In the field, we quickly realized the complex is incredibly complicated: the concept of isostasy is incredibly prevalent in the region and helps to continuously raise ranges higher. Features called half-grabens further complicated the “horst and graben” structure leaving fascinated drag-folds in their wake. Additionally, the entire normal fault system is irregularly and infrequently cross cut by strike-slip faults which didn’t help much to orientate yourself. Crustal expansion alters its thickness making it on average 30 - 35 km thick in the Mojave; the worldwide average continental crust thickness is 40 km. Even before the creation of the basin and range, volcanic activity has been prevalent in the area, with the first evidence dating back to the Proterozoic. Igneous intrusions and eruptions have both irregularly shown themselves in the region, becoming more frequent ~250 - 60 million years ago during the separation of Pangea; we saw outcrops and previously active volcanoes on multiple occasions.

Because of its arid climate and long history, the Mojave Desert has preserved all three rock types: metamorphic, igneous, and sedimentary; some metamorphosed rocks act as the basement rock, or the foundation for the complex with their protoliths (original rock) having been created from igneous and sedimentary basement rock. Much of the sedimentary rocks in the basin and range formed 1 billion to 250 million years ago during a time when the entire complex was beneath the precambrian sea; 750 million years of deposition is by far the longest period that any rock type was continuously deposited. More recent sedimentary rocks come from precipitates due to the lack of water and increased heat. Igneous rocks in the complex come from batholiths, or magmatic intrusions and relatively recent volcanic activity; the extensive crustal thinning and extension in the basin and range could have expedited intrusive and volcanic activity in the region. The entire complex is a testament to the power of tectonic activity and the subsequent faulting gives way to the creation of more metamorphic rock; because of the intense pressures and temperatures at depth in a fault zone, metamorphic rocks like mylonite are able to form. 

A mylonite from a fault at Mountain Pass Mine. There is a calcareous infiltration, presumably dolomite because of deposits of magnesium at Mountain Pass

Joke: How fast can a geologist run? A mile-a-night.

For this field class we created a rough rock type map so we could potentially date any formation we found in the field, provided we knew the rock type.

Geomorphology of Locations Visited

        Over the last million years the Mojave has been flooded and dried multiple times, even glaciers had existed in the Mojave for an ephemeral point in time; this previously flowing water is responsible for a lot of the geomorphology in the region. We explored and experienced first hand many of the unique geologic features the desert had to offer. At every location, we couldn’t help but notice the surrounding ranges; Many of the ranges we saw were composed of carbonate sedimentary and granitic igneous rocks, the desert studies center specifically sat at the base of a limestone range to the west.

In Death Valley we saw a rich tapestry of lithologies in the mountains. Sedimentary, igneous, and metamorphic rocks all composed the eastern mountain range. Again, these ranges occurred from normal faulting which lifted some features higher than others. The basin and range fault belt is inherently listric, or shows curved faulting; as this process continues for long periods of time isostasy is able to take effect and uplift metamorphic rock to an equal level as younger rocks.

         Desert varnish was also a ubiquitous sight in the Desert. As rock sits for very long periods of time, bacterial microorganisms extract and cement elements like iron or magnesium to the surface of a rock. Varnish takes a great deal of time to form and the reason for its prevalence in the desert is due to the arid conditions. Another fascinating torture rocks are subjected to is sand blasting. Rocks deposited in wide open areas can be subjected to high wind levels that carry fine particulate matter that exotically weathers the rock.

Just in the backyard of the Desert Studies Center, is Soda Lake, the largest playa in the region. A playa is a flat area found in the lowest part of a basin where temporary lakes form during wet periods. They were once permanent lakes during the last glacial period and now contain a record of layered sediments and salts that were transported there via fluvial processes. One interesting feature of playas are the polygonal fissures that tend to form as a result of chemical reactions involving salt and water. Over the week we spent in the Mojave, geology was a major aspect of every location.

At Cima Dome, we ate our lunch and enjoyed the sun on big exposed patches of bedrock, a common feature of pediments. Pediments are areas of low topographical relief found at the base of ranges. On Cima Dome, the exposed bedrock is covered in a thin layer of alluvium, and you can see that erosional processes are at work. More specifically, grussification is a process by which granite is weathered by freeze-thaw cycles that create fractures in the rock and cause small pieces or sheets to be cleaved off. Interestingly, many of the exposed patches of granite bedrock on Cima Dome were rounded because the angular faces with more exposed edges had already been worn down. 

The Mojave is home to volcanoes, specifically cinder cones. We intended to visit two different cinder cone locations but we were only able to see one due to unkempt roads; the volcano we did visit was Pisgah Crater. At Pisgah we saw large collapsed domes of basalt, fine volcanic sediment, volcanic bombs, and two equally sized peaks. The volcanic province was circular towards the east and propagated further south. The reason for this shape was due to the way the cinder cone erupts; as lighter mantle magma reaches the surface it only melts through more dense rock until it explosively erupts. The difference in density causes irregular flows, denser magma cannot move above less dense material; for example: the equally sized peaks actually used to be a singular peak when the volcano erupted. Lighter magma eventually ran out leaving only denser magma that wasn’t light enough to erupt at the surface, so instead it erupted from the side carrying the dome with it. This characteristic is seen all throughout Pisgah Crater with even denser magma having erupted further south. 

A trip favorite was our tour through Mitchell Caverns. Caves and Caverns are an unnatural occurrence in the Mojave Desert, Mitchell Caverns is unique in the very specific conditions it requires to form. The caverns are located in the Providence Mountain range which is a sedimentary range that consists of carbonate rock at its peak; back in the pleistocene, this peak carbonate, composed of marble and limestone, was eaten away by acidic rainwater. The dissolved calcium carbonate was then carried by water infiltration to form the caves we see today. Stalactites and stalagmites are both products of this method of infiltration and the ones that formed at the Mitchell Caverns are incredibly old and delicate. The caverns were also used by the Chemehuevi Indians and at one point even housed giant sloths. 

At Dumont Dunes, we were amazed by the height and ever evolving faces of the star-shaped dunes. Sand dunes seem to appear out of nowhere but there are actually many factors at play in their creation. Sand particles from across the desert are picked up by prevailing winds and become a part of the “saltating bedload” as they bounce along the surface. They accumulate into dunes when the wind is forced up and over a barrier such as a mountain range, and the sand grains are left behind. For the dunes to persist, there needs to be a constant source of sand. For the case of Dumont Dunes, Soda Lake at the terminus of the Mojave River provides that source. Most dunes in the Mojave Desert region are classified as active or migrating dunes. Sand is simultaneously deposited and eroded, constantly molding the dunes into new shapes.  

Again in Death Valley, we saw stream channels scraping down the sides of the mountain ranges that blossomed into alluvial fans stretching out across the basin. Most streams in the Mojave only flow after periods of rain, and flash floods are common. It is standard for channel size and depth to increase as you move downslope, as that's where the most water has accumulated and is moving at its fastest. Alluvial fans are triangular shaped sediment deposits found at the base of mountains in most arid environments. They are a result of fast moving water confined to a very narrow channel reaching the base of a streambed and suddenly being able to dump out all the sediment within (called alluvium). Active alluvial fans are very dynamic and their shape changes as the ideal path of the flowing water shifts, or avulses. The narrowest point at the top of the fan is known as the apex and the widest edge at the bottom is known as the apron. You can expect to see grain size sorting at play, with the largest grains (sometimes as large as boulders) near the apex and the smallest grains out in the apron. When the aprons of alluvial fans converge, it is called a bajada.