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 Wyoming 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 Wyoming.]|
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.
|Devil's Tower National Monument|
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.
Sweetwater Rocks, Granite Mountains:
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.
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
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.
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.
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, 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 National Park
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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