America's Volcanic Past
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| "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 |
Location Map - Southern California National Parks and Monuments
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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 |
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Cima Dome:2
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.
Granite Mountains:3
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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