Archive for March 17th, 2011

Back to granite on Cima Dome

March 17th, 2011

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.

A tuff afternoon

March 17th, 2011

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.

A massive pile of sand: the Kelso Dunes

March 17th, 2011

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.

Deep in the heart of a lava flow

March 17th, 2011

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.