America's Volcanic Past
|"Though few people in the United States may actually experience an erupting volcano, the evidence for earlier volcanism is preserved in many rocks of North America. Features seen in volcanic rocks only hours old are also present in ancient volcanic rocks, both at the surface and buried beneath younger deposits." -- Excerpt from: Brantley, 1994|
Volcanic Highlights and Features:
|[NOTE: This list is just a sample of various Mojave features or events and is by no means inclusive. All information presented here was gathered from other online websites and each excerpt is attributed back to the original source. Please use those sources in referencing any information on this webpage, and please visit those websites for more information on the Geology of the Mojave National Reserve.]|
The volcanic eruptions that rocked what is now Mojave National
Preserve coincided with a period (about 20-5 million years ago) of intense
plate tectonic activity.
During this episode, the Earth's crust was literally
ripped apart, radically altering the landscape of the Southwest.
As the crust stretched, fractures formed along
weak zones and mountain range-sized blocks jolted
and slid against each other along faults. Some of
these enormous blocks of crust rose up, forming
rows of high, elongate mountains. Other blocks slid
down, forming the low valleys. Together, the linear
mountain ranges and intervening valleys define the
Basin and Range Province.
The thinned, faulted crust made it easier for
magma to rise up and follow weaknesses in the
rock. Where magma reached the surface volcanoes
grew and great globs of magma solidified to form
plutons beneath them. In some places within the Preserve these geologically
young plutons can be seen right at the surface. These plutons 'froze' at
depth, then were almost immediately lifted up to the surface as mountains
rose along new faults. This is truly a dynamic place!
Excerpt from: USGS/NPS Geology in the Parks Website, 2001
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.
Cima Volcanic Field:3
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.
Cinder Cones National Natural Landmark:1
The skyline of Cima Dome is interrupted by the conical outlines of dozens of remarkably well-preserved volcanic cinder cones and black basalt lava flows. The earliest began about 7.6 million years ago and eruptions continued until at least 10,000 years ago, near the end of the most recent ice age.
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 parallels 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 highlands during much of this period and erosion gradually stripped off Paleozoic and Mesozoic sedimentary rocks overlying the granites.
Hole In The Wall:1
Visitors to Mojave National Preserve are fascinated by the brightly colored, fantastically sculpted rocks at Hole in the Wall. Little do they know that these intricate forms hold the key to a devastating and violent episode in Mojave's geological past. About 18.5 million years ago, a powerful volcanic eruption blasted outward from the nearby Woods Mountains. Propelled by the force of rapidly rising and expanding gasses, a ground-hugging cloud of ash and rock fragments spread out at near super-sonic speeds across the countryside. Some of the rocks thrown out by the blast are 14-20 meters (60 feet) across - the largest ever documented! Hot, suffocating ash buried every living thing in the path of the blast. An area of over 600 square kilometers was covered with ash and rock fragments so hot that they welded together after they reached the ground. The toasted and fossilized remains of birds, mammals, and plants lie entombed beneath the volcanic tuff that forms the colorful cliffs of Hole in the Wall.
Peach Springs Tuff:3
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 airfall 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.
Van Winkle Mountains:2
20 million-year-old volcanic rocks cap the Van Winkle Mountains.
Wild Horse Mesa Tuff and Hole-in-the-Wall:3
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.
Woods Mountain Caldera:3
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.
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