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

The previous chapter may 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. The Lobato Formation
      1. Basalt
      2. Pyroxenes
      3. Olivine
      4. Lobato Mesa and Clara Peak
        1. Aa and Pahoehoe
  2. The Paliza Canyon Formation
    1. Los Alamos Canyon
    2. San Miguel Mountains
      1. Andesite
    3. Cochiti Canyon and environs
      1. Dacite
    4. The Keres Highlands
  3. Canovas Canyon Rhyolite
    1. Stagnation and bimodal volcanism
    2. Rhyolite
  4. The La Grulla Plateau

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. This means that "first Jemez eruption" is partly a matter of definition. Furthermore, subsequent eruptions may have buried any traces of the earliest Jemez eruptions. But most geologists place the beginnings of the Jemez Volcanic Field at about 14 million years ago.

A second burst of rifting

The first phase of rifting along the Rio Grande Rift lasted from about 30 to about 20 million years ago. This was followed by a pause lasting about six million years. When rifting resumed 14 million years ago, the most active displacement  shifted east to the Canada de Cochiti Fault Zone in the southern 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 ancient suture where the Yavapai and Mazatzal Precambrian provinces were fused together. The mantle here seems to be fertile for production of magma.

Geologists divide the volcanic rock that has been erupted in the Jemez over the last fourteen million years into three groups. The Polvadera Group is composed mostly of formations of the northern Jemez dating from before the first known supervolcano eruption, 1.85 million years ago. The Keres Group is composed mostly of formations of the southern Jemez dating from roughly the same time period as the Polvadera Group. The Tewa Group is composed of formations younger than about 1.85 million years old, which dominate the central Jemez. There is some 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 assign all the older volcanic rocks of the Jemez to the Keres Group.

The Lobato Formation

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)

The Lobato Formation was erupted between 13.3 million and 7.6 million years ago, with the period of highest activity from 10.8 to 9.1 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 consolidated 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 13.3 million years. Santa Clara Canyon is on tribal lands of the Santa Clara Tribe, and the tribe 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.

A somewhat younger Lobato Formation basalt flow is 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 comes down to the road cut.

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

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, or perhaps even a basanite. It's impossible to be certain without a laboratory analysis.

Basalt

I introduced basalt in the first chapter of this book, but now that our story has arrived at the time of the Lobato Formation, it's time to look at this variety of igneous rock more closely.

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 hot mantle that rose into the Rio Grande Rift, producing 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 forms picrobasalt. Magma more enriched in alkali metals forms trachybasalt. Most basalt is relatively "dry", with a water content between about 0.1% and 1.5%. The more alkaline basalts tend to have the higher water content.

Basalt consists mostly of plagioclase feldspar and pyroxenes. There are often a few small phenocrysts, and these are used to further classify basalts. Most of the basalt of the Lobato Mesa Formation is olivine basalt. The Lobato Mesa 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 as 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 ferrosilite, (Fe,Mg)SiO3; 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 and Mg2SiO4, 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 extra electrons required by this structure are balanced by the iron and magnesium. Olivine crystals are typical deep green in color, but are unstable under surface conditions, slowly decomposing 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 decomposed to iddingsite.

The presence of olivine indicates that a volcanic rock is quite low in silica. In particular, quartz is almost never found in the same volcanic rock as olivine. 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. In some uncommon intrusive rocks, the olivine can become rich enough in iron that it is able to coexist with quartz.

Lobato Mesa and Clara Peak

From Gallina Mesa, one can look out over Lobato Mesa and the high plateau to its west.

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

The meadow in the center 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 itself until its branches reach National Forest land to the southwest, south, and east.

To the east of Gallina Mesa is the Clara Peak volcanic center of the Lobato Mesa Formation. Clara Peak itself is 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. 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.

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 this 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 desert climates (such as the modern Jemez area) 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.

The view from here is spectacular.

Clara Peak panorama
Los Cerros panorama. 36 01.356N 106 15.737W

That's Santa Clara Canyon in the foreground and extending up into the Tschicoma highlands at right. Tschicoma Peak itself is on the skyline in the fifth frame, with Polvadera Peak at extreme right.

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 with
          dike on Los Cerros
Cinder with dike on Los Cerros. 36 01.604N 106 15.400W

Cinder is formed when basalt magma is erupted that contains considerable dissolved gas. When the 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. 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 and are here intruded by a younger basalt dike. There are no really well-preserved cinder cones in the Jemez area, though there are some excellent young cinder cones in other locations in the Southwest, such as Capulin Mountain.

As we continue descending the road to the east, we find that the heart of the shield volcano has been exposed by faulting and erosion.

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 presumably 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

And here we see the other side of the vent.

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

Los Cerros gabbro
        intrusion, up close

Diabase
Diabase from the Lobato Formation, Los Cerros. 36 01.680N 106 15.247W

The gabbro here is relatively fine grained, since it cooled close to the surface (a hypabyssal rock) 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.

The intrusion is notable for the presence of some lenses of much more coarsely crystalline rock.


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 the last bits of liquid magma in the vent. This would have been unusually rich in water, which helps silicate minerals crystallize into larger crystals. This is not quite coarse enough to be described as a gabbroic pegmatite, but close.

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.

Here is some scoria from the remnants of a cinder cone buried under subsequent flows.

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. 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.

The Lobato Mesa Formation caps the mesas forming the highlands south of the Abiquiu area.

Northeast
          Jemez

Northeast Jemez. Looking south from 36 14.769N 106 22.017W

At left, Abiquiu Mesa and, at a higher level, El Alto. On the right side of the second frame, Arroyo de los Frijoles with Polvadera Peak on the skyline. The third frame is filled by Canones Mesa with its exposures of Abiquiu Formation, and Cerro Pedernal, beloved of artist Georgia O’Keefe, stands at the right side of the third frame. Of the features visible here, only El Alto is underlain by flows of the Lobato Mesa Formation. The other flows are at least four million years younger. All will be revisited later in the book.

A contrasting view is this, from the northeast.

The
          Jemez from the northeast

The Jemez Mountains from the northeast. 36 02.451N 106 03.877W

Lobato Mesa itself forms the eastern escarpment of the plateau, visible to the right in this panorama, and it can be reached via a forest road up its west face.  The basalt capping the mesa is shown below.

Lobato Mesa olivine basalt

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

Olivine basalt
          from Lobato Mesa

A nice gray basalt, with 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 rather than a tholeiite. (The geologic map for the area does not say.)

Here is the view from the eastern escarpment of Lobato Mesa.

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.

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 west. We saw a closer view of this area in the last chapter. In the distance just to the right of the escarpment is Sierra Negra.

To the east, beyond the hills of Ojo Caliente Member, are ridges cored with basalt dikes. These appear to be the same age and composition as the Lobato Mesa Formation, and point to an eruptive center to the east that is now almost completely eroded away.

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, but I have no way to take photographs through the binoculars. These flows range from 10.2 to 10.8 million years old and are composed of olivine basalt with sizable feldspar phenocrysts.

Here is the view of El Alto from the east side of Lobato Mesa.

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. We'll have more to say about this area later in the book. 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 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, mafic volcanism had spread to what is now the southern Jemez. However, the volcanic activity here did not long remain mafic. Soon after the first basalts were erupted, there was 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.

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.

The Paliza Canyon Formation is one of the largest in the Jemez and accounts for as much as half of the original volume of the Jemez volcanic field. I'll describe it in some detail, starting in the east and working my way clockwise around the caldera.

Los Alamos Canyon

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 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, 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.

There are no further exposed outcroppings of the Paliza Canyon Formation to the south until one reaches Frijoles Canyon north 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 shows that this is actually 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 are underlain by andesite of the Paliza Canyon Formation.

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, 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.

Let's return now to the panorama of the San Miguel Mountains. Behind and to the right of the San Miguel Mountains is a part of Aspen Ridge, one of the most prominent exposures of the Paliza Canyon Formation. Aspen Ridge, and Peralta Ridge to its west, form the heart of what geologists call the Keres Highlands of the southern Jemez. These are underlain by Paliza Canyon Formation and other formations of the Keres Group.

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 is part of a low ridge extending east to Rabbit Hill. The Sangre de Cristo Mountains are visible on the distant skyline to the 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. 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 probably 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. However, it is more likely that the beds have been tilted to the west by 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; if I read the geologic map correctly, these cliffs are 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.

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 Pico 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 Pico. Volcaniclastics are beds of broken rock fragments produced by volcanic activity and subsequent erosion, and 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. Older still are some alkaline basalt 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, 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 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 — a mixture of quartz and iron oxides formed by alteration of olivine.

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.

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. 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

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.

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 has 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.

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.

The Keres Highlands

Paliza Canyon Formation underlies much of the southern rim of the Valles caldera and the region to the south. This is a region of rugged parallel 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 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 is Bearhead Peak.

Further west, State Road Four enters the Valles caldera north of Rabbit Mountain. Rabbit Mountain itself 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 typical of high-silica lava. Here's a closer look at a sample

Paliza Canyon Formation hornblende-biotite dacite sample
Paliza Canyon Formation. Looking north from 35 48.5785N 106 27.897W

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

South of this area, the Paliza Canyon Formation is buried under younger Bandelier Tuff, with only occasional outcrops in canyons. West and southwest are the exposures of Aspen Ridge and Peralta Ridge that form the heart of the Keres Highlands. 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 (not to be confused with 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. All are underlain by Paliza Canyon andesite dating to between 8.78 and 9.44 million years old, but Los Griegos also has exposed basalt beds along its eastern flank.

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

If I’m reading my geological map correctly, the cliffs above center are 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.

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.

Down section 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.

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 radiometric age of 8.05 million years. 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 may have thrown off the radiometric dating slightly.

To the west of Los Griegos, the caldera rim is buried under younger El Cajete Pumice beds. The next photograph was still taken further west, from a point along the south caldera rim, looking back east along the rim.

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

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 Forest Road 10 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

Cerro del Pino itself is a dome of Paliza Canyon Formation biotite-hornblende dacite, similar to the exposures south of Rabbit Mountain but significantly older at 9.42 million years. Again, 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.

Paliza Canyon basalt is exposed southwest of Cerro del Pino. This is probably the most convenient location for collecting a sample.

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

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 in the rightmost frame.)

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

Just east of this outcrop are sedimentary beds that may be Paliza Formation volcaniclastics.

Paliza Formation volcaniclastic beds?

Probable Paliza Formation volcaniclastic beds. Near 35 43.009N 106 37.146W

There is a small chance these are much younger river or lake deposits, but Paliza Formation volcaniclastics are mapped in the area and that seems the more likely interpretation. Thes are particularly fine beds, likely reworked by water.

Just north of this location is a small dacite dome. 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\

East of the dacite dome, and further up Paliza Canyon, 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.

Volcaniclastics are also 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.

The lower flows of the Paliza Canyon Formation tend to be olivine basalt, similar to that of the Lobato Formation. These are most prominently exposed in the mesas east and southeast of the village of Ponderosa. Borrego Mesa itself is underlain by basalt of the Paliza Canyon Formation, 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

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 is 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, some of the oldest flows of the Paliza Canyon Formation have now been dated back as much as 13 million years and the basalt of Chamisa Mesa is now assigned to the Paliza Canyon Formation.

East 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

There are also a few exposures on the flanks of Borrego Mesa of the Jurassic Entrada, Todilto, Summerville, and Morrison Formations. We saw some of these in earlier chapters.

There are no exposures of the Paliza Canyon Formation to the west. Bandelier Tuff rests directly on Paleozoic and Mesozoic rocks that must have formed a highland at the time of the Paliza eruptions. We must head north into San Diego Canyon to pick up the Paliza Canyon Formation again, in the canyon walls near Hummingbird Music Camp.

Debris
          avalanche
Paliza Canyon debris avalanche.  Looking north from 35 48.924N 106 40.087W

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 andesite 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 north 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 north rim of the caldera.


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.

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 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 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 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.

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. I'll have quite a lot to say about tuff later in the book, but for now, it is sufficient to know that tuff is 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 Peak Springs, and more isolated outcrops as far east as the San Miguel Mountains. These are collectively known as the Canovas Canyon Rhyolite.

With limited radiological data, 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

Earlier I noted that 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 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 or picritic basalt. Picritic basalt is characterized by a high olivine content, arising from a very low silica content without the elevated levels of alkali oxides seen in basanite. We saw an example of a basanite in the La Cienega area in the last chapter, but picritic basalt is rarely erupted on continents and none is found in the Jemez Area.

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. But it 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. We'll have quite a bit to say about pyroclastic flows later in the book.

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

This sample has a thick crust of quartz blebs.

Silica blebs on Canovas Canyon Rhyolite

I'd speculate that the nearly pure quartz encrusting this sample is hydrothermal in origin, deposited by silica-rich fluids some time after the rock originally cooled.

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

Another outlying area of exposure of Canovas Canyon Rhyolite is found far to the east, in the San Miguel Mountains. The most accessible 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.

Across Sanchez Canyon, east of Cerro Balitas, tuffs of the Canovas Canyon Formation are exposed on the southern flanks of Cerro Pico.

"Pink
          tuff"
"Pink tuff". 35 43.967N 106 23.945W

The cliffs partway down the side of the canyon, to the left and right, are Tshirege Member, Bandelier Tuff, which are remnants of flows that likely filled the ancestral Sanchez Canyon. However, the light pink tuff on top of the far side of the canyon, just above and right of the center of the photograph, is the "pink tuff" of the San Miguel Mountains. This has been identified as a tuff of the Canovas Canyon Rhyolite. Alas, this can be reached only by driving along the fairly rotten road to St. Peters Dome, then hiking a trail to north of Cerro Pico, then bushwacking across the rugged slopes of Cerro Pico to this location. 

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 it 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 Plateau


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.

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, there is 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 probably Cerro Pavo, a dacite dome about 7.63 million years old. Beyond is probably Redondo Peak in the center 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.

Next page: What goes up must come down

Copyright © 2014-2015 Kent G. Budge. All rights reserved.