The previous chapter may be found here.
Paliza Canyon volcaniclastics. 35 45.620N 106 22.335W
In this chapter, we will look at the formation of vast beds of volcanic sediments accompanying early volcanism in the Jemez volcanic field towards the end of the Tertiary Period.
Gravity is a relentless leveling force, and, over geological time spans, it can bring down the highest mountain. This is particularly true where the climate favors erosion and the rocks underlying the mountain are soft or otherwise easily decomposed. The eroded sediments can produce widespread and thick beds, as they do in places in the Jemez.
In addition, volcanoes tend to eject a lot of solid particles
into the air during eruptions. These, too, must come down
somewhere, and they form a variety of rock types described as pyroclastic.
Pyroclastic rocks lie somewhere in the borderlands between igneous
rocks and sedimentary rocks. As the products of volcanic activity,
they seem like they would be igneous rocks; but as rocks formed
from deposits of broken rock fragments, they seem like fair game
to be claimed as sedimentary rocks. Early geologists had some
difficulty deciding how to classify them, particularly since it
was uncertain exactly how various kinds of pyroclastic rocks
formed. Those composed of microscopic clasts cemented together
were lumped together as tuffs.
With further observation of active volcanism and study of prehistoric pyroclastic rocks, geologists came to understand that pyroclastics formed by more than one process, and that some were clearly igneous rocks while others were clearly sedimentary. Pyroclastic rocks whose clasts were cemented by their own residual heat, or by components of the hot gases that were erupted with the clasts, were clearly igneous and became known as ignimbrites. Those that had been reworked by wind or water became known as volcaniclastics, and are usually regarded as sedimentary. Tuffs turned out to include rocks from both groups. There are also cinder and pumice beds, which when fresh and poorly consolidated are usually considered igneous but become sedimentary when they are cemented by ground water, and maar deposits, which are right on the border between igneous and sedimentary rocks and are probably best regarded as sui generis, in a class of their own. Examples of all these pyroclastic rocks are found in the Jemez area.
Most magma contains considerable dissolved gas, mainly water vapor and carbon dioxide. The gas remains dissolved in the magma so long as the magma remains deep underground and under pressure from the weight of the overlying country rock. However, when the magma approaches the surface and the pressure begins to drop, the gases begin bubbling out of the magma.
In a low-silica magma, gas molecules move easily through the magma to join existing gas bubbles, instead of forming new gas bubbles. As a result, the bubbles are few and large. When the magma reaches the surface, these large bubbles burst relatively gently. The lava may fountain to a considerable height, but the individual blobs of lava are fairly large and fall to the surface close to the vent. Now free of gas, the lava can pool to form flows. This is the kind of eruption that tourists in Hawaii view from fairly close up in reasonable safety.
In a high-silica magma, gas molecules have great difficulty diffusing to existing bubbles and tend to form new bubbles instead. The magma becomes a foam of microscopic bubbles. At the same time, the magma is rapidly solidifying. The solidification is so fast that crystals do not have time to form, and the magma solidifies into volcanic glass instead. If the glass forming the walls of the bubbles is not strong enough to withstand the pressure of the gas within, the rock disintegrates into fine dust as the bubbles shatter. The final product is a hot mixture of volcanic gases in which is suspended countless tiny shards of volcanic glass. These shards continue to emit residual gas for some time, and this helps keep the shards suspended in the mixture.
Clasts formed from degassing magma are collectively known as tephra,
regardless of size. The finest clasts, under 2mm in diameter, are
classified as ash. Those between 2mm and 64 mm (2.5
inches) in diameter are known as lapilli, while larger
tephra are classified as either volcanic bombs or volcanic
blocks, depending on whether they are still partially molten
when they reach the surface (bombs) or are already largely
The mixture of magma, tephra, fragments of country rock, and hot gases accelerates rapidly as it approaches the surface, sometimes reaching velocities of over 600 m/s (2000 feet per second) by the time it leaves the vent. This jet of material is known as an eruptive column. What happens thereafter is determined by whether the eruptive column mixes with enough of the surrounding air to become less dense than the atmosphere.
Eruption of Mt. Pinatubo. USGS
If enough air is entrained to make the eruptive column less dense than the atmosphere, then buoyant forces will cause the eruptive column to continue rising, often to more than 30km (20 miles). This is well into the stratosphere. Such an eruptive column is called a convective eruptive column or a plume. Once the column reaches an altitude where its density is equal to the surrounding air, it slows rapidly and begins to spread out to form an umbrella or mushroom cloud.
As the eruption column expands and cools, the ash begins to fall back to the earth. The coarsest clasts will tend to fall out close to the vent, while finer ash may be deposited a considerable distance downwind. Ash from the Jemez area has been identified as far as 500 km (300 miles) downwind. The very finest ash particles that reach the stratosphere may remain there for years.
Ash fall beds have a number of distinctive characteristics. They tend to form an even blanket of uniform thickness over the existing terrain, and the clasts are typically well-sorted. The ash is typically white to grey in color, which often makes an ash bed stand out in a sequence of sedimentary beds. Because the ash is already quite cool when it is deposited, fresh ash beds are unconsolidated. However, because the ash is rich in colloidal silica, it will rapidly become cemented if it moistened by rain or groundwater, and so ash beds are often well preserved in the geological record.
Pumice fall resembles ash fall in all but the size of the clasts. If a high-silica magma is a bit less viscous when it erupts, perhaps because it is unusually hot or rich in water vapor, it may resist disintegrating to ash under its internal pressure and can then solidify as a glassy foam of larger bubbles. Such rock is known as pumice, and it ranges in size from lapilli to small volcanic blocks. Pumice contains so much void space that it will usually float on water.
The sample is about 10 cm (4 inches) long and so is classified as
a volcanic block rather than a lapillus. Under close examination,
we find that many of the voids are elongated along the length of
the sample, suggesting the pumice was stretched while still
partially molten. This is quite common. Under the loupe, the
pumice shows a few phenocrysts of quartz and feldspar, some tiny
fragments of mafic rock, and even a few bits of older pumice
entrained in the younger rock.
Like ash fall beds, pumice fall beds are fairly uniform in
thickness and tend to be well-sorted, and they are unconsolidated
when fresh but can become well-consolidated after they absorb
Pumice and ash falls beds that have become indurated in this
manner should probably be regarded as sedimentary rocks, since
they have undergone a form of diagenesis. Indurated ash beds are
one of the rock types that earlier geologists identified as tuff.
Ignimbrites are formed from volcanic ash deposited in a pyroclastic flow. These form when an eruption column does not entrain enough air to become less dense than the surrounding atmosphere. The result is column collapse. The hot mixture of gas and particles falls back to the surface and flows along the ground, often for distances of many kilometers. Because the mixture is heavier than air, it tends to pool in low areas. The clasts in the flow are kept suspended in part by their own emissions of residual hot gas, but mostly from rising air entrained and heated at the front of the flow. When the supply of heat runs out, the particles settle onto the surface. The resulting rock formation is called an ignimbrite, though earlier geologists described it as tuff, and the name continues to be widely used for this rock type.
Welded tuff showing fiamme. 35.54.857N
If the pyroclastic flow is hot enough, the clasts will still be
slightly molten when they settle on the surface. The clasts will
partially merge and there may be local flow, which will tend to
smear out the larger clasts in the flow (often fragments of
pumice) in a distinctive way (fiamme). Such an ignimbrite
is known as a welded tuff.
Unwelded tuff showing intact pumice and lithic clasts. 35 52.233N 106 13.390W
If the flow is not as hot, so that the clasts are fully solidified when the settle on the surface, they will retain their individual identity to produce an unwelded tuff. However, the hot gases trapped in the flow will cement the clasts together as they cool, so that an unwelded tuff is typically still solid rock, albeit a rock that is usually softer than a welded tuff. Further deposition of cement by ground water can further indurate either kind of tuff, blurring the distinction between igneous and sedimentary rock, but ignimbrite tuffs are usually placed on the igneous side of the line.
Pyroclastic flows can reach speeds of over 300 km/h (200 miles per hour). Faster still are the hot pressure waves (blasts) that move ahead of them and to their sides, and which are one of the most serious hazards of volcanic eruptions. Behind the blasts are clouds of hot, turbulent gases with a relatively low ash content known as surges, which can produce characteristic surge deposits.
We will see examples of all these kinds of pyroclastic flows in
the pages that follow.
Lahars are volcanic mud flows. They are formed when heavy
rain mixes with fresh volcanic ash to form a dense mixture that
can flow for miles down existing slopes and river channels, then
harden like concrete when it comes to rest. We will have much more
to say about lahars in the next chapter. Lahars are probably on
the sedimentary side of the line between igneous and sedimentary
Reworked tuffs are formed over longer periods of time when ash is exposed to wind and water, and they are definitely sedimentary rocks. The Abiquiu Formation contains many beds of reworked tuff. The cementing of these rocks is brought about by the same processes of diagenesis that are at work in other kinds of sedimentary rock, though the abundance of silica in the ash aids the process and can produce very hard rock.
I mentioned in the previous chapter that some of the domes around the San Miguel Mountains came from a later stage of the eruption there. These domes are part of a formation called the Bearhead Rhyolite, named for its type location at Bearhead Peak, that is generally younger (6-7 million years old) and considerably richer in silica than the Paliza Canyon Formation. The Bearhead Rhyolite is found in the same general area as the Paliza Canyon Formation, with the Bearhead Rhyolite typically lying on top of the Paliza Canyon rocks. In some places, erosion has exposed lava vents in the Paliza Canyon Formation that are filled with Bearhead Rhyolite.
One of the more accessible outcroppings of Bearhead Rhyolite is along Forest Road 298, well south of the side road to St. Peter's Dome. The road passes west of Cerro Balitas, a plug of Bearhead Rhyolite and tuffs that we saw earlier in our panorama of the area.
Cerro Balitas looking north from 35 43.311N 106 24.162W
The foreground is Tshirege Member, Bandelier Tuff. The darker ridge behind it is Paliza Canyon Formation, while Cerro Balitas itself is Bearhead Rhyolite with a cap of Peralta Tuff. The road cuts through an outcropping of Bearhead Rhyolite as it passes west of Cerro Balitas.
Outcrop of Bearhead Rhyolite. 35 44.049N 106 24.555W
The geologic map for this area maps this as hypabyssal rhyolite, which cooled just below the surface.
Here is a photograph of a sample of Bearhead Rhyolite from the
outcropping shown above.
Bearhead Rhyolite. 35 44.049N 106 24.555W
There are signs of flow banding, wihch shows that the magma was still slowly flowing up to the time it solidified. Under the loupe, the sample shows sparse phenocrysts of feldspar, quartz, and biotite mica. The ground mass is very fine grained, looking almost like a siltstone.
Bearhead Rhyolite is most extensive in the southern Jemez, but thin dikes (too small to show on the relief map) are widespread and occur even in the north caldera moat area. For example, a dike of Bearhead Rhyolite can be seen in the road bed of Forest Road 36 inside the south caldera rim:
Bearhead Rhyolite dike in road bed. 335 49.860N 106 28.862W
This is just within the Valles Preserve boundaries, so this
sample was photographed in place. Another dike occurs along Forest
Road 208 on Aspen Ridge.
Bearhead Rhyolite along Aspen Fault. 35
48.078N 106 30.199W
This exposure occurs along the Aspen Fault. The exposure can be traced a considerable distance north along the fault. Many exposures of Bearhead Rhyolite in the southern Jemez appear to lie along faults of the Cochiti Fault Zone, which was active at the time the Bearhead Rhyolite was erupted. These faults gave the rhyolite magma an easy path to the surface.
Here‘s an unusually well-exposed contact between a Bearhead Rhyolite dike and the surrounding Paliza Canyon dacite.
Bearhead Rhyolite dike in Paliza Canyon
47.642N 106 29.127W
The light-colored Bearhead Rhyolite is visiblein the foreground, with an abrupt transition to dark Paliza Canyon dacite futher from the camera.
Here is an outcrop of Bearhead Rhyolite in the northern moat:
Bearhead Rhyolite. 35 58.526N 106 31.282W
There are small outcrops of Bearhead Rhyolite throughout the eastern side of the northern moat area. Geologists interpret these dikes as feeder conduits connecting overlying domes and flows (now mostly eroded away) with a vast body of rhyolite underground, the Bearhead Batholith. It's kind of conjectural that it's actually down there, but the conjecture is a good one.
The Bearhead Rhyolite takes its name from Bearhead
Peak, one of the major eruptive centers of this formation.
Bearhead Peak is one of the most inaccessible points in the
southern Jemez, being surrounded by deep canyons and rugged
terrain with no forest road nearby. However, one can get a distant
view of Bearhead Peak from the Veterans'
Memorial Overlook at Kashe-Katuwe National Monument. This is
located on the south rim of Canada Canyon and overlooks
spectacular scenery to the north, where several canyons converge
in an area underlain by Peralta Tuff.
Panorama from Veteran's Memorial Scenic Overlook. 35 40.398N 106 27.162W
The peak near the center of the second frame is Bearhead Peak.
In the foreground, dark cliffs mapped as Bearhead Rhyolite are
visible across the canyon on the right side of the first frame and
the left side of the third frame. The rounded hill between is
mapped as Cochiti Formation. The third and fourth frames show
Peralta Tuff in the lower slopes, with numerous tent rocks, while
the darker pink to orange cap forming the mesa tops is Bandelier
Tuff. The contacts between the two members of the Bandelier Tuff,
and between the lower member of the Bandelier Tuff and the
underlying Peralta Tuff, are well displayed in the third frame.
(Click for a higher-resolution view.) We'll have more to say about
the Cochiti Formation, and the Bandelier Tuff in later sections of
The overall picture geologists get is that mafic volcanism ceased
in the Jemez around 7 million years ago, and did not resume again
until about 4 million years ago. However, felsic volcanism
continued for about another million years.
I discussed the phenomenon of bimodal magmatism in the last chapter. While the magma that produced Paliza Canyon Formation originated mostly in the mantle, the magma that produced the Bearhead Rhyolite was mostly melted crust. In an area like the Jemez, in which magma repeatedly rises from the depths, a considerable amount of high-silica magma can build up to form a giant magma chamber in the middle crust. We'll see the consequences later in this book.
The Bearhead Rhyolite is not significantly different in
composition from the Canovas Canyon Rhyolite that preceded it. It
is more widespread and is generally found overlying Paliza Canyon
Formation flows. This may seem like a somewhat arbitrary
distinction. However, it is common for otherwise similar
formations to be separated by marker beds of distinctive
character. The marker bed that distinguishes the older Canovas
Canyon Rhyolite from the younger Bearhead Rhyolite is the Peralta
Tuff Member of the Bearhead Rhyolite.
The Peralta Tuff is thought to be about 7 million years old. It forms thick beds in some parts of the southeast Jemez, particularly around Kasha-Katuwe Tent Rocks National Monument. This is a relatively new national monument, established as almost one of the last acts of President Clinton in office. It is located in the southwest Jemez north of Cochiti Pueblo, and one must drive through the pueblo to reach it. The monument itself is on Bureau of Land Management land and is administered by BLM, a break with the usual rule for national parks and monuments.High-silica volcanism, such as that which produced the Bearhead Rhyolite, is violent. The magma is extremely viscous and tends to be saturated with dissolved gas, and this makes for explosive volcanism. Such volcanism is responsible for the Valles caldera and the Bandelier Tuff that dominates the Los Alamos area. However, no caldera has been identified in the southern Jemez, and geologists speculate that the rapid widening of the Rio Grande Rift (which peaked in this area around 12 million years ago and was still quite active at 7 million years) tended to open fissures through which the magma was able to erupt before enough magma build up for a caldera eruption. The eruption was still violent enough to produce a great variety of pyroclastic rocks.
The area around Kasha-Katuwe Tent Rocks National Monument is
underlain by two formations, the Peralta Tuff and the Cochiti
Formation. The Peralta Tuff is mostly ash and pumice falls, but
with a number of ignimbrite beds (one source says 14) and some
beds of volcanic sediments reworked by floods and streams. Some of
the pyroclastic flows likely originated from Bearhead Peak.
The distinctive feature of Kasha-Katuwe Tent Rocks is, unsurprisingly, the tent rocks. These form when relatively soft beds of ash are capped by somewhat harder beds that partially protect the beds underneath from erosion.
The tent rocks are eroded out of the Peralta Tuff. The Cochiti Formation forms the thin darker layer on top.
A local sign explains some of the geology. Click to enlarge.
A spectacular feature of the monument is the slot canyon that has
been eroded into the Peralta Tuff. The trail to the slot canyon
first passes south of a magnificent exposure of the Peralta Tuff
in a cliff side.
Peralta Tuff. Looking northwest from 35 39.607N 106 24.546W
Note the hoodoo atop the cliff, left of center, and the second hoodoo halfway down the cliff. (Click to enlarge.) These form when a boulder protects the softer beds immediately underneath from erosion. The darker bands are reworked beds with considerable soil mixed in, while the purer white bands are individual pyroclastic flows or ash fall beds.
These beds are also exposed in the entrance to the slot canyon
and throughout its length.
This closeup shows the varying texture of the beds, which are
sometimes very fine grained and sometimes contain quite large
clasts. The walking stick for scale is about five feet tall.
The bed towards the bottom with some very large bounders in it is
probably a lahar. Lahars tend to be very poorly sorted.
This large tree looks very much as if flash floods have eroded away several feet of soil from around the roots. I don't know the actual history.
The slot canyon has a magnificent, eerie beauty.
Here there is a resistant layer that has eroded out of the softer
beds above and below. My guess is that this layer is particularly
rich in volcanic ash, which provides a source of silica for
cementing the grains tightly together. Possibly this is a thin
lahar flow between pumice falls. Note again my walking stick for
Here is a "mushroom" consisting of a resistant silica-rich bed above less resistant sediments. Much larger examples are found in the monument among the tent rocks, but it was interesting to see this one up close.
At first glance, the "mushroom" looks like a pumice bed, but on
close examination, one finds that the individual clasts are
actually solid rhyolite.
Some slabs of conglomerate on the canyon floor looked for all the
world like concrete. Concrete is, after all, artificial
conglomerate. I have sometimes mistaken heavily weathered concrete
for conglomerate in the field, and I doubt I'm the first
geologist, amateur or professional, to have done so.
Some slabs are so heavily opalized (cemented together with amorphous silica) that they are glossy in luster, and nearly as hard as quartzite.
It's even possible that this is actually a solid block of flow
banded rhyolite of the Bearhead Peak Rhyolite that has been
carried into the slot canyon by a flash flood. However, the
visible grains and layering suggest opalized sediment beds.
I mentioned Cerro Balitas earlier, since there is hypabyssal Bearhead Rhyolite exposed on its northwest flank. The rest of the hill is underlain by Peralta Tuff showing a variety of facies.
The lower slopes of the hill on its northern flank are underlain
by Tsherige Member, Bandelier Tuff. This is exposed as solid
bedrock at the bottom of the numerous gulleys which, together with
the heavy and thorny scrub growth, make hiking around the hill
Gully in solid rock
The Tsherige Member forms the relatively level surface north of
the hill, dissected here by the headwaters of Sanchez Canyon.
Panorama of San Miguel Mountains from the southwest
The southern Sierra de los Valles are the distant skyline in the first frame. St. Peter's Dome appears as the flat topped mountain near the right edge of the second frame. Cerro Pico dominates the third frame. The flank of Cerro Balitas fills the final frame.
Partway up the flank, the Peralta Tuff is exposed. It is rich in
lithic clasts, mostly of mafic rocks of the Paliza Canyon
Formation on which Cerro Balitas and its outlying flows sit.
Bearhead Rhyolite? 35 44.174N 106 24.392W
Northwest of Cerro Balitas one comes across a striking
outcropping of tuff.
Peralta Tuff with high lithic content. Near Near 35 44.103N 106 24.007W
This is a superb example of a lithic-rich tuff. Some of the lithic clasts are football-sized:
Peralta Tuff with very large lithic blocks. Near 35 44.103N 106 24.007W
The two dark rocks at lower right are actually partially embedded in the white tuff around them. It is an indication how energetic the eruption was that it could sweep up such large blocks and carry them along.
Peralta Tuff with coarse lithics. Near 35 44.103N 106 24.007W
This lithic tuff resembles a conglomerate, but cemented by red-hot ash rather than minerals brought in by ground water.
Looking back at the outcrop.
Lithic Peralta Tuff. Near 35 44.103N 106 24.007W
There is a large flow of very lithic-rich tuff to the southeast from
Lithic Peralta Tuff. Near 35 44.103N 106 24.007W
Really lithic-rich tuff. 35 43.967N 106 23.945W
Strictly speaking, to qualify as a lithic tuff, the tuff must
have more volume of lithic fragments than of ash or phenocrysts.
The earlier examples don't quite make it, but this exposure
qualifies as a lithic tuff.
Block and ash flow. Near 35 43.967N 106 23.945W
This is a lithic tuff, but the bits of rock appear to be composed of Bearhead Rhyolite, like the tuff itself, rather than Paliza Canyon Formation, as in the earlier examples. This is very likely an example of a block and ash flow, typically of high-silica volcanism. Magma high in silica is extremely viscous, flowing only with the greatest difficulty. As a result, when it erupts from a vent, it tends to dome up rather than flow away from the vent. Such domes can become tens of meters high. When the face of a dome becomes steep enough, the solid outer crust can collapse, exposing the gas-rich liquid magma within. This explodes into red-hot volcanic ash, which mingles with the fragments of the solid outer crust to produce a flow like the one here. The mechanism is a little like the famous eruption of Mt. St. Helens, but on a much smaller scale. Or you can think of the famous eruption of Mt. St. Helens as the Mother of All Block and Ash Flows. I'm easy.
Here is a panorama of Cerro Balitas seen from the southeast.
Southeast flank of Cerro Balitas. 35 43.837N 106 23.706W
Notice that the thin tuff beds at the head of the small canyon dip fairly steeply to the southwest (left) but the beds beyond, on the far side of the canyon and in Cerro Balitas itself, are level. It's hard for me to imagine that there isn't a fault running down the canyon, but the only fault mapped in this area is just over the ridge on the far side of the canyon. Go figure.
More block and ash flows.
Block and ash flow on southeast flank of Cerro Balitas. 35 43.837N 106 23.706W
And one more. This doesn't really show any geology I haven't already photographed, but it is probably the most Ansel Adams moment from this expedition.
Block and ash flow on southeast flank of Cerro Balitas. 35 43.754N 106 23.614W
Here is a view from beyond the end of the tuff flow. Note that
the flow is fairly steep at its face, making it an example of a
Southeast end of Cerro Balitas tuff beds. Looking northwest from near 35 43.501N 106 23.273W
While scrambling up the flow, I ran across some impressive surge deposits.
Surge beds. 35 43.546N 106 23.394W
Surge deposits are common at the start or the end of a tuff
eruption. Surge deposits differ from ordinary pyroclastic flows in
that they are thinly bedded and well sorted, and likely formed
from a mixture of gas and particles that was richer in gas. The
particles are kept suspended by the turbulence in the flow more
than by their own emission of gas. Such flows are relatively low
in density and are more able to cross topographic barriers than
ordinary pyroclastic flows.
This deposit was located towards the top of the ridge. It is clear that the ridge was formed by multiple flows, and not a single eruptive episode.
The upper slopes of Cerro Balitas are a difficult climb, but some
boulders detached from the upper slopes are more accessible. These
prove to be similar to the lithic tuff we've seen earlier.
Tuff boulder. 35 44.050N 106 24.097W
So Cerro Balitas is a knob of tuff surrounded by tuff, except for the outcrop of solid hypabyssal rhyolite around the northwest flank. This may be the actual eruptive center, which has mostly eroded away, leaving Cerro Balitas as a remnant.
Over geologic time scales, it is almost inevitable that groundwater will react with the rocks through which it passes, altering the chemistry of both. Where the water is cool and relatively pure and the flow rate is modest, such reactions will be slow, but when the water is hot, acidic, and constantly flowing, the reactions will be relatively fast and dramatic. This results in hydrothermal alteration of the rock. Geologists recognize many forms of hydrothermal alteration, depending on the temperature and chemistry of the water.
In many places, the Paliza Canyon Formation shows hydrothermal
alteration. For example, here's an outcropping of Paliza Canyon
Formation andesite at the base of the Valles caldera north rim.
Hydrothermally altered andesite. 35 58.310N 106 31.102W
At first glance, this looks like another local outcrop of the
Bearhead Rhyolite. However, closer examination reveals that it was
probably originally andesite. The rock has been altered by
hydrothermal processes. A few feet further along the road, we see
some andesite that escaped alteration that shows the contrast.
Unaltered versus hydrothermally altered andesite. 35 58.310N 106 31.102W
The dark rock at the top is unaltered andesite, while chunks of altered andesite litter the slope under it. The local nature of the alteration suggests that there was a small fault or other weakness in the rock under the altered andesite that allowed hydrothermal fluids from the underlying batholith to move upwards.
It is no coincidence that Bearhead Rhyolite intrusions are found
in the same part of the caldera north rim as hydrothermally
altered rocks. This is an area where part of the Bearhead Rhyolite
Batholith is close to the surface. During crystallization of a
batholith, a great deal of volatiles (water vapor with many
different dissolved gases, including acid sulfur oxides) is
"sweated" by the solidifying mass of rock. This can do weird and
wonderful things to the overlying rock.
In the case of the rocks found here, the hydrothermal fluids were both hot and highly acidic, and their effect was to leach sodium and potassium out of the rock. Ferrous iron was oxidized as well, producing the reddish staining.
A little further to the east is another example of hydrothermally
Hydrothermally altered outcrop. 35 58.628N 106 30.334W
Close up of hydrothermally altered conglomerate. 35 58.628N 106 30.334W
Part of the outcropping is a coarse conglomerate containing
angular fragments that cannot have come far from their source.
These are embedded in a matrix of equally granular sand grains
that are cemented together with silica and hematite. (The hematite
is responsible for the orange color.) These are likely rift fill
sediments, which form thick beds east of this location and north
of Cerro Trasquilar. These beds are assigned to the Santa Fe Group
and are sediments that had already been deposited in the Rio
Grande Rift before the Paliza Canyon Formation was erupted on top
of them. Here the sediments have been hydrothermally altered and
cemented by fluids from the Bearhead batholith.
Next to the conglomerate is a knob of volcanic rock so altered
that its original type is uncertain. It could be anything from
Paliza Canyon andesite to a local intrusion of the Bearhead
Close up of hydrothermally altered volcanic rock. 35 58.628N 106 30.334W
Hydrothermal alteration is pervasive in the Bland area, but extends north clear to the southern caldera wall. There are outcrops of altered dacite south of Rabbit Mountain along a southern spur of Forest Road 36.
Hydrothermally altered dacite. 35 49.299N 106 28.299W
Here's a sample
Hydrothermally altered dacite. 35 49.299N 106 28.299W
The white veins are massive quartz deposited in fractures in the rock. The rock itself shows signs of extensive oxidation of iron minerals, and there are scattered pockets of a translucent greenish mineral, which occasionally forms small tabular crystals. I'm not mineralogist to identify these with certainty, but their relatively high hardness suggests epidote, a common alteration product.
There is no question about the identity of the hydrothermal
minerals at Las Conchas, immediately south of the highway, where
Paliza Canyon Formation basalt has been hydrothermally altered.
The unaltered andesite looks like this.
Paliza Formation basalt at Las Conchas. 35 48.838N 106 31.533W
This is basalt of the Paliza Canyon Formation, which has been dated to a hair over 8 million years in age. It is also the base of the south wall of the caldera in this location. The outcrop in the photograph is not visibly altered, other than oxidization of scattered olivine crystals to rusty brown iddingsite.
Unaltered Paliza Formation basalt at Las Conchas. 35 48.838N 106 31.533W
But just a short distance east, the basalt shows minerals deposited in cavities in the rock.
Altered Paliza Formation basalt at Las Conchas. 35 48.856N 106 31.436W
This sample appears to have had quartz deposited in it.
Quartz altered Paliza Formation basalt at Las Conchas. 35 48.856N 106 31.436W
Quartz alteration is not particular extreme. Somewhat more extreme is epidote alteration.
The epidote is the green mineral appearing in patches and lining
cavities in the basalt. It is a ferrous calcium aluminosilicate
mineral, getting its green color from its iron content.
Epidote altered Paliza Formation basalt at Las Conchas. 35 48.856N 106 31.436W
Relief map of the Jemez with Cochiti Formation outcroppings highlighted in red
As the high-silica eruptions of the Bearhead Rhyolite began to die out, around 6 million years ago, erosion of the eruptive centers produced a large quantity of sediment that was deposited on top of the Peralta Tuff. This is the Cochiti Formation. It underlies a large area of the southeast Jemez, grading northward into the volcaniclastics of the Paliza Canyon Formation. The distinction is that the volcaniclastics were formed at the same time as the Paliza Canyon Formation flows and domes and are rich in volcanic ash, while the Cochiti Formation formed by subsequent erosion and is poor in ash. However, the distinction is not a sharp one. This is reflected in geological maps of the Jemez, where a formation mapped as Cochiti Formation at the boundary of one quadrangle is sometimes mapped as volcaniclastics of the Paliza Canyon Formation in the neighboring quadrangle.
The Cochiti Formation caps the Peralta Tuff at Kasha-Katuwe Tent Rocks. The contact is particular clear just north of the main tent rocks as seen from the west.
Cochiti Formation overlying Peralta Tuff. Looking east from 35 39.667N 106 25.656W
The contact between the Cochiti Formation and the underlying Peralta Tuff is obvious. The less easily eroded Peralta Tuff is here a cliff-forming formation, while more easily eroded Cochiti Formation is a slope-forming formation that supports considerably more vegetation.
Outcrops of the Cochiti Formation can be examined at close range
at numerous locations along the road from Cochiti
Pueblo to the Veterans'
Memorial Overlook. Here is a particularly fine exposure.
Exposure of Cochiti Formation. South of
39.713N 106 26.545W
The lighting here makes the formation look almost handsome. The usual impression one gets in the field is of nondescript brown dirt. More formally, it is described as poorly-sorted silt, sand, and gravel of up to cobble size in a tan matrix.
For comparison, here is a photograph of volcaniclastics of the
Paliza Canyon Formation exposed further north, along Dome Road.
Contact in road cut. 35 42.800N 106 23.406W
The hill behind this road cut is the location of a source vent for the Paliza Canyon Formation. To the right in the road cut is solid porphyritic andesite. This has a radiometric age of 9.33 million years, making it one of older Paliza Canyon flows in this area.
To the left are the volcaniclastics, assigned by some geologists
to the Paliza Canyon Formation and by others to the Cochiti
Formation. These deposits appear to be from an ash-rich mud flow,
or lahar, formed shortly after the eruption of a Paliza
Canyon Formation volcano. I would be inclined to assign these to
the Paliza Canyon Formation rather than the Cochiti Formation.
We'll revisit lahars again later in the book.
The question is sometimes asked: What is the oldest mountain range in the world? Giving an answer is a bit problematic. Some mountains ranges in continental shields are composed of Archean rock over three billion years old. But the age of the rock is not necessarily the age of the mountain range itself. In New Mexico, the Precambrian cores of the principal mountain ranges are up to 1.8 billion years old, but these mountains are believed to have been thrown up during the Laramide orogeny, which began just 60 million years ago. Erosion quickly attacks the mass of rocks thrown up by mountain building, and the Laramide mountains of New Mexico are in places deeply eroded, exposing rock which, based on the kinds of minerals it contains, must once have been many kilometers below the surface. Several kilometers of erosion in tens of millions of years is not at all unusual.
Of course, the mountains themselves are not much more than a couple of kilometers higher than the surrounding terrain, and likely never were. As the tops of the mountains eroded, their deep, buoyant roots pushed the mountains back up to maintain isostatic balance. Because the difference in density between continental crust (2.7 g/cm3) and the underlying mantle (3.4 g/cm3) is not much more than 20%, it takes at least four kilometers of mountain roots to balance a kilometer of mountain. This means that for a mountain two kilometers high to be permanently eroded flat, some eight kilometers of rock must be eroded away.
The granite cores of the Sierra Nacimiento at the western edge of the Jemez likely follow this pattern. There is some evidence this area has been thrown up repeatedly since the Mississippian, but the most recent period of uplift goes back at least thirty million years. This is enough time that the Sierra Nacimiento have been deeply eroded, as is clear when they are viewed from the west.
Southern Sierra Nacimiento. Looking west
from near 35
42.697N 106 57.303W
Erosion has stripped the upper layers of this block of crust to
expose Precambrian basement rocks that were once far below the
surface. The high point in the range, at the left side of the last
frame, is Pajarito Peak, which is mostly underlain with gneiss
with a quartz monzonite cap. These beds are around 1.6 billion
years old. The range north of Pajarito Peak is underlain by the
Joaquin Granite, with a radiometric age of 1.4 billion years.
The volcanic rocks of the Jemez are particularly susceptible to
erosion, since they include loosely consolidated volcaniclastic
and ash beds as well as lava flows whose minerals are not entirely
stable at the surface. When these flows were first erupted, they
likely had the smooth surfaces that are still apparent on some of
the younger domes of the volcanic field, which we will see more of
in succeeding chapters. The older formations of the Polvadera and
Keres Groups have been deeply eroded in the millions of years
since they were erupted, preferentially along faults associated
with the Rio Grande Rift. The fault zones break up the rock,
making it more susceptible to erosion. A particular clear example
is upper San Juan Canyon.
Map of upper San Juan Canyon and associated faults
The deep canyon in the lower left part of this relief map is
lower San Juan Canyon. The upper part of the canyon turns sharply
east to follow an unnamed east-west fault, then turns sharply
north again to follow the Upper San Juan Canyon Fault. To the
east, upper Paliza Canyon and upper Peralta Canyon also follow
major north-south faults.
The rugged terrain of the southern Jemez suggest that erosion has
been extensive, so that little of the original surface is left.
Nevertheless, most geologists do not believe that the original
terrain could have been much higher than the remaining ridge
crests and peaks. Local domes that obviously correspond to
eruption centers, such as Cerro del Pino southeast of San Juan
Canyon, are still discernible as high points of the terrain,
though they have likely been stripped of their outer mantles. As
we shall see in the next chapter, it seems to take about four
million years for a fresh eruptive center to show obvious signs of
Next page: Dacite and dacite, what is
Copyright ©2015 Kent G. Budge. All rights reserved.