Virtual Field Trip: Geology

Geologist Fraser Goff points out a lava flow he’s about to investigate. (Photo by Siobhan Niklasson)

One of the perks of being a field geologist is that they often get to spend time outside! Follow along with a geologist as he spends a day mapping rocks in the Jemez Mountains, and then try your own hand at observing rocks and mapping soils

Click the links below to take a virtual geology field trip!

Read more Virtual Field Trip: Geology


Valles Caldera National Preserve is a fascinating place to explore volcanology and geology. Almost every fall, local geologists Fraser and Cathy Goff lead a geology tour of the Valles Caldera for PEEC. We’re lucky to have this gem in Northern New Mexico! (Photo by Eric John Peterson)

This week on Take It Outside, take some time to appreciate the geologic history of our area, learn how rocks are used by living creatures, and maybe start a rock collection!

Summer Nature Challenge:

Participate in our Summer Nature Challenge! Every week, participants who complete the challenge can earn a sticker. If you finish all nine weeks, you’ll earn a bonus sticker! Find our archive containing all of our past Take It Outside activities here.

Download the challenge sheet here to print out and complete at home. At the end of the challenge, you can either bring it to the nature center or mail it to us at 2600 Canyon Rd, Los Alamos, NM 87544.

If you don’t have a printer or prefer to work online, you can tell us about your experiences in the Google Form below or email your stories and pictures to

Blog Post:

Geologists Fraser and Cathy Goff have spent years studying the Valles Caldera volcano. In this week’s blog post, they provide an introduction to how the Valles Caldera formed and why it is one of the most famous volcanoes in the world! Read their blog post here.

The theme for our June photo contest also happens to be the Valles Caldera! If you haven’t already, vote for the winner of this month’s contest here.

Do you want to learn how to participate in our monthly photo contest? Find out more here. We’re accepting submissions for our July contest until Tuesday, June 30. Next month’s theme will be insects! Please send your submissions to

Outdoor Challenges:

Campers in our 2016 Backpacking Adventure for Teens summer camp hold up obsidian that they found while hiking. What shapes, sizes, and colors of rocks can you find this week? (Photo by Beth Cortright)

We’re posting three outdoor challenges today that you can enjoy throughout the week!

Tell us about your experiences with one, two, or all three of them! You can do this in the Google Form below, by writing or drawing about them on our summer challenge sheet, or by sending an email to


Challenge #1:

Collect and categorize rocks. There’s a temptation to jump straight to identifying rocks, but all identification begins with careful observation. Bring home some rocks that catch your attention: maybe they sparkle, or have an interesting shape, or are a little different from the other rocks you’ve seen. Then, sort them by characteristics.

First, look at the rocks:

  • What colors do you see? What shapes?
  • Patterns: Are the rocks striped, or polka dotted, or do they have any other patterns?
  • How heavy does the rock feel when you lift it? Does it have air pockets?
  • What textures can you feel?

Next, see if you can find any crystals in the rocks:

  • How big are the crystals?
  • What different colors do you see?
  • Can you see any shiny faces, or are the crystals rounded or uneven?

Try drawing one of your favorite rocks! Use colors if you can, and add as many details as you can. See if someone else can guess which rock you’ve drawn.


Challenge #2:

Explore soil separation in this challenge. Rocks are the foundation of our soil! Go outside and gather a container of soil. Try to find the following parts, using a magnifying glass if you have one:

  • Pieces of rock of different sizes
  • Sticks, leaves, and other plant matter
  • Insects or worms
  • Air pockets
  • Water (can you feel any dampness?)

Pour a scoop of soil into a transparent container with a lid. Fill the rest of the container with water. Close the lid, and shake the soil thoroughly. Watch what happens as the soil settles. Draw the layers you see.

Challenge #3:

White Rock Canyon is a good place to look for petroglyphs in Los Alamos County! (Photo by Craig Martin)

Discover how rocks and life intersect. Rocks are part of our ecosystem. See if you can find any of the following signs of how rocks interact with living things in our environment:

  • Lizards or snakes sunning themselves on rocks
  • Animal burrows under rocks
  • Signs that squirrels sit on rocks to snack
  • Tree roots growing into rocks
  • Lichens growing on rocks
  • Insects and other creatures in the soil
  • Ant hills covered with sand grains
  • Fish or other aquatic life hiding among rocks
  • Signs of people using rocks (cliff dwellings, petroglyphs, rocks in homes and gardens)

What else can you find? Let us know in the form below!


Want to Learn More?

Share Your Experience:

Tell us about your outdoor experiences! We’d love to see your photos, too. Please send them to or share them on Facebook or Instagram with the hashtag #peectakeitoutside. If you’d like this to count for the Summer Nature Challenge, be sure to include your name and email address.

The Volcanology of the Valles Caldera

Figure 1: Center of the geologic map of the Jemez Mountains (Smith et al., 1970), showing the Valles Caldera, about 22 km across. Los Alamos is to the east. Valle Grande is but one valley of many within the caldera. Major colors: Yellow = post-caldera lava domes, orange = Bandelier Tuff pyroclastic flows; green = pre-caldera lava domes such as Pajarito Mountain west of Los Alamos; other colors = various volcanic, sedimentary, and crystalline rocks. RP = Redondo Peak, the resurgent dome (uplifted central structural dome) of the caldera; note that Redondo Peak is mostly uplifted Bandelier Tuff that filled the initial caldera “hole;” CdM = Cerro del Medio, the first of the post-caldera lava domes. A more recent and detailed color geologic map of Valles Caldera (Goff et al., 2011) and other books can be obtained at PEEC bookstore. (Modified Image by Fraser and Cathy Goff)

By Fraser & Cathy Goff

In Northern New Mexico, we are fortunate to be situated very close to the Valles Caldera: one of the most famous volcanoes in the world (Fig. 1). A caldera is a large volcanic depression, more or less circular, the diameter of which is many times greater than subsequent post-caldera eruption vents. 

Based on a seminal publication by scientists of the U.S. Geological Survey (Smith and Bailey, 1968), the Valles is known as the world’s “type” resurgent caldera. A resurgent caldera has a structurally uplifted central dome (Redondo Peak, Fig. 1) and a ring of post-caldera lava domes, like Cerro del Medio. A “type” example means that this volcano was the place where this kind of volcanism was first described.

There are three large, relatively young calderas in the United States: Yellowstone, WY (0.64 million years old), Long Valley, CA (0.76 million years old), and the Valles (1.25 million years old). Although the Valles is the oldest and the smallest of the three, the eruptions that produced it were anything but small. About 400 cubic kilometers (95 cubic miles) of hot pyroclastic flows erupted in a period of a few months or less and formed a near-circular apron of consolidated ash, the Tshirege Member of the Bandelier Tuff. A pyroclastic flow is a fast-moving cloud of volcanic gases, ash, and chunks of pumice and lava. Well-known examples include the eruption of Vesuvius that buried Pompeii in pumice fall deposits, and the Mount St. Helens eruption in 1980. The pyroclastic flows from the eruption of Vesuvius wiped out Herculaneum on the west flank of the volcano. The flows erupting from the Valles reached temperatures of about 400 °C (752 °F)! The pyroclastic flows were dispersed at velocities approaching about 200 km/hr (124 miles/hr), filling valleys in the pre-existing terrain and forming thick deposits of tuff that we now call the Pajarito and Jemez Plateaus, east and west of the Valles, respectively.

From their work at the Valles, Smith and Bailey (1968) published the “standard model” of large caldera formation, a model that is used to compare and contrast calderas to this day (Fig. 2). Magmas of the Bandelier type erupt at 700 to 900°C (1292 to 1652 °F) and contain substantial amounts of dissolved water, up to 6 percent of the magma by weight. Hot, water-rich magma is less dense than surrounding rocks, so the magma slowly rises toward the Earth’s surface. 

When the magma rises close to the surface, the pressure of the overlying cold rock can no longer prevent the dissolved water from forming water vapor (steam) in the magma, just like when you slowly open a shaken soda bottle. Then the magma explodes violently, producing superheated clouds of steam, gas, volcanic ash, and mineral fragments from crystals in magma, pumice, and fragments of the surrounding rock. Together, they form pyroclastic flows (Fig. 3). Ash fills the sky, and pumice (gas-inflated magma fragments that quench in the cold atmosphere) rains down. When the flows come to rest, they solidify, forming “welded tuff” (Fig. 4).

For a good visual of pyroclastic flows, check out this photo of a truck fleeing the famous 1991 eruption of Mount Pinatubo, Philippines.

Figure 2: Six stages of large caldera formation: I. Regional tumescence cracks landscape above rising magma chamber (pink), II. Eruption of explosive magma through faults, particularly ring-faults, to form pyroclastic flows (gray), III. Deposition of pyroclastic flows (thin yellow layer) and subsidence of caldera floor along ring-faults above evacuated magma chamber, IV. Formation of first intra-caldera lakes in caldera basin (blue) and eruption of first post-caldera lava (red), V. Resurgence (structural uplift) of central caldera floor, additional eruptions of lava, and formation of more lakes, VI. Eruption of a ring of post-caldera lava domes along the ring-fault system (these lavas are shown in yellow on Fig. 1). Crustal rocks are shown in tan; pre-caldera volcanic and sedimentary rocks are shown in darker yellow. (Image Credit: Goff, 2009)
Figure 3: Distribution of wind-blown fine ash from the three young caldera eruptions in the U.S.A., Tshirege ash in blue (Goff, 2010). The southerly prong of Tshirege represents identified pumice rafts along the ancestral Rio Grande as far south as El Paso, TX. Since this map was published, Tshirege ash has been identified in a 3-m-thick paleo pond deposit in Saskatchewan, Canada (Westgate et al., 2018) extending the depositional footprint of Tshirege ash another 1000 km to the north. The Canadian ash is correlated to the Tshirege ash by age, mineralogy, chemistry, and paleomagnetism. 1980 St. Helens ash is also shown. (Image Credit: Goff, 2010)

The Valles Caldera was preceded by the comparably sized Toledo caldera, which erupted 1.62 million years ago and produced the Otowi Member of Bandelier Tuff. The Otowi is the darker orange unit on the geologic map of Fig. 1, but the paired tuff sequence is exposed in deep canyons all around the Valles (Fig. 4). Because the two calderas formed nearly on top of each other, formation of the Valles nearly obliterated geologic evidence for the Toledo caldera.

Figure 4: Exposure of two Bandelier Tuffs about 1 km east of “the Y” on NM 502; white = Otowi from Toledo caldera; orange = Tshirege from Valles caldera. Sequential explosive pulses cause the tuff to form layers. The bottom layer is mostly pumice that “rains” out before being overrun by pyroclastic flows. The tuff in the upper cliffs is colored orange from oxidation of iron-bearing minerals. The Bandelier Tuff overlies dark gray basalt lava (2.4 Ma) at the bottom of the photo where Fraser is standing. (Photo by Cathy Goff)
Figure 5: White, laminated, fine-grained lacustrine (lake) deposits in Valle San Antonio, northern Valles Caldera (view looking NE toward caldera wall). The deposits contain siliceous cell-wall remains of diatoms, one-celled algae, that flourish in silica-rich waters of volcanic lakes. These fossils are visible with a hand lens. (Photo by Fraser Goff)

Because calderas form large topographic depressions, they often contain lakes that capture rain and snow (Fig. 2). Crater Lake, OR is such a lake in a small caldera, but Lake Yellowstone and Lake Crowley are large intracaldera lakes in the Yellowstone and Long Valley calderas. We know that Valles contained several lakes during its history because many fine-grained lacustrine (lake) deposits are exposed in and around the large valleys (Fig. 5). A well core obtained from Valle Grande in 2004 intersected 75 m of lake deposits that accumulated 300 to 520 thousand years ago. The Valle Grande lake deposits are hidden by only a few meters of younger valley-fill sediments.

Cerro del Medio (Fig. 1), the first Valles ring-fracture lava dome, is a famous archeological site for acquisition of obsidian (Fig. 6). Ancestral Puebloans mined this lava extensively for high-quality, crystal-free natural glass to make tools. Formation of compositionally pure glass requires that the lava erupted at a temperature above which crystals can form in the magma, about 900 °C (1652 °F). During and immediately after eruption, the viscous lava chilled (“froze”) quickly, avoiding growth of all but the tiniest black crystals of iron oxides, which make obsidian appear black. Native American tribes traded valuable “Valles” obsidian throughout the Southwest United States (Shackley, 2005).

Figure 6: Cerro del Medio obsidian found as a cobble in gravel by a stream. This is actually a core stone, recognized by percussion marks, that was used by Native Americans to make tools. Because it is an artifact, it must remain where found. (Photo by Fraser Goff)

 The immense quantity of heat released from crystallizing magma beneath the Valles (nominally 900 °C or 1652 °F) creates relatively shallow underground reservoirs of hot water; some of these waters reach the surface as hot springs and fumaroles (Fig. 7). Crystallizing magma also releases steam and acidic gases such as hydrochloric acid, hydrogen fluoride, carbon dioxide, hydrogen sulfide, and sulfur dioxide. These acidic fluids react with rock, forming a variety of secondary minerals such as clays, iron oxides, sulfur, sulfates, pyrite, and other minerals. 

Ancient calderas, such as those in southern Colorado, are well-known hosts for gold-silver-copper-lead-zinc-molybdenum ores. Mining of ores in these eroded calderas resulted in many “boom towns” (i.e., Creede, Silverton, Platoro, etc.) and fabulous wealth for very few. Geothermal wells drilled into the Valles Caldera from 1962 to 1988 intersected superheated waters (about 300 °C or 572 °F) and small intervals of such ore minerals, but because subsurface temperatures are still hot, it is impossible to economically mine these intervals. Valles Caldera became a National Park (Valles Caldera National Preserve) in 2016, so mining and geothermal exploration activities are now prohibited.

Figure 7: Scientific core hole VC-2A erupting steam and geothermal brine at Sulphur Springs, western Valles caldera, May 1987. The fluid originates from a 210 °C crack in intracaldera Bandelier Tuff at 490 m depth. Geothermal gases reaching the surface from this reservoir fluid form fumaroles and create natural sulfuric acid that reacts with rock. A wasteland of kaolin, sulfur, gypsum, alunite, pyrite and other interesting minerals surrounds the hot spring area. Sulphur Springs recently became part of Valles Caldera National Preserve. (Photo by Fraser Goff)

Further Reading

Goff, F., 2009, Valles caldera – A Geologic History: University of New Mexico Press, Albuquerque, 114 p.

Goff, F., 2010, The Valles caldera – New Mexico’s supervolcano: New Mexico Earth Matters, Winter 2010, p. 1-4.

Goff, F., and Goff, C.J., 2017, Overview of the Valles Caldera (Baca) geothermal system, in (McLemore, V.T., et al., eds.), Energy and Mineral Resources of New Mexico: New Mexico Bureau of Geology and Mineral Resources, Memoir 50F, 65 p.

Goff, F., Gardner, J.N., Reneau, S.L., Kelley, S.A., Kempter, K.A., Lawrence, J.R., 2011, Geologic map of the Valles caldera, Jemez Mountains, New Mexico: New Mexico Bureau of Geology and Mineral Resources, Geologic Map 79, 1:50,000 scale, color, w/30 p. booklet.

Shackley, M.S., 2005, Obsidian: University of Arizona Press, Tucson, 246 p.

Smith, R.L., and R.A. Bailey, 1968, Resurgent cauldrons: Geological Society of America, Memoir 116, p. 613-662.

Smith, R.L., Bailey, R.A., Ross, C.S., 1970, Geologic map of the Jemez Mountains, New Mexico: U.S. Geological Survey, Miscellaneous Investigations Map I-571, 1:125,000 scale, color.

Westgate, J.A., WoldeGabriel, G., Halls, H.C., Bray, C.J., Barendregt, R.W., Pearce, N.J.G., Sarna-Wojcicki, A.M., Gorton, M.P., Kelley, R.E., and Schultz-Fellenz, E., 2018, Quaternary tephra from the Valles caldera in the volcanic field of the Jemez Mountains identified in western Canada: Quaternary Research, 1-16.

Week 6, Day 1: Geology

Bandelier Tuff is commonly found in and around Los Alamos. Lower flows are whitish and soft — Ancestral Pueblo people cut caves into them. Upper flows are orange-tan-gray and tend to form cliffs. You can see examples of the lower and upper flows in this photo of Frijoles Canyon! (Photo by Sally King/NPS)

Welcome to week six of Take It Outside! This week, there is a lot to celebrate! It’s the 50th anniversary of Earth Day, and PEEC’s 20th birthday! Please see the schedule of our birthday festivities, and join us to celebrate!

If you haven’t already, please share your feedback on Take It Outside with us by filling out this evaluation form.

Birthday Challenge:

We wanted to serve you cake for our birthday, but because of the COVID-19 restrictions, we can’t do that. So celebrate with us by baking and enjoying your own birthday cake at home!

Take a photo of the completed cake and send it to or post it on Facebook or Instagram with the hashtag #PEECTurns20 by midnight on Wednesday, April 22. Then, we will vote to decide whose cake looks the most delicious! Try to use what you already have at home to limit grocery trips. The more creativity the better!

We would love to see alternative “cakes” too, like mud pies, paintings, sculptures, crocheted cakes, or whatever you can come up with! Use anything you have on hand. The winner will receive a PEEC hat and will be announced during Friday’s birthday bash livestream!

Blog Post:

Geologists Fraser and Cathy Goff explore the geology of Los Alamos and some of the formations you can look for the next time you head outside! Read their post here.

Explore crystals in today’s craft! (Photo by Denise Matthews)


Minerals are the building blocks of rocks. With the right combination of temperature, pressure, and time, minerals will form their amazing and unique crystal structures.

Build your own crystal using sugar or Epsom salt by following these steps. Extend this by building crystals with borax or alum and compare all the different crystal shapes that are revealed by each mineral!

Outdoor Challenge (Beginner):

Make mud pies! Get up to your elbows in our local dirt and explore it with your sight, smell, and sense of touch. Can you find any crystals in the dirt? Think of a few adjectives to describe how it feels. Then add water and describe it again. Can you form it into a shape? Use a bucket or container to shape it, add decorations from nature, and if you like, submit it to our cake contest by emailing a picture to

If you need some inspiration or visuals to get started on your mud pie, check out this post.

Outdoor Challenge (Advanced):

Take a walk and see if you can identify some of the local geologic features mentioned in today’s blog post. You can find descriptions of each type and typical locations in PEEC’s geology guide.

  • Chunky Rhyodacite. Can you see large, white feldspar crystals?
  • Cliff-forming Bandelier Tuff. Local kids call it “chalk rock” because it crumbles to powder. Try using it like chalk!
  • Dark-colored, fine-grained Basalt. This is found on both sides of White Rock Canyon.
  • Beds of gravel or even boulders transported by water.
  • Signs of faulting. Look for a sudden lateral change in the appearance of the rocks, as in Figure 7 in the blog post.

Much of the visible geologic history of the Jemez Mountains is volcanic, and there have been many different kinds of volcanoes in this area. Can you imagine what it would have been like to be here when these rocks were forming? Or what it would have been like to be here during a flood strong enough to transport huge boulders? Try to imagine how it would have looked, sounded, smelled, and felt!

Other Resources:

Share Your Experience:

Tell us how you like to celebrate the Earth! We’d love to see your photos, too. Please send them to or share them on Facebook or Instagram with the hashtag #peectakeitoutside.

Join us tomorrow to learn about our local ecosystem!

The Geology of Los Alamos

Article and Images by Fraser and Cathy Goff

In these days of quarantine, there’s no better time to explore the fabulous geology of Los Alamos. Geology is the science concerned with the solid Earth, the rocks of which it is composed, and processes by which they change over time. 

The major rock types in Los Alamos formed from volcanic eruptions. These rocks have been faulted and eroded over time to shape the rocks we see today. These rocks and faults are displayed on the following geologic map of the area (Fig. 1).

Fig. 1: A geologic map of western Los Alamos’s major rocks. North is up. This map is pulled from this geologic map. Green = Rendija Canyon lavas (Ttrc), orange = Bandelier Tuff (Qbt), and yellow = various sediments, mostly alluvial fans and stream deposits (Qoal, Qal). Thin black lines are faults with the ball and bar symbol on the down side of the fault.  AP = Ashley Pond, LAC = Los Alamos Canyon, PC = Pueblo Canyon, and RC = Rendija Canyon.

The oldest volcanic rocks are tall ridges and bluffs found on the west and north sides of town. These formed during extensive eruptions of Rendija Canyon lava flows (Fig. 2). Their sources are further west of town, though the exact source has not been determined, and they are about 5 million years old. Rendija lava flows form a rock called “rhyodacite.”

Fig. 2: View facing west from the Los Alamos Nature Center toward Rendija Canyon lava highland and LA Hill.
Fig. 3: Texture of Rendija Canyon lava showing white feldspars.

During the eruption, they were sticky, viscous liquids that flowed slowly like taffy and stacked hundreds of meters thick. The lavas contain conspicuous crystals of white feldspar, that are about 2 cm long or less, and smaller, clear quartz (Fig. 3). The best place to see Rendija flows are at the Natural Arch off Mitchell Trail (Fig. 4) or the trail to Los Alamos Hill from 48th street.

Fig. 4: The Los Alamos Arch off Mitchell trail is an eroded window formed in Rendija lava. (Photo by Rachel Landman)
Fig. 5: Texture of dense Bandelier Tuff showing lens of partially melted pumice.

Bandelier Tuff is the younger volcanic unit found in Los Alamos and is composed of layered pyroclastic flows. Pyroclastic flows are mixtures of hot gas, ash, pumice, crystals, and pre-eruption rock fragments that move downslope at high velocity — up to about 500 km/hr (300 miles/hr). The tuff formed 1.25 million years ago during explosive eruptions of the Valles Caldera west of town.

Lower flows are whitish and soft — Ancestral Pueblo people cut caves into them. You can find examples of these caves along the Main Loop Trail at Bandelier National Monument. Upper flows are orange-tan-gray (Fig. 2) and tend to form cliffs. The tuff contains many crystals, but they are small and easily missed. They are clear quartz and clear iridescent-blue feldspar (Fig. 5). The best place to see layered tuff is from along the fence outside the Los Alamos Nature Center, but many other Los Alamos trails also descend into canyons and ravines of tuff.

Fig. 6: Bed of boulders overlies Bandelier Tuff; rock hammer on tuff for scale.

Sedimentary deposits found in the area are mostly boulders, gravels, and sands of eroded volcanics (Fig. 6). Coarse boulder layers speak to the force of ancient flash floods. The lower part of Rendija Road past the shooting range, and the outcrops across the highway from Totavi gas station are some of the best places near Los Alamos to see these boulder layers.

Solid black lines on the above geologic map are faults (Fig. 1). A fault is a fracture between two blocks of rock. If dashed or dotted, the fault evidence is inferred or hidden. If solid, geologists found evidence for rock rupture and displacement. Many faults in Los Alamos cut Bandelier Tuff (Fig. 7) and thus are 1.25 million years old. There is a shallow basin in western Los Alamos called the Diamond Drive graben, which is bounded on both sides by faults that dropped this basin relative to the rocks on either side (Figs. 1 and 8). 

Fig. 7: View north of North-South fault line (red) cutting Bandelier Tuff, east end of Walnut Street and west of Middle School; displacement is roughly 35 m down to west.

The swarm of faults west of town is part of the Pajarito Fault Zone (Fig. 1), one of the most active fault groups in New Mexico. Occasionally, small earthquakes are generated at depth along this zone and are felt by local residents.

As you hike and bike the trails, look around and ponder how this geologic landscape formed! And, don’t forget your social distancing!

Fig. 8: View east from top of LA Hill across Diamond Drive graben (see Fig. 1).