Geology of the Jemez Area, Chapter 7: Volcaniclastics and Erosion

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.
  1. What Goes Up Must Come Down
    1. Tephra
    2. Ash and pumice falls
    3. Ignimbrites
    4. Lahars and reworked tuffs
  2. The Bearhead Rhyolite
    1. The Peralta Tuff
      1. Kasha-Katuwe Tent Rocks
      2. Cerro Balitas
  3. Hydrothermal Alteration
  4. The Cochiti Formation
  5. Erosion

What Goes Up Must Come Down

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 solidified (blocks).

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.

Mount Pinatubo eruption column
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.

Ash and pumice falls

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.

Sample of El
        Cajete Pumice
Pumice. From White Rock Canyon near 35 49.407N 106 11.089W

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 water. 

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 with fiamme

Welded tuff showing fiamme. 35.54.857N 106 30.554W

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.

Otowi Member,
          Bandelier 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 and reworked tuffs

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 rock types.

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.

The Bearhead Rhyolite

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

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
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
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
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:

          Rhyolite in road bed

Bearhead Rhyolite close p
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.

Bearhed Rhyolite along Aspen Fault

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 Formation

Bearhead Rhyolite dike in Paliza Canyon Formation. 35 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
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.

          Memorial Scenic Overlook panorama
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 this book.

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 Peralta Tuff

Digital relief map of Peralta Tuff exposures in the Jemez
Relief map of the Jemez with Peralta Tuff outcroppings highlighted in red

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.

Kasha-Katuwe Tent 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.

Tent rocks
Peralta Tuff at Kasha-Katuwe Tent Rocks. Looking north from 35 39.386N 106 24.694W

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.

More helpful signage

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
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.

Slot canyon entrance
Entrance to slot canyon. 35 39.705N 106 24.402W

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.

texture of beds

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.

Tree with exposed roots
35 39.855N 106 24.389W

The slot canyon has a magnificent, eerie beauty.

Slot canyon

Slot canyon

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 scale.


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.

Mushroom structure

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.

Opalized ash

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.

Cerro Balitas

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 somewhat challenging.

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 southwest
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.

Cerro Balitas Bearhead Rhyolite
Bearhead Rhyolite? 35 44.174N 106 24.392W

Northwest of Cerro Balitas one comes across a striking outcropping of tuff.

Peralta Tuff with lithic content
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 lithic content
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 lithic content
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.

Peralta Tuff with lithic content
Lithic Peralta Tuff. Near 35 44.103N 106 24.007W

A sample:

Peralta Tuff with lithic content
Lithic Peralta Tuff. Near 35 44.103N 106 24.007W

There is a large flow of very lithic-rich tuff to the southeast from Cerro Balitas.

Extremely lithic tuff
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.

Further down the flow, the nature of the tuff changes.

          and ash flow
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.

          flank of Cerro Balitas
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
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
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 high-aspect flow.

Peralta Tuff flow
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.

Hydrothermal Alteration

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
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 and
          hydrothermally altered andesite
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 altered rocks.

          altered outcrop
Hydrothermally altered outcrop. 35 58.628N 106 30.334W

Hydrothermally altered conglomerate
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 Rhyolite.

Hydrothermally altered volcanic rock
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.

          altered volcanic rock
Hydrothermally altered dacite. 35 49.299N 106 28.299W

Here's a sample

Hydrothermally altered volcanic rock
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.

          Formation basalt at Las Conchas
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
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
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
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.

Epidote altered Paliza Formation basalt at Las Conchas
Epidote altered Paliza Formation basalt at Las Conchas. 35 48.856N 106 31.436W

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.

The Cochiti Formation

Digital relief map of Cochiti Formation exposures in
        the Jemez Mountains
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.

          Formation overlying Peralta Tuff
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.

Cochiti Formation in arroyo south of Veterans' Memorial

Exposure of Cochiti Formation. South of 35 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.

Cochiti Formation in
          road cut
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 San Juan Canyon and associated

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 erosion.

Next page: Dacite and dacite, what is dacite?

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