A little vignette of desert ecology

Mark Wilson March 18, 2011 9:12 am


ZZYZX, CALIFORNIA–While exploring the Amboy Crater lava fields on Wednesday, we noticed these small and very active “yellow” beetles. With a little research we discovered they are Desert Spider Beetles (Cysteodemus armatus) that feed on the nectar from a variety of flowers. As you might have guessed by now, the yellow color on the heads and abdomens of the beetles is actually from the pollen of a particular yellow flower blooming in abundance the day of our visit. These beetles turn out to be important pollinators for several flower species.

Some of the many flowers near Amboy Crater. I need my flower expert Mother's help to identify them!

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Back to granite on Cima Dome

Mark Wilson March 17, 2011 11:50 pm

A granite exposure near Teutonia Peak on Cima Dome. Note our jackets and hands in pockets!

ZZYZX, CALIFORNIA–Our last stop of the rapidly-cooling day was on the huge Cima Dome east of Zzyzx in the Mojave National Preserve. The dome is so large (about 70 square miles) that it is impossible to detect when you are actually on it, but easily visible from miles away. It apparently is the eroded root of a granitic intrusion formed during subduction in the Jurassic to Cretaceous. The alkali granite exposed here is very similar to that of the Granite Mountains we saw yesterday.

Potassium feldspar crystals in the coarse alkali granite of Cima Dome.

The soil of Cima Dome is derived almost entirely from the underlying alkali granite.

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A tuff afternoon

Mark Wilson March 17, 2011 11:45 pm

Lindsey Bowman and Becky Alcorn on the Hole-In-The-Wall tuff deposits.

ZZYZX, CALIFORNIA–After lunch we took a long drive south and east to the Hole-In-The-Wall visitor center and trail. Exposed here are diverse and colorful rocks called tuffs that were formed by pyroclastic eruptions from volcanoes roughly 18.5 million years ago. These eruptions of hot gases and ash swept the surrounding countryside depositing thick masses of complex rock. Plants and animals were incorporated in the ash flows, so we occasionally find charcoal in the tuffs as well as various other volcanic products.

A piece of charcoal from a burnt tree in a tuff at Hole-In-The-Wall.

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A massive pile of sand: the Kelso Dunes

Mark Wilson March 17, 2011 11:41 pm

ZZYZX, CALIFORNIA–Later in the morning the Wooster Geologists visited a favorite location: the Kelso Dunes in the Mojave National Preserve. We arrived before noon so we could work up a hearty appetite for lunch by climbing the dunes first. The Kelso Dunes are made almost entirely of medium to fine sand grains derived from the dry bed of the Mojave River (and ultimately the San Bernardino Mountains where it originates).  The most common minerals are clear quartz and white to pink potassium feldspar, with a smaller but prominent component of black magnetite that often concentrates on dune crests (see above).  Most of the sand accumulated at the end of the last ice age and has been blowing around in place since then.  No new sand is being added to the dunes today. The highest dune rises 200 meters above the valley floor — and it is a hard slog up to the top.  (And much faster going down!)

Dune grass baffling sediment and refracting the waveforms of magnetite-rich sand.

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Deep in the heart of a lava flow

Mark Wilson March 17, 2011 9:51 pm

ZZYZX, CALIFORNIA–This morning the Wooster Geologists enjoyed an ancient lava flow from the inside. We found our way to a lava tube near the center of the Mojave National Preserve and explored the interior with flashlights and flash cameras. It helped that there were a few “windows” in the basalt roof where sunlight could stream in. As Meagen Pollock (she who lives for basalt) explained, a lava tube is formed when a flow cools on its exterior portions while the lava is still moving.  When the lava drains out at the end of the flow, the result is a long tunnel of basalt.  Lava dripped from the ceilings, making the igneous equivalent of stalactites.  As the flow receded, it left horizontal “bath tub rings” along the side of the tube. It was fun to speculate on how many lava tubes remain undiscovered beneath the many square kilometers of basalt exposed in the Mojave Desert.

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Igneous delights at Amboy Crater and in the Granite Mountains

Mark Wilson March 16, 2011 11:13 pm

Greg Wiles on the rim of Amboy Crater.

ZZYZX, CALIFORNIA–This afternoon the Wooster geologists studied two very different magma products in the southern part of our field area. After lunch we drove to Amboy on historic Route 66 and then hiked up to the rim of Amboy Crater. Here we saw the extrusive, mafic rock basalt at its finest in the cinder cone itself and the lava flows across the valley.

Desert iguana on the basalt near Amboy Crater.

After Amboy, we traveled north to the Granite Mountains and examined wonderful alkali granites weathering into rounded boulders. The feldspar crystals in these intrusive felsic rocks were extraordinarily large and numerous, and there were many xenoliths scattered throughout. The countryside here was lush with desert vegetation that made our all-too-brief stop most enjoyable.

An exposure in the Granite Mountains, California.

The feldspar-rich alkali granite in closer view.

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Structural geology and mineralogy at Calico

Mark Wilson March 16, 2011 10:58 pm

ZZYZX, CALIFORNIA–It was surprisingly cool this morning in Zzyzx as we left for our day of fieldwork, but we were not surprised by the wind at our first stop, Calico Ghost Town outside Barstow. Every time we’ve been there it has been blustery. Calico is an old silver mining site with a complex geological structure complimented by hydrothermal mineralization driven by dacite intrusions roughly 17 million years ago. Shelley Judge showed us how strike-slip faulting (ultimately a result of movement on the San Andreas Fault to the west) produced compressional folds in the Barstow Formation. Meagen Pollock talked about how the silver rich veins were formed by thermal alterations of the sedimentary rocks on the flanks of the Calico Mountains. We then spent a little time in the town itself eating our packed lunches and enjoying cold sarsparilla!

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Plunging into Lake Manix

Mark Wilson March 15, 2011 11:11 pm

ZZYZX, CALIFORNIA–East of Barstow and west of Afton Canyon was a very large pluvial lake during the Pleistocene. This Lake Manix was hundreds of feet deep, and its catastrophic drainage through Afton Canyon about 185,000 years ago must have been a great spectacle. This afternoon we explored one of the southern shores of this ancient lake, and then climbed down through its eroding bottom sediments.

Shoreline of Pleistocene Lake Manix. The dark rocks to the left apparently are remnants of an alluvial fan delta which extended into the lake shallows. The light-colored sediments below and to the right are from the lake itself. The white band in the foreground appears to be a type of coastal tufa formed by the agitated lake waters mixed with waters coming from the fan.

Lake Manix bottom sediments consisting mostly of fine silts and clays.

The plateau above the lake sediments includes windswept desert pavements and beautiful ventifacts (wind-carved stones) like this one.

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A morning with the Barstow Formation

Mark Wilson March 15, 2011 10:48 pm

Greg Wiles explores the top of a wadi in Owl Canyon. The debris here consists of blocks of Barstow Formation eroded during recent floods.

ZZYZX, CALIFORNIA–It is hard to believe that the Mojave region once had vast lakes, erupting volcanoes and a diverse mammalian fauna including camels and horses. The most important record of this time is the Barstow Formation (Miocene — about 15 million years ago). This unit contains river and lake sediments along with volcanic ash, all of which produce a diversity of colors leading to the name “Rainbow Basin” for the best exposures outside Barstow, California.

The Wooster geologists spent the morning with the Barstow Formation in Owl Canyon and Rainbow Basin. The highlights included a group analysis of dozens of rock specimens, a desert tortoise crossing the road, and meeting old friends Buzz and Phyllis Sawyer from Barstow.

The Wooster geologists in Rainbow Basin with the Barstow Syncline in the background. Image by Buzz Sawyer.

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Death Valley Day

Mark Wilson March 14, 2011 11:01 pm

ZZYZX, CALIFORNIA–Our Wooster geological crew awoke to a spectacular sunrise over the Soda Lake playa this morning. We drove north from Zzyzx through Baker and Shoshone into Death Valley by way of Jubilee Pass. The weather could not be better with daytime temperatures in the low 80s and brilliant blue skies. Shelley Judge gave us an overview of the tectonics that formed Death Valley, Meagen Pollock helped us sort out the poikilitic textures in basalts along Artists Drive, Greg Wiles discussed the declining levels of Lake Manly, and I helped out with interpretations of sedimentary structures in Miocene deposits exposed along Golden Canyon. Students kept their end of the bargain with challenging questions. A most excellent day.

Part of Death Valley's charm is in its extremes. This is Telescope Peak on the western side of the valley. It is 11,049 feet above sea level. Note the snow cover.

And here are salt deposits below Telescope Peak at 275 feet below sea level, baking in the relentless sunlight.

The Wooster students on the 2011 Mojave Desert field trip.

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