cut away by the Pajarito Fault, so that its remnants are buried under the Bandelier Tuff to the east. This is supported by studies of the volcaniclastic beds in the area, which show that volcanic debris was shed to the southwest from a source somewhere to the  northeast. However, the beds have also been tilted to the west by movement along the fault.

Looking east, we see red beds of the Gallisteo Formation at the foot of the San Miguel Mountains. This older formation has been exposed by the extensive displacement on the Pajarito Fault. In effect, we are looking at a complete cross section of the San Miguel volcano on this side of the mountain.

Towards the southeast is Tetilla Peak in the far distance and the southeast spur of St. Peter's Dome in the foreground. Cochiti Reservoir is visible as well.

To the south is Cerro Picacho, and, to its right, Cerro Balitas. Notice the cliffs on the east side of Cerro Picacho, formed from the Peralta Tuff of the Bearhead Rhyolite. This is the same unit that forms most of the tent rocks at Kasha-Katuwe Tent Rocks National Monument. We'll revisit this again in the next chapter.

Here's a closer view of the Gallisteo Formation red beds. We saw these in the last chapter.

Gallisteo Formation east of St. Peter's Dome
Gallisteo Formation from St. Peter's Dome. 35 45.441N 106 22.135W

There is a road cut in the andesite just below the ranger station.

Porphyritic andesite at summit of St. Peter's Dome
Road cut near summit of St. Peter's Dome. 35 45.453N 106 22.239W

Note the thin beds dipping to the west, from a possible vent just to the east. The rock here is a porphyritic andesite, with large phenocrysts of plagioclase feldspar. If you click to get the full resolution version of this picture, you can make out a bore hole on the boulder right of center where a geological sample was taken. This outcrop has been dated as 8.69 million years old.

An outcrop just south of the parking area is mapped as clotted andesite:

Clotted
          andesite near summit of St. Peter's Dome
Andesite outcrop near St. Peter's Dome. 35 45.910N 106 22.033W

Clotted
          andesite near summit of St. Peter's Dome
Clotted andesite from near St. Peter's Dome. 35 45.910N 106 22.033W

Clotted andesite is andesite containing large clumps of phenocrysts. My sample happens not to have any clots in it, alas.

The character of the San Miguel Mountains is evident from the trail from the parking area past Boundary Peak. Looking south:

Clotted andesite
          near summit of St. Peter's Dome
St. Peter's Dome. Looking south from 35.765407N 106.3629086W

The top “layer” of the mountains is identified as clotted andesite. Beneath is a rugged layer of biotite dacite, which is somewhat richer in silica than andesite and contains biotite (iron mica) in small quantities. Below that are smoother slopes of volcaniclastics, with a thin bed of plain vanilla andesite towards the bottom. The same beds are visible on the ridge to the north.

Bedding of San Miguel
          Mountains
St. Peter's Dome. Looking west from 35.764275N 106.3591687W

The volcaniclastics have a much subtler bedding, but it visible when the light is at the right angle.

Bedding of San Miguel
          Mountains
St. Peter's Dome. Looking south from 35.764095N 106.3566617W

This layering is entirely typical of stratovolcanoes.

How long did it take Nature to construct this edifice? There is a hornblende dacite plug on the northeast slopes of the San Miguel Mountains

Clotted andesite near
          summit of St. Peter's Dome
Dacite plug. Looking south from 35.76409N 106.3521437W

that has been cut in half by a strand of the Pajarito Fault on its east side. It has a radiometric date of 9.48 million years. This vent is like the source of a dacite ash bed near the base of the mountain on its north side.

Clotted andesite near
          summit of St. Peter's Dome
Dacite ash bed. Looking northwest from 35.76573N 106.3594503W

Ash beds are particularly excellent for dating, because they often contain potassium feldspar crystals that can be analyzed with great precision. An intrepid geologist fetched a sample of this ash bed and it was dated at 9.47 million years old, indistinguishable from the vent. Since the beds atop St. Peter’s Dome have been dated at 8.69 million years, it took a little under a million years for the entire thick sequence of volcanic beds from low in Capulin Canyon to high atop St. Peter’s Dome to be erupted.

The volcaniclastic beds that make up much of the volume of the San Miguel Mountains are well exposed west of Boundary Peak.

Clotted andesite
          near summit of St. Peter's Dome
Volcaniclastic beds. Looking northwest from 35.76573N 106.3594503W

Similar beds crop out at many locations along the Boundary Peak trail and the St. Peter's Dome trail to Cerro Picacho and beyond. The beds generally show numerous broken clasts of volcanic rock in a well-cemented ashy matrix.

To the south of the San Miguel Mountains, in the area around Cerro Pico and in Sanchez Canyon, there are exposures of some of the oldest volcanic rocks of the Jemez field. These are overlain by thick beds of Paliza Canyon volcaniclastics.

Panorama
Panorama from southeast of Cerro Balitas. 35 43.360N 106 23.082W

Cerro Picacho dominates the left side of the panorama, while to the right, we look down Sanchez Canyon to its confluence with White Rock Canyon.The volcaniclastics form the very rugged terrain south of Cerro Picacho. There are great thicknesses of volcaniclastic beds of the Paliza Canyon Formation stretching west from the San Miguel Mountains.

Some of the old volcanic rocks in Sanchez Canyon have radioisotope ages in excess of 12 million years, but these may be mistaken; more recent dating makes them much younger, around 9.6 million years old. Older still are some basanite flows on the southeast flank of the San Miguel Mountains that are interbedded with Santa Fe Group sediments; these have been dated to 18 million years old or older, and may represent the very earliest beginnings of Jemez volcanism. However, because they are interbedded with Santa Fe Group sediments, and because their chemistry is distinctly different from any younger Jemez flows, they are usually regarded as pre-Jemez flows. The geology of the San Miguel Mountains is complex and of considerable scientific interest, but the area south and east of the mountains is designated wilderness area with no roads, few trails, and challenging hiking that makes it difficult to investigate.

The ridge from which this photograph was taken has exposures of one of the more unusual rocks of the San Miguel Mountains, olivine andesite:

Andesite
Olivine andesite. 35 43.360N 106 23.082W

Olivine
          andesite
Olivine andesite. 35 43.360N 106 23.082W

Under the distinctive rusty weathering surface, the rock does indeed look like an andesite. It is somewhat vesicular, typical for the top of a flow, and there are scattered blobs of what looks like iddingsite, indicating that this is an olivine andesite.

Olivine andesite is of special interest because it breaks the rules. There isn't a reasonable composition for magma that gives you the intermediate silica content of andesite in equilibrium with olivine. Olivine is normally found only in low-silica rocks. Geologists have concluded that this is a non-equilibrium composition, probably formed when a basaltic magma containing phenocrysts of olivine mixed with a high-silica magma on its way to the surface. The olivine phenocrysts didn’t have time to react with the excess silica in the surrounding magma to convert to pyroxenes, which is strong evidence that the mixing occurred just before the eruption of this magma. Such non-equilibrium compositions are so common that some volcanologists believe that magma mixing is one of the most important triggers for an eruption.

Although the San Miguel Mountains stand out from the remainder of the southern Jemez, and were clearly an important and long-lived eruptive center, they are not distinct from the rest of the Keres Group either in age or composition. The volcanic beds here are particularly impressive from a combination of subsidence around 9 million years ago that allowed volcaniclastics to accumulate, followed by tilting and uplift of the San Miguel Mountain block around 6 million years ago that has exposed an impressive cross section of the beds.

Cochiti Canyon and environs

I mentioned that thick volcaniclastic beds extend west of the San Miguel Mountains. These are beautifully exposed in upper Cochiti Canyon, which is easily viewed from the Dome Road.

Cochiti Canyon
          panorama
Cochiti Canyon. 35 45.999N 106 25.150W

The canyon is rimmed with Bandelier Tuff, but the slopes and wild terrain in the bottom of the canyon are Paliza Canyon Formation volcaniclastics. A thin ash bed in the canyon wall, just visible halfway down the slope to the left, has been dated at 9.5 million years old, indistinguishable from the dacite vent we saw earlier in the San Miguel Mountains.. You can see that the terrain over which the Bandelier Tuff erupted was heavily eroded and quite irregular. The upper part of the modern canyon, to the right, coincides with a paleocanyon that was later filled with a great thickness of Otowi Member, Bandelier Tuff.

The next picture shows Tshirege Member, Bandelier Tuff, to the right, sitting directly on a topographic high of the Paliza Canyon Formation volcaniclastics beds.

Tshirege on volcanicalstics
Bandelier tuff on Paliza volcaniclastics.  Looking northwest from 35 45.087N 106 24.716W

Volcaniclastics come in several varieties, and geologists who have closely studied this area have divided the beds accordingly. Autoclastic flow breccia are beds of broken andesite with occasional layers of massive lava. Typically 70% of the bed will be broken rock and 30% will be massive lava. These beds are typical of the central peak of a composite volcano. Their presence here confirms that one or more andesitic volcanoes erupted very close to this area.

Pyroclastic flow deposits in this area probably resulted from collapse of andesitic or dacitic domes, which send red-hot rock and ash tumbling down the slopes of the volcano as block and ash flows. These are characterized by igneous clasts in a well-cemented ashy matrix. The clasts often show fracturing that is interpreted as cooling fractures, showing that the clasts were red-hot when the beds were deposited. We'll see some excellent examples of these later in the book.

Debris-flow deposits have more rounded clasts and a less well-cemented matrix indicating that the material was cool and probably water-saturated when deposited. We saw such deposits in the Espinaso Formation in the last chatper, and we'll see more examples later on. A few such flows are almost entirely ash and pumice and represent explosive eruptions.

Hyperconcentrated-flow deposits were deposited from a mixture of water and volcanic debris in which water predominated. They resemble stream deposits, but from a particularly swift and muddy stream rich in material eroded off the volcano.

The volcaniclastic beds may extend for great distances in the subsurface to the north and east. Drilling on the Pajarito Plateau has revealed subsurface beds composed of volcaniclastics of the Paliza Canyon Formation.

The area northeast of Cochiti Canyon and west of the San Miguel Mountains contains several exposures of Paliza Canyon Formation andesite and dacite protruding above the Bandelier Tuff.

Forest Road 298 south of Graduation Flats passes a dome of hornblende dacite.


Paliza Canyon Formation
        hornblende dacite knoll
Knoll of hornblende dacite of Paliza Canyon Formation. 35 47.684N 106 25.235W

Hornblende dacite
Horblence dacite of the Paliza Canyon Formation. 35 47.684N 106 25.235W

This sample show large white crystals of plagioclase in a dark matrix, with some smaller crystals of hornblende clearly visible in the geologist's loupe. Most dacite has fairly large plagioclase phenocrysts, but the ground mass is unusually dark in this specimen.

We have seen that the flows of the Paliza Canyon Formation overlie rocks as young as the Tertiary Santa Fe Group (in the southern Jemez) and as old as the Permian Abo Group (in the western Jemez.) Rifting had already thrown down and buried the older rocks further east, whereas they were exposed at the surface by erosion to the west. The Paliza Canyon Formation also thickens dramatically from west to east, from the thin beds in the west rim of the caldera to the very thick volcaniclastic beds of Cochiti Canyon. This reflects the vertical displacement across the western margin of the Rio Grande Rift.

If the Lobato Formation is the oldest rock unambiguously associated with the Jemez volcanic field, first erupting about 14 million years ago; and the volcanic activity then spread south, with the Paliza Canyon Formation first erupting 13 million years ago; then what came in the area between? Most geologists think that mafic volcanism occurred in the entire area from Lobato Mesa to Borrego Mesa from about 13 to 8 million years ago. However, the area between the southern Jemez and the northeastern Jemez is taken up by the Valles Caldera to the west, the Sierra de los Valles further east, and, beyond that, the Pajarito Plateau. These features are all significantly younger than the Lobato and Paliza Canyon Formations, and they have mostly buried or destroyed any older rocks. However, there are outcroppings of andesite in Guaje Canyon (9.6 million years old) and in Santa Clara Canyon (7.8 million years old). All these outcroppings are presently inaccessible because of flood damage to the access roads or because they are on tribal lands.

Los Alamos Canyon and points east

Los Alamos Canyon is a deep canyon located just south of the city of Los Alamos. The canyon is crossed by Omega Bridge, which connects the city with Los Alamos National Laboratory and the road to the Jemez Mountains. The laboratory ran a small research reactor in the lower canyon for many years, while the city built an ice skating rink near the bridge, where the steep canyon walls shelters the rink from the winter sun. Further up the canyon, a small reservoir was constructed, with a beautiful hiking trail and some picnic areas.

From its upper reaches to just below the reservoir, the canyon bottom exposes older rocks of the Jemez volcanic field. An exposure of andesite buttresses the south end of the Los Alamos Reservoir dam.

Paliza Canyon
          andesite at Los Alamos Reservoir

Paliza
          Canyon andesite at Los Alamos Reservoir
Paliza Canyon Formation andesite buttressing Los Alamos Reservoir dam. 35 52.987N 106 21.234W

This andesite has been dated at 8.72 million years old. It has the right age and composition to be an outcrop of the Paliza Canyon Formation, and is part of a flow that is elsewhere buried under younger rocks of the Pajarito Plateau.

Across the canyon, a large outcrop of andesite towers over the reservoir

Paliza
          Canyon andesite at Los Alamos Reservoir
Paliza Canyon Formation andesite north of Los Alamos Reservoir dam. 35 53.041N 106 21.285W

This outcrop sits just to the right of a fault that cuts across the canyon, with the downthrown side to the left (west). Here a small sliver of crust has dropped a few tens of feet.

Younger flows are found interbedded with the Santa Fe Group sedimentary beds in wells drilled in the Buckman area. These are about 8 to 9 million  years in age, the same as the Paliza Canyon Formation. Whether they should be included in the Keres Group or the Santa Fe Group is a question for which there is no definitive answer.

Canovas Canyon Rhyolite

Digital relief map of Canovas Canyon Rhyolite exposures
        in the Jemez Mountains
Relief map of the Jemez with Canovas Canyon Rhyolite outcroppings highlighted in red

On the southern end of Borrego Mesa, two basalt flows of the Paliza Canyon Formation are exposed, the older correlating with the basalt of Chamisa Mesa. Between the flows is a substantial bed of rhyolite tuff, formed from high-silica volcanic ash. There are additional extensive exposures of rhyolite flows, domes, and tuff beds south and east of Borrego Mesa, particularly around Bear Springs Peak, and more isolated outcrops as far east as the San Miguel Mountains. These are collectively known as the Canovas Canyon Rhyolite.

With limited radiometric dating, it was natural for geologists to conclude that the basalt of Chamisa Mesa was the oldest in the southern Jemez, and was followed by the Canovas Canyon Rhyolite. This in turn was followed by the oldest basalt flows of the Paliza Canyon Formation. Improved mapping, dating, and petrological analyses have largely shattered this pretty picture. It now appears that the Canovas Canyon Rhyolite includes flows with ages from over 12 million years to around 8 million years in age, roughly the same age range as the Paliza Canyon Formation. The two formations erupted over the same extended period of time.

However, the Canovas Canyon Rhyolite is chemically distinct from the Paliza Canyon Formation. If one extrapolates the composition of Paliza Canyon Formation dacites into the rhyolite field, the resulting composition differs in subtle but significant ways from the Canovas Canyon Rhyolite. Geologists interpret this to mean that, whereas the Paliza Canyon Formation magmas originated in the mantle before differentiating as they rose to the surface, the Canovas Canyon Rhyolite formed from lower crustal material that was melted by the heat of basaltic magma that did not make it to the surface.

Stagnation and bimodal volcanism

The Lobato Mesa Formation and Paliza Canyon Formation basalts were formed from magma that originated in the hot asthenosphere rising into the Rio Grande Rift. Because this primitive magma was significantly less dense than the surrounding solid rock from which it melted, it was subject to buoyant forces that drove it towards the surface.

However, the journey was likely neither swift nor uninterrupted. As the magma rose, the country rock around it became increasingly less dense. At first, this did not matter much, since the magma was also expanding and becoming less dense as the pressure dropped. However, the magma eventually reached the Moho, the boundary between crust and mantle, where the density of the country rock jumped from the 3.4 g/cm3 of the upper mantle to the 3.0 g/cm3 of the lower crust. This is close to the density of primitive magma, and so it is likely that most primitive magmas stagnate at the lower boundary of the crust, underplating the crust. Only in rare cases is the magma able to continue rising through deep fractures in especially cold, dense crust to form unusually silica-poor rocks such as basanite. We saw an example of a basanite in the La Cienega area in the last chapter, and some basanite flows are interbedded with Santa Fe Group beds on the southeast flank of the San Miguel Mountains.

Magma that stagnates at the Moho slowly cools, and it may also assimilate small amounts of material from the lower crust. The two are connected: Heat lost by the cooling magma goes into heating the country rock. Either way, the effect is to increase the silica content of the magma, by adding new silica-rich material and by crystallizing out silica-poor minerals. This can  lower the density of the magma enough for it to regain buoyancy. Where the crust is thin and highly fractured, as it is along the Rio Grande Rift, the magma can rise in quantity to the surface and erupt as basalt lava.

Such eruptions are self-limiting in most continental settings. Basalt lava has a maximum density of about 3.1 g/cm3, while the rocks of the upper crust have a density of less than 2.7 g/cm3. The magma is able to make it to the surface only because the country rock is rigid enough to form a "pipe" for the heavy magma, and because the magma contains considerable dissolved gas, which begins bubbling out as the pressure drops. The gas bubbles lower the effective density of the magma and help drive it to the surface. But as the country rock begins to soften, it tends to seal up the path to the surface, and the release of gas in an eruption increases the density of the remaining magma. The eruption ceases long before the supply of magma is exhausted. The magma remaining in the vent effectively plugs it, so that subsequent eruptions usually find a different path to the surface. As a result, continental basaltic volcanic fields are typically monogenetic, meaning that they are composed of many small eruptive centers, most of which only erupt a few times. Each such cone is relatively small, typically from 0.25 to 2.5 km (0.15 to 1.5 miles) in diameter.

The remaining magma deep underground will tend to spread out at the level where its density matches the country rock. In some cases the magma quietly cools and solidifies at this level, forming various kinds of intrusive bodies. If the magma spreads as a relatively thin layer sandwiched between the beds of the country rock, it forms a sill.

Sill

Sill in Chinle Formation beds in Hagen Basin. 35 16.504N 106 17.147W

There are likely some sills underlying the Jemez volcanic field, but because the field is young, these have not been exposed by erosion yet. The sill here is significantly older than the Jemez, at around 30 million years old, but much younger than the Triassic beds it has intruded.

Magma stagnating in the crust can undergo additional differentiation as it cools and low-silica minerals crystallize out. It can also assimilate more crust material. The body of cooling magma, known as a magma chamber, may eventually become buoyant enough to resume its rise to the surface, and can erupt as andesite or dacite. This was the origin of most of the flows of the Paliza Canyon Formation. But the magma can also melt a significant amount of high-silica crustal rock without mixing. The magma becomes zoned, with a layer of low-silica magma from the mantle at the bottom and a layer of high-silica magma from melted crust at the top.

This is a plausible explanation for a common geological phenomenon called bimodal volcanism. It is surprisingly common for a volcanic field to erupt low-silica lava followed by high-silica lava, often with little intermediate-silica lava. The southern Jemez are not the most extreme example; here the the first eruptions have compositions ranging all the way from basalt to dacite, followed by rhyolite. In other volcanic fields, such as Yellowstone, and in other parts of the Jemez, there is much less of the intermediate composition rocks. The low-silica magma shows chemical signatures of having come primarily from the mantle, while the high-silica magma shows chemical signatures of having formed from melted crust rather than from differentiation of mantle magma.

Melting of crustal rock by underlying stagnated mafic magma also explains how so much rhyolite can be produced in continental volcanism. Simple differentiation of primitive magma from the mantle is an inadequate source, since fully 90% of a basaltic magma must crystallize out before the remainder has a rhyolitic composition.

Rhyolite

Rhyolite is the most silica-rich of all extrusive volcanic rocks, with a silica content of at least 69% (more for rhyolite poor in alkali oxides). It is normally very fine-grained or even glassy (having no discernible crystal structure at all), though it may contain phenocrysts of quartz, feldspar, hornblende, or biotite. 

The high silica content makes rhyolite magma extremely viscous, with about the consistency of cold roofing tar. Rhyolite magma can hardly make it out of its eruptive vent intact. Instead, the dissolved gases in the magma typically turn it into a froth of tiny bubbles that blows itself apart on contact with air, forming volcanic ash that consists of vast numbers of tiny fragments of solidified bubble fragments. Under the microscope, these appear as concave bits of volcanic glass. When the ash falls to the surface while still hot enough to be soft, it can compact under its own weight to form a solid rock called tuff. It is very common, though, for the ash to fall some distance from the vent as solid particles, or to form the deadliest of all volcanic phenomenon, the pyroclastic flow.

The viscosity of rhyolitic magma depends strongly on the content of dissolved gases. Rhyolitic magmas are quite "wet", with a water content of up to 7%. This is because the minerals that crystallize first from a primitive magma are "dry" minerals with no room for water in their structure, so the water is concentrated in the residual melt. As I discussed in Chapter 1, water tends to break up silica networks and lower the viscosity of magma, so as a rhyolitic magma loses its gas content, it becomes even more viscous. On the other hand, carbon dioxide tends to make a magma more viscous, but carbon dioxide has low solubility in silica melts and tends to bubble out of the magma while it is still underground. This may actually be a factor in triggering high-silica eruptions.

The exposures of the Canovas Canyon Rhyolite tend to be located in some of the most inaccessible terrain in the southern Jemez, much of which can be reached only by difficult forest roads or on tribal lands closed to the public. However, one of the largest exposures, at Bear Springs, is accessible without too much difficulty via Forest Road 266. The intrepid explorer is advised to map out the route in advance and use a GPS system to navigate, since there are a number of poorly marked forks in the road.

The most prominent exposure of the Canovas Canyon Rhyolite is probably Tres Cerros West, located not far northeast of Bear Springs.

Tres Cerros West

Tres Cerros West. Looking east from 35 41.796N 106 33.696W

The mountain is underlain by massive rhyolite, while there are rhyolite tuff beds exposed in a road cut to the southwest.

Canovas Canyon Rhyolite tuff bed in road cut

Canovas Canyon Rhyolite tuff bed in roadcut. 35 41.517N 106 33.652W

Canovas Canyon Rhyolite crops out on the hill just to the north of the Bear Springs ranger station.

Canovas Canyon Rhyolite north of Bear Springs

Canovas Canyon Rhyolite just north of Bear Springs. 35 40.501N 106 33.012W

Samples from the area include lithic tuff

Lithic
          tuff

and flow banded massive rhyolite.

Flow banded massive rhyolite

The Canovas Canyon Rhyolite is very thick in this area, though since its base is not exposed, we do not know the full thickness. The lower beds and the peaks corresponding to eruption centers are massive flows, with tuff beds overlying the massive flows in valley bottoms where the hot ash apparently ponded. The transition between the two is gradual enough to suggest that the two erupted together. This is rather unusual for rhyolite, as is the thinness of the flows; rhyolite is usually so viscous that it can barely make it out of the ground, and it usually forms very steep flows and domes. One wonders if the rhyolite was unusually hot or wet or both.

In any case, the rhyolite eruption appears to have been effusive rather than explosive, in contrast with the rhyolite that produced the Bandelier Tuff. Geologists have speculated that the crust was being rifted apart here so quickly 8 to 10 million years ago that magma had no time to build up for a big explosion. It was released gradually and relatively gently through the many fissures produced by the rifting.

Further west, the thick beds of Canovas Canyon Rhyolite at Bear Springs disappear under younger and less silicic flows of the Paliza Canyon Formation. This is the field criterion for identifying a tuff or rhyolite in the southern Jemez as Canovas Canyon Formation. A good example is an outcrop of tuff in the nothern wall of middle Paliza Canyon.

Canovas Canyon Rhyolite tuff in upper Paliza Canyon

Canovas Canyon Rhyolite tuff beds in upper Paliza Canyon. 35 43.907N 106 35.747W

These beds are identified as part of the Canovas Canyon Rhyolite by their stratigraphic location: The skyline visible above the tuff is Paliza Canyon basalt. Dark boulders of Paliza Canyon basalt from the upper cliffs litter the colluvium apron below the tuff beds. Few fragments of tuff are present here; they are much softer and quickly erode away.

There is a very large block of tuff on the slope with a rather striking surface.

Tuff block with unusual erosional surrface

I don’t know if this is an usual erosional surface or the cooling surface of the original flow.

Here are the beds close up.

C anovas
          Canyon Rhyolite tuff beds

Canovas
          Canyon Rhyolite tuff beds

The beds include some that are very lithic-rich.

Lithic-rich Canovas Canyon Rhyolite tuff beds

Canovas Canyon Rhyolite tuff bed sample

This fine-grained bed looks like a surge deposit.

Canovas Canyon Rhyolite tuff bed surge deposits

Canovas Canyon Rhyolite tuff bed surge bed sample

while the upper beds are more massive.

Canovas Canyon Rhyolite massive tuff beds

Canovas Canyon Rhyolite massive tuff sample

Nearby, on the west escarpment of Borrego Mesa, reworked volcanic ash assigned to the Canovas Canyon Formation overlies much older beds of Jurassic age.

Reworked tuff
          on Borrego Mesa
Canovas Canyon Rhyolite on Borrego Mesa. Near 35 40.631N 106 38.612W


Canovas Canyon reworked tuff

Another outlying area of exposure of Canovas Canyon Rhyolite is found far to the east, in the San Miguel Mountains. The most accessible dated exposure is southeast of Cerro Balitas, and can be reached by a short if strenuous hike from the Dome Road.

Canovas Canyon Rhyolite
Canovas Canyon Rhyolite southeast of Cerro Balitas. Near 35 43.301N 106 22.991

The outcrop here is very close to the point where my geologic map indicates Canovas Canyon Rhyolite was sampled and dated at 10.5 million years in age. Here's a sample.

Canovas Canyon Rhyolite
Canovas Canyon Rhyolite southeast of Cerro Balitas. Near 35 43.301N 106 22.991

Under the loupe, the sample shows phenocrysts of feldspar and blebs of what appears to be iddingsite, but it is particularly striking for the numerous sizable flakes of biotite. It looks more like a dacite than a rhyolite, but the Canovas Canyon Rhyolite does include some low-silica rhyolites bordering on dacite.

The furthest northeast exposure of Canovas Canyon Rhyolite is found on the northeast foot of the San Miguel Mountains, where it has been exposed in a landslide scarp.

Canovas
          Canyon Rhyolite

Canovas Canyon Rhyolite
Canovas Canyon Rhyolite on northeast foot of San Miguel Mountains. Near 35.76424N 106.3505707W

Notice the resemblance to some of the Canovas Canyon tuff beds in Paliza Canyon, shown earlier.  Further to the east the rhyolite is exposed in a small arroyo.

Canovas Canyon Rhyolite

Canovas Canyon Rhyolite
Canovas Canyon Rhyolite east of San Miguel Mountains. Near 35.761738N 106.3453987W

This outcrop appears to lie below sandstone beds mapped as Tertiary Santa Fe Group beds. It has not been dated but may be an intrusion younger than the beds, or the beds may be relatively young rift fill.

The picture we have so far of Keres volcanism is of initial low-silica eruptions coming almost directly from the upper mantle. This hot magma softened the crust through which it passed, sealing the fissures through which it had formerly erupted, and hot magma began to accumulate in pockets deep in the crust. This magma cooled and differentiated, with low-silica minerals crystallizing out to leave andesitic magma. Further rifting in the area allowed this magma to reach the surface, along with high-silica magma formed as independent pockets of melted crust, which erupted as Canovas Canyon Rhyolite. Finally, some of the andesitic and rhyolitic magma mixed to form relatively small quantities of dacitic magma that erupted as domes and short flows.

Much of the magma solidified underground, and some of this was eventually exposed by erosion to form one of the more economically interesting outcrops in the southern Jemez, the Bland intrusion. We'll learn more about the Bland intrusion in the next chapter.

The La Grulla Formation


Digital relief map of La Grulla Formation exposures in the
      Jemez Mountains
Relief map of the Jemez with La Grulla Formation outcroppings highlighted in red (andesite), green (basalt), and yellow (dacite).

The La Grulla Plateau is a large subalpine plateau that is reached via Forest Road 100 from Youngsville. As the road begins to ascend the plateau, there is a beautiful view across Canones Canyon towards Polvadera Peak to the southeast.

Polvadera panorama
Polvadera panorama. Looking southeast from 36 06.874N 106 30.570W

The mesa across Canones Canyon is Mesa del Medio. To the left one sees the mesa underlain by Santa Fe Group with a cap of Bandelier Tuff. To the right this gives way to basalt flows of the La Grulla volcanic center. Polvadera Peak is right of center and Cerro Pelon is the smaller peak just left of center. Both are dacite domes of the Tschicoma Formation.

Nearby the road cuts through andesites of the La Grulla Plateau. These were long lumped together with the Lobato Mesa Formation to the east or the Tschicoma Formation to the southeast, but geologists now think these were separate pulses of volcanic activity, with the La Grulla eruptions peaking a bit later than Lobato, about 7.5 million years ago. This reflects a general shift in the Jemez from volcanism along a northeast-southest line from Borrego Mesa to Lobato Mesa to a north-south line from the Keres Highlands to the La Grulla Plateau. There are also chemical differences between the La Grulla flows and the Lobato Mesa flows. The Grulla Plateau andesites and dacites are richer in alkali oxides than their Tschicoma Formation counterparts. Geologists have concluded that the La Grulla andesites and dacites formed from straightforward differentiation of primitive magmas while the Tshicoma Formation formed from differentiated magma that mixed with basalt magma.

La Grulla
        andesite under a cliff of Bandelier Tuff
La Grulla andesite. 36 06.697N 106 30.599W

La
            Grulla andesite
La Grulla andesite

The La Grulla Plateau formed from an area that was flooded with basalt and andesite lava. The resistant lava protected the area from erosion, and subsequent faulting to the east and west left the plateau standing more or less by itself except on the south, where it was buried under Tschicoma lava that now forms the Valles Caldera north rim. The plateau has low rolling hills and long meadows marking faults, mostly trending north or northeast. It's a wonderful place to camp.

Here is a fault valley with a small lake.

La Grulla
        Plateau
Fault valley on the La Grulla Plateau. 36 06.244N 106 32.327W

And here is a photo looking along a fault meadow.

Fault
        meadow
Looking northeast along a fault trace. 36 07.236N 106 32.822W

There is a spectacular lookout at Encino Point which provides a view of much of the interesting geology in the area. Here's a panorama.

Encino Point
        panorama
Panorama from Encino Point. 36 07.621N 106 33.114W

On the far skyline is the San Pedro Mountains, a northern extension of the Sierra Nacimiento, where a huge block of crust has been thrust up by tectonic forces. Mesa Naranja is visible to the right, and above it on the skyline is Mesa Alta. Both mesas are underlain by Permian and Triassic sedimentary rocks.

Underneath the ranger lookout, the escarpment exposes a cross section of an endogenous dome associated with the La Grulla eruptions.

Encino dome
Encino dome. Looking north from 36 7.197N 106 33.495W

The dome is located at the center of the picture. It consists of a mass of dacite which pushed up from beneath while in the liquid state. One can see that the andesite beds have been forced upwards above the dome. (Click to enlarge.) This suggests that this dome never reached the surface.

Looking northwest, one sees the basalt dikes of Los Barrancos, the rugged area below the escarpment. This was long thought to be a landslide area, but we now know that the slabs of hardened lava are actually dikes and that this area was a volcanic center that was broken up by faulting and eroded to its current form.

Los Barrancos
Encino barrancos. Looking northwest from 36 7.197N 106 33.495W

At the northern end of La Grulla Plateau, the andesite flows lie atop an extensive hydromagmatic deposit, produced when magma intruded rock that was saturated with water. The magma and rock exploded (a phreatomagmatic eruption) to leave large chunks of country rock embedded in a matrix of cinders.

Hydromagmatic
          deposits
Hydromagmatic deposits. Near 36 07.924N 106 32.712W

Hydromagmatic
          deposits
Hydromagmatic deposits. Near 36 07.924N 106 32.712W

Hydromagmatic
          deposits
Hydromagmatic deposits. Near 36 07.924N 106 32.712W

The continuing heat from the eruption and steam from groundwater tended to bake the whole mess together; in this case, the cement is largely calcite, which forms occasional large crystals of dogtooth spar.

On the way out, the road passes another mountain meadow strewn with andesite boulders.

Mountain meadow on
        the La Grulla Plateau
La Grulla Plateau. Looking south from near 36 05.613N 106 32.516W

The broad hill in the middle distance is part of the Four Hills eruptive center, and is underlain by hornblende dacite 7.35 million years old. Beyond is the north rim of the Valles caldera.

The vista from the headwaters of Polvadera Creek, near here, shows one of the most famous landforms created by the La Grulla eruptions, Cerro Pedernal.

Polvadera Creek
        overlook
Polvadera Creek overlook. Looking north from near 36 01.267N 106 26.931W

That's Cerro Pedernal in the center, and Polvadera Peak again off to the right.

Closer up:

Cerro Pedernal
Cerro Pedernal from the south

Cerro Pedernal has a hard cap of La Grulla basalts underlain by an impressive rock column of Permian, Triassic, Cretaceous, and Tertiary sediments, covering some 300 million years of the Earth's history. However, there are a number of gaps (unconformities) in the rock column. We discussed some of these in earlier chapters of this book.

The La Grulla flows extended south into what is now the Valles caldera, and La Grulla andesite is exposed in the north rim of the caldera.

La Grulla
          andesite in Valles caldera north rim

Close view of La Grulla andesite in caldera north rim
La Grulla andesite in caldera north rim. 35 59.995N 106 30.127W

There are multiple flows making up the plateau and the rim of the caldera. A higher flow show similar prophyritic character but a gray rather than red color.

La
          Grulla andesite on Valles north rim

La
          Grulla andesite on Valles north rim
La Grulla andesite in caldera north rim. 36 00.037N 106 30.109W

This is a reminder that color is often the least diagnostic feature of an igneous rock. Further east along the rim, the flows transition to a biotite-hornblende dacite that is more silica-rich, more massive, and has sparser phenocrysts.

La Grulla dacite
La Grulla dacite in caldera north rim. 35 59.983N 106 29.766W

The transition from La Grulla dacites to Tschicoma dacites occurs somewhere east of this point and is poorly defined.

Paleomap of the Lower Keres Group

This map shows the lower units of the Keres Group (Lobato Mesa, Paliza Canyon, Canovas Canyon, and La Grulla) and the age ranges of the beds they rest on.

Keres Group paleomap
Paleomap of Keres Group. Keres Group in red, Santa Fe Group contacts in yellow, pre-Rift Tertiary and Mesozoic contacts in green, Paleozoic contacts in blue

Most of the Keres Group rests on Santa Fe Group sedimentary beds. Only the westernmost contacts, in the Rio de la Guadalupe valley, rest on older Abo or Madera beds. However, the Santa Fe Group beds vary greatly in thickness, from just a few meters thick in the western caldera rim to hundreds of meters thick to the east. This indicates that the Sierra Nacimiento was uplifted enough to expose Paleozoic beds on its eastern flank, but only in a narrow zone that gave way further east to rift fill sediments. The Keres Group was erupted largely through these sediments. The presence of inverted topography and of Santa Fe Group beds under Keres flows in canyons that are now eroded down to pre-Santa Fe beds shows how much uplift and erosion has taken place just in the last ten million years or so.

Next page: What goes up must come down

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