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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Select Volcanic Highlights and Features:
| [NOTE: This list is just a sample of various Sierra Nevada 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 Sierra Nevada.] |
California's Sierra Nevada ("snow-capped mountain") is a
west-tilting 350-mile-long block of granite. Extending
from 14,494 feet (Mount Whitney, the highest peak in the lower
48 states) in the east to near sea level in the west, it contains
the spectacular Yosemite and Sequoia National Parks.
The massive granite intruded the crust
in Mesozoic time and was uplifted and faulted in the Tertiary
during formation of the Basin and Range province to the east.
Eroded residue from the Sierra Nevada has filled the Central
Valley of California, giving rise to both extensive agriculture
and the 1849 Gold Rush.
Excerpt from: USGS A Tapestry of Time and Terrain Website, 2001 |
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Pacific Mountain System:1
This region is one of the most geologically
young and tectonically active in North America. The
generally rugged, mountainous landscape of this
province provides evidence of ongoing
mountain-building.
The Pacific Mountain System straddles the
boundaries between several of Earth's moving
plates -- the source of the monumental forces
required to build the sweeping arc of mountains that
extends from Alaska to the southern reaches of
South America. This province includes the active
and sometimes deadly volcanoes of the
Cascade Range and the young, steep mountains of
the Pacific Border and the Sierra
Nevada.
Although the Sierra Nevada and Cascade Range form
a nearly continuous barrier
along the western edge of the United States, the
two ranges really have very
little in common. They have been and continue to
be formed by quite different
geological forces and processes.
| Sierra Nevadas Vulcanism |
Volcanism in the Sierra Nevadas:1
About 150 million years ago, in the Jurassic period of geologic time,
the sediments that had been deposited in the sea and later compressed
into sedimentary rocks began to be lifted above sea level.
Then began a time of explosive volcanic eruptions that spewed
forth vast quantities of lava and volcanic ash, deeply
burying the sedimentary rocks. The layered sedimentary and
volcanic rocks were then deformed and, in many places,
folded intricately, changing or metamorphosing them into tougher,
very resistant metamorphic rocks.
Examples of these
metamorphic rocks can be seen at Minaret Summit and
along the road descending to Agnew Meadow.
Concurrent with the folding and deformation of the layered rocks,
molten rock bodies were intruded into these rocks under conditions
of great heat and high pressure deep below the Earth's surface;
some of this molten rock, or magma, reached the surface to form new volcanoes.
This intrusive activity ended about 80 million years ago,
in the Cretaceous period. The molten rock cooled slowly and in
time solidified into the
granite core of the Sierra Nevada.
The episode of deformation, intrusion, and volcanic eruption
ended in the formation of a mountain range that can be
considered an ancestral Sierra. Erosion followed,
and by the end of Cretaceous time, about 60 million years ago,
most of the older volcanic and metamorphic rocks had been worn away,
exposing the granite core of the range. The area had a low relief
compared to the mountains of today and posed little obstacle to streams
draining westward from the interior of the continent.
About 25 million years ago, this lowland area began to be uplifted
and tilted toward the southwest, a construction which would
eventually lead to the present Sierra Nevada.
About 3 million years ago the river channel north of Minaret Summit
was filled by lava flows - reddish-colored, layered rocks visible
on the slope above the road descending to Agnew Meadows.
This blocked the former channel and isolated the present
San Joaquin drainage basin from the area east of the crest.
Also about this time the Long Valley-Mono Lake area began to
lag behind as the present range continued to rise and the
growth of the east-facing Sierra Nevada escarpment further
blocked any renewal of a trans-Sierra San Joaquin River.
The Sierra Nevada continues to rise today.
| Sierra Fault Block |
The Sierra Fault Block:3
The Sierra is a tilted fault block nearly 400 miles long.
Its east face is a high, rugged multiple scarp,
contrasting with the gentle western slope (about 2 degrees)
that disappears under sediments of the Great Valley.
Deep river canyons are cut into the western slope.
Their upper courses, especially in massive granites of the
higher Sierra, are modified by glacial sculpturing,
forming such scenic features as Yosemite Valley. The high
crest culminates in Mt. Whitney with an elevation of
14,495 feet above sea level near the eastern scarp. The
metamorphic bedrock (still partly capped by Tertiary volcanics),
contains gold-bearing quartz veins of the
Mother Lode system; a north-south structural trend is
predominant in the western flank and northern end of
the Sierra. The northern Sierra boundary is marked
where bedrock disappears under the Cenozoic volcanic
cover of the Cascade Range.
| California's Volcanic Past |
| Sierra Nevada National Parks and Monuments |
Excerpts from:
1) USGS/NPS Geology in the Parks Website, 2001
2) USGS A Tapestry of Time and Terrain Website, 2001
3) California Geological Survey Website, 2002
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