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The REALLY Ancient SouthWest



California Geology and Volcanoes



The one major geologic event that has shaped California since ancient times - and still shapes her landscape today - is the plate tectonics that have caused the San Andreas Fault. The pressure caused by the Pacific plate crushing northward along the North American plate is best seen along the San Andreas Fault, but it effects land for thousands of miles on either side. This force caused the land to fold and uplift - creating the mountain ranges we see today - the coastal ranges, the San Bernardino ranges, and the Sierra Nevada. And these Sierra Nevada - and the Panamint Range behind them, have caused such a sever rain shadow that Death Valley has been registered as the hottest - a lowest - place on earth. This pressure also caused the Rocky mountains to rise, and the Colorado Plateau to rise and tilt, creating an include that ran its rivers into the canyons that cut through the southwest today. All of this shifting and movement starts in California and spreads west.

Death Valley Area
Split Cinder Cone
Less than 300,000 years ago, a chamber filled with solid crystals and searing molten basaltic rock simmered beneath Death Valley. Magma rose toward the surface, following weaknesses in the Earth's crust. Nearing the surface, the black lava encountered the fractured earth of the Death Valley Fault zone. Lava quickly made its way through the fault-weakened rock and burst out of the valley floor as a fiery fountain of scorching lava and gas. Lava fountains threw blobs of molten basalt hundreds of feet into the air. Although lava erupted at 1200 degrees C (2200 degrees F), most of the molten, airborne globs cooled and solidified to form cinders before reaching the ground. Most cinders fell very near the central vent, building a small cone. ... Split Cinder Cone was probably built over a very short time; its birth and death probably spanned less than a few decades. Although the little volcano lay quiet, the Death Valley Fault zone continued to move as it had for almost three million years. The wrenching force of this very active fault pulled one part of the volcano to the southeast, while the other part was pulled toward the northwest. Eventually, the crust could no longer resist the wrenching motion of the fault and the cinder cone began to be ripped into two pieces. Each time the fault moved, the two sides of the cone moved farther apart.

Black Mountains
The face of the Black Mountains along Artist's Drive is made up of the multicolored rock of the Artist Drive Formation. Aprons of pink, green, purple, brown, and black rock debris drape across the mountain front, providing some of the most scenic evidence of one of Death Valley's most violently explosive volcanic periods.

Ubehebe Crater
(Less than 10,000 years ago) ... Over a dozen volcanoes dot the landscape of Ubehebe volcanic field. Ubehebe Crater is the largest and youngest volcanic feature. ... Following weaknesses in the Earth's crust, searing basaltic magma rises upward. A fault along the base of Tin Mountain, responsible for uplift of the entire mountain range, lies in the path of the molten mass, providing an easy escape route to the surface. Magma worked its way through the fault-weakened rock where it met water-soaked bedrock and alluvial fan sediments. In an instant, water flashed to steam. A sudden, violent release of steam-powered energy blasted away the confining rock above. ... The largest of these eruptions produced Ubehebe Crater, over a half a mile wide and 770 feet deep. ... The eruptions that created Ubehebe Crater blasted through older conglomerate layers, now revealed in the crater walls. ... Over a dozen other explosion craters and tuff rings in the Ubehebe Crater field are the result of this type of hydrovolcanic eruption.

Mojave Desert Area
Granite Mountains
Some of the more striking rock formations in the Mojave National Preserve lie in the Granite Mountains. These granitic rocks have eroded into unusual rounded shapes that include spires, perched boulders, and curved cliff faces. ... Granitic rocks represent the roots of ancient continental-margin volcanic systems. Most of the granitic rock in the Mojave Desert is late Mesozoic in age (80 to 180 million years old). The Mojave National Preserve lies within a belt of late Mesozoic granites that parallel the western continental margin from Mexico to Canada. The granites formed at depth within a volcanically active mountain range comparable in geologic setting to the Andes Mountains chain in South America. The granitoids formed by the slow cooling and solidification of molten magma bodies that developed above sinking slabs of oceanic crust overridden by the edge of the continent. At least 55 or 60 million years elapsed between the crystallization of the last Mesozoic magma bodies and deposition of the youngest-preserved overlying strata. The Mojave National Preserve probably formed a highland during much of this period and erosion gradually stripped off Paleozoic and Mesozoic sedimentary rocks overlying the granites.

Cima Dome
The northern half of Mojave National Preserve is dominated by a broad sloping desert upland called Cima Dome. The dome is the exposed remains of a massive body of granite that formed deep underground long ago and was slowly forced to the surface.

Van Winkle Mountains
20 million-year-old volcanic rocks ... cap the Van Winkle Mountains to the east.
Peach Springs Tuff
The oldest volcanic rocks preserved in this area, the Peach Springs Tuff, erupted 18.5 million years ago from a volcano near the southern tip of Nevada. The air fall deposit settled on pediment surfaces and fluvial deposits. The Peach Springs Tuff event approximately coincided with the beginning of basin formation in the region, which may explain why it is overlain in several places by shallow lake deposits.

Woods Mountain Caldera
Local volcanism in the Woods Mountains area began ... About 17.8 million years ago, viscous siliceous magma approached the surface of the volcano. A plume of ash was spewed high into the atmosphere. Then the volcano exploded with devastating force. ... Two similar explosive cycles followed within less than 100,000 years. The resulting deposits formed a flat plateau extending from the Pinto Mountains to Blind Hills and from Wildhorse Mesa to the Hackberry Mountains. Such a large volume of Wild Horse Mesa Tuff was ejected from the volcano's magma chamber that overlying strata collapsed downward, forming a cone-shaped depression called a caldera. The Woods Mountains caldera, the most well preserved caldera in the Mojave, was 10 kilometers wide and 4 kilometers deep. It was largely in-filled with collapsed tuff and younger light-colored (rhyolite) flows.

Wild Horse Mesa Tuff and Hole-in-the-Wall
About 17.8 million years ago, a powerful eruption blasted outward from a volcanic center in the Woods Mountains in the Eastern Mojave. Propelled by the force of rapidly rising and expanding superheated gases, a ground-hugging cloud of ash and rock fragments spread out at near-supersonic speed across the countryside. Hot, suffocating ash buried shallow lakes and stands of trees. The remains of birds, mammals, and plants are preserved as fossils in the sediments below the ash layer. The May 18, 1980 lateral blast from Mount St. Helens was somewhat analogous. The deposits from three closely spaced, violent eruptions comprise the rock unit called the Wild Horse Mesa Tuff which forms the cliffs of Hole-in-the-Wall.

Cima Volcanic Field
Volcanic eruptions in the Cima field first began about 7.6 million years ago and continued until at least as recently as 10,000 years ago (based on the K-Ar dating method), near the end of the most recent ice age. The field is characterized by basalt, which is a black to dark gray volcanic rock formed from lava rich in magnesium and iron. ... Each of the 40 cinder cones in the volcanic field represents one or more sites from which lava erupted.

Click here to see the map of California's potentially hazardous volcanoes still a concern today.



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