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!
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 firstname.lastname@example.org.
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.
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 email@example.com.
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 firstname.lastname@example.org.
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.
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
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.
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!
Tell us about your outdoor experiences! We’d love to see your photos, too. Please send them to email@example.com 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.
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).
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.
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).
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.
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.
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).
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.”
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.
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.
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).
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!
Fraser and Cathy Goff are PEEC volunteers that lead geologic tours through the Valles Caldera. The Valles is one of three active calderas in the mainland United States and the hot magma is only three miles below your feet when standing in the Valle Grande. Fraser Goff, retired geologist for LANL and adjunct professor at UNM, has been working on the Preserve for many years and has authored numerous books and papers dealing with the geology of the caldera. With the help of his geologist wife, Cathy Goff (retired from US Geological Survey), he conducts fact-filled tours of the east and central parts of the Preserve and provides thought-provoking answers to the public’s curiosity about this wonderful landscape. We hope you enjoy learning more about volunteers, Fraser and Cathy Goff.