Far from being static, the geology of Earth is dynamic, in constant motion and change. The crust is a puzzle made up of tectonic plates, shifting against one another over a molten layer of magma. During the changing relationship of the plates, Earth’s crust has undergone compression and stretching, creating vast breaks in the surface known as faults. Periods of mountain building uplifted the great mountain chains of the world. Since the Precambrian Era, the Mojave Desert has experienced many different phases over almost incomprehensible time periods.

Geology

All three families of rock are evident in the Mojave Desert — igneous, sedimentary, and metamorphic. The earliest rock, related to the metamorphic gneiss and schist at the bottom of the Grand Canyon, has little exposure in the Mojave, but can be seen at Saddle Island in Lake Mead National Recreation Area (NRA) and parts of Death Valley National Park (NP). Changed from preexisting rock by immense temperature and/or pressure (without melting), this metamorphic rock is ancient — from the Precambrian Era, formed about 1.6 to 1.8 billion years ago.

Once Upon A Time

It is hard to imagine that during the Paleozoic Era (570 – 225 million years ago) much of the American Southwest was covered by shallow seas. However, fossil impressions of marine creatures in limestone and dolomite reveal the ancient beginnings of many sedimentary rocks. Throughout this period, fluctuating seas deposited thousands of feet of sediment, then retreated, exposing these layers to erosion. Different layers of marine deposits can be seen in banded mountains throughout the desert.

During the Mesozoic Era (225 – 65 million years ago) mountains were uplifted, revealing the Paleozoic layers. Evaporating bodies of water left behind salt (halite), calcium (gypsum), and many other evaporates that have been mined by humans for hundreds of years for industrial and domestic use. Rivers and transient streams carried great amounts of eroded material into the lowlands.

The climate became even more arid than it is today, expanding great belts of sand dunes. Jurassic winds carried the sand, creating the cross-bedding patterns still evident in the Aztec sandstone found in Red Rock Canyon National Conservation Area (NCA), Valley of Fire State Park, and Lake Mead NRA. It continues into Arizona and Utah where it is called Navajo sandstone. Red, beige, and even white, this sandstone has a propensity for fanciful erosion by wind and water. Spectacular arches can be formed. Beehives, goblins, and castles appear to the human imagination in other rock formations.

The beginning of the Cenozoic Era (65 million years ago – present) heralded another tectonically active period. Several different types of faulting deformed the layers of sediment and the metamorphic rock beneath. Normal faults produced vertical lifting and dropping, while strike-slip faults shifted blocks laterally. Thrust faults folded older layers over younger deposits, producing miles of horizontal displacement. This topsy-turvy construction can be seen at the Keystone Thrust Fault at Red Rock Canyon NCA and in the Muddy Mountains of Lake Mead NRA.

Magma On The Move

Along with the faulting, volcanic activity increased across the landscape. Igneous rock is derived from molten layers of magma beneath the Earth’s crust. Under great pressure, the magma rises towards the surface through weak points such as faults. If the molten rock solidifies before reaching the surface, it is intrusive, forming tabular sills, dikes, and large bodies called plutons. These rocks are large-grained from slow cooling. Erosion of less resistant surface rock exposes the plutonic formations. Plutonic rock can be seen all over the Mojave Desert, and Joshua Tree National Park has especially fantastic examples.

Extrusive igneous rock is magma that reaches the surface, in the form of lava and explosive material. Ash, cinder, lapilli, and volcanic bombs are violently expelled by volcanoes and vents when lava is very thick and filled with gas. Cinder cones, calderas, and stratovolcanoes are formed in this way. Shield volcanoes are formed by less viscous lava. Extrusive igneous rock is usually finegrained from cooling quickly on the surface.

The most recent period of volcanic activity in the Mojave Desert occurred only three thousand years ago and was centered at the Ubehebe Crater in Death Valley NP. The hot springs in Black Canyon at Lake Mead NRA and the cinder cones and lava flows found in the Mojave National Preserve (NPres) are reminders of this active period.

Shake, Rattle, And Roll

For Mojave Desert residents earthquakes are active reminders of the Earth’s dynamic nature. An earthquake is a shaking of the Earth caused by the release of energy as rock suddenly breaks or shifts under stress. Although earthquakes can happen anywhere, most occur along tectonic plate margins and are associated with faults.

The Mojave Desert lies near the boundary between the Pacific and North American plates. As these plates slide past each other stress is created, forcing rocks to break along faults. A magnitude 7.6 earthquake, centered in the western Mojave Desert, rattled a wide area on June 28, 1992. It was the largest to strike California in forty years and was also the world’s largest earthquake for 1992. Aftershocks from this quake were felt for many years.

Whittling The Earth

In this arid region of little precipitation and sparse vegetation, erosion is a major process shaping the desert. Water can seep into the smallest crack and percolate between grains of stone. Acids, minerals, and gases are carried in solution to react with the rock, causing chemical alterations such as oxidation of iron and manganese which produces many hues of red and purple. The rain that does come is often in torrential thunderstorms that carry large amounts of debris through washes, forming alluvial fans below the mountains and filling in the desert basins. Wind also distributes material, molding sand dunes, such as those found at the Kelso Dunes in the Mojave NP, and abrading landforms into sculptural shapes, like the arches at Valley of Fire. Gravity continually draws material downward, forming aprons of talus debris along the skirts of the mountains. Erosion is not always a natural process. Human activities, such as overgrazing, deforestation, construction, and water diversion, are recorded on the land. Bicycles, off-road vehicles, and indiscriminate blazing of new trails destroy the protective layer of desert pavement and promote erosion of fragile desert soils. Removal of even seemingly unimportant rocks spoils the experience for others and is illegal in National Park Service areas.

The fascinating rock formations of the Mojave Desert are inviting, but be careful when climbing. Many people are injured or killed in climbing accidents every year. Explore and enjoy the desert safely and sanely.