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 |
Volcanic Highlights and Features:
[This list is just a sample of
various Wyoming volcanic features or events and is by no means inclusive.]
Excerpt from: |
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Rocky Mountains:6
The Interior Plains:6 The Interior Plains is a vast region that spreads across the stable core (craton) of North America. This area had formed when several small continents collided and welded together well over a billion years ago, during the Precambrian. Precambrian metamorphic and igneous rocks now form the basement of the Interior Plains and make up the stable nucleus of North America. With the exception of the Black Hills of South Dakota, the entire region has low relief, reflecting more than 500 million years of relative tectonic stability. |
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| Devil's Tower National Monument |
Devils Tower:2
Although Devils Tower has long been a prominent landmark in northeastern
Wyoming, the origin of the mammoth rock obelisk remains somewhat obscure.
Geologists agree that Devils Tower formed from molten rock forced upwards from
deep within the earth.
Debate continues, however, as to whether the rock
cooled underground or whether Devils Tower magma reached the surface. Current
research supports the conclusion that Devils Tower was not a volcano, but was
injected between sedimentary rock layers and cooled underground. The
characteristic furrowed columns are the result of contraction which occurred
during the cooling of the magma. Geologic estimates have placed the age of
Devils Tower at greater than 50 million years, although it is likely that
erosion uncovered the rock formations only one or two million years ago.
| Fossil Butte National Monument |
Fossil Butte National Monument:3
Three ancient great lakes existed in the region of Wyoming,
Utah, and Colorado 50 million years ago: Lake Goshute, Lake
Uinta, and Fossil Lake, the smallest. All are gone today,
but they left behind a wealth of fossils in lake sediments that
turned into the rock layer known as the Green River Formation,
made up of laminated limestone, mudstone, and volcanic ash.
The
fossils are among the most nearly perfectly preserved
remains of ancient plant and animal life in the world. Some of the most
extraordinary of these fossils came from Fossil Lake,
represented today by a flat-topped remnant of rock that stands where the
center of Fossil Lake once was. Fossil Butte National Monument
preserves the butte and its invaluable, fascinating record of the
past.
| Grand Teton National Park |
The Grand Tetons:4
The geologic story of this range starts with the formation of
the rocks that make up the mountains, rocks
far older than the mountains themselves.
| Granite Mountains |
Sweetwater Rocks, Granite Mountains:
1
The Granite Mountains in central Wyoming
provide an impression of an old
mountain range submerged in a sea of young
(Tertiary) sediments. Some of the
rocks in this ancient craton are reportedly
as old as 3.6 billion years.
This is a great place to visit if you
like to search for mineral specimens.
Drive along Highway 287 north of Rawlins,
or east of Lander, until you reach
Jeffrey City. From Jeffrey City,
you will need to go north across the Sweetwater
River. You may get the impression that you
just drove into the outback of Africa
or Australia when you survey the terrain,
but there will be no kangaroos in site.
Just a little west of Jeffrey City, the Graham Ranch road
will take you into
some very interesting areas. In this area,
rubies, sapphires, jasper, and jade have
been found. One ruby deposit in this
area was mapped by the
Wyoming State Geological Survey over a strike
length of 5,000 feet. Several
spectacular jade specimens have been
found in this region, some weighing more
than 200 pounds.
As you visit the area, be careful
not to trespass on valid mining claims.
| Jackson Hole |
Jackson Hole:4
Vast clouds of volcanic ash
blew into the Teton region from west and north,
beginning more than 20 million years ago.
White ash accumulated on the sinking floor of Jackson Hole
9 million to 10 million years ago,
leaving deposits nearly one mile thick.
| Kirwin Mining District |
Kirwin Mining District, Wood River Area, Absaroka
Mountains:1
The Kirwin Mining district lies within
the Absaroka Mountains along the
eastern margin of Yellowstone National Park.
Access is by the Wood River
trail, 33 miles southwest of Meeteetse, Wyoming.
Take a backpack and some good shoes.
You will also be in bear country, so take the necessary
precautions. This deeply dissected plateau is cut by
numerous streams and rivers surrounded by
steep mountains formed primarily of Tertiary volcanic rock.
You should be in good shape for this hike
as you have several miles of hiking before
you reach Kirwin, but it is all along the Wood River trail.
The Kirwin district is underlain by a
large, copper-silver porphyry that was
initially prospected in the 1800s for silver,
copper, and lead. Some of the old
lode mines in the area still yield
some spectacular silver specimens.
| Leucite Hills |
Leucite Hills:1
Gemstones:
gem-quality olivine (peridot) in the
Leucite Hills of western Wyoming,
where more than 13,000 carats of peridot were recovered during
reconnaissance. Much of the peridot is very high quality.
| Signal Mountain |
Signal Mountain:4
Between 6 million and 600 thousand years ago,
fiery incandescent clouds of gaseous molten rock
originated in what is now central
Yellowstone Park and flowed southward on both
sides of the Teton Range.
Remnants of this flow are
exposed on Signal Mountain and on the north
end of the Teton Range.
| South Pass |
South Pass, Southern Wind River Mountains:1
The South Pass region is part of an Archean
(>2.5 billion year old) granite-greenstone belt
with several old gold mines and a few ghost towns. Gold
was discovered along Strawberry Creek near the
Sweetwater River in 1842.
Estimates suggest that as much as 350,000 ounces
of gold were recovered from
the region, and many nuggets are found each year.
The South Pass greenstone belt consists
primarily of metamorphosed sedimentary
and volcanic rock deposited in
an ancient sea. These rocks were
compressed into a regional synclinorium
and intensely folded such that most
rock units are now sitting on end.
South Pass consists of rolling hills
on a gradual sloping pediment.
Near the foothills, you will be about 8,500 feet
above sea level where a portion
of the granite-greenstone terrain lies
within the national forest. Keep your eyes
open in this area for beryl associated
with granite pegmatites.
To get there, drive 30 miles south of Lander
along Highway 28.
|
Yellowstone -
Yellowstone National Park |
Yellowstone Plateau:7
The Yellowstone Plateau spans the continental divide between the
Northern and Middle Rocky Mountains, at an
average elevation of about 2,400
meters. The plateau lies at the center of
one of the Earth's largest
volcanic fields,
entirely postdating 2.5 million years ago. The total
volume of magma erupted from the Yellowstone Plateau
volcanic field since
2.5 million years ago probably approaches 6,000
cubic kilometers.
Yellowstone Hot Spot and Caldera Formation:8
A few hotspots
are thought to exist below the
North American Plate.
Perhaps the best known is the hotspot presumed to
exist under the continental crust in the region of
Yellowstone National Park
in northwestern Wyoming. Here are several calderas
(large craters formed by the ground collapse accompanying explosive volcanism)
that were produced by three gigantic eruptions during the past two million years,
the most recent of which occurred about 600,000 years ago.
Ash deposits from these powerful eruptions have been mapped as far away as
Iowa, Missouri, Texas, and even
northern Mexico. The thermal energy
of the presumed Yellowstone hotspot fuels
more than 10,000 hot pools and springs, geysers
(like Old Faithful), and bubbling mudpots (pools of boiling mud).
A large body of magma, capped by a hydrothermal
system (a zone of pressurized steam and hot water),
still exists beneath the caldera.
Yellowstone National Park:5
Yellowstone's vast collection of thermal features provides a constant
reminder of the park's
recent volcanic past.
Indeed, the caldera provides
the setting that allows such features as Old Faithful to exist and to exist in
such great concentrations.
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