The formation of volcanoes in the U.S. is a product of a variety of tectonic settings. The continental U.S. lies entirely on top of the North American Plate and contact with neighboring plates, such as the Pacific plate, produce a variety of tectonic activity and strain within the North American plate. In the Western interior of the U.S., this tectonic activity has led to extension across a wide region of the West and transtension at the base of the Sierra Nevada Mountain Range.
Volcanic Regions of the United States
Extension and transtension zones aren't the only kind of tectonic setting that has created volcanic activity in the United States. The U.S. has six major volcanic regions related to tectonics:
- The subduction zone region of the Pacific Northwest, which has created the Cascade Volcano Range, extending from northern California all the way up to Southern British Columbia.
- The subduction zone region of the northern Pacific, which has created the Aleutian Arc and the volcanoes of Alaska's Aleutian Island Chain.
- The extensional region of the western interior, which has created a region with a high number of cinder cone volcanic fields.
- The transtensional region at the base of the Sierra Nevada mountain range in California, which has created the Long Valley Caldera.
- The hotspot region within the Pacific Plate, which has created the volcanoes of the Hawaiian Island Chain.
- The hotspot region within the North American plate, which has created Yellowstone Caldera and the world's largest known hyrdrothermal system.
How Extension and Transtension Zones Work
Extension occurs within continental crust when tectonic forces are pulling and tearing the continent, and therefore the crust, apart. In the Western U.S., this extension began during the mid-Tertiary when a subduction zone situated off western North America slowly evolved into a transform plate boundary which is now the San Andreas Fault. This tectonic activity destroyed a lot of crust at the convergent plate boundary, or the subduction zone, and the formation of the San Andreas Fault added a lot of stress and strain to the North American plate. The result was the stretching and tearing of the North American plate in the region of the Great Basin, which is where the states of Montana, Idaho, Wyoming, Nevada, Utah, Arizona and California now reside. Transtension occurred in California, where the transform plate boundary, the San Andreas Fault, crept into the regions of extension.
The Cinder Cone Volcanic Fields of the West
Extension across the Western U.S. caused the crust to thin and allowed for multiple areas where magma was able to escape through the tearing crust. For this reason, the Western U.S. is home to a very high number of cinder cone volcanoes, some of them considered still potentially active. The number of cinder cones is so high in number, that many times they are grouped together and referred to as volcanic fields. Two volcanic fields that are considered to be still potentially active are the San Francisco Field located in Arizona and the Bandera Field located in New Mexico. The San Francisco field alone contains about 600 individual cinder cones, including the famous Sunset Crater Volcano.
The Long Valley Caldera
Transtension in the Western U.S. caused the formation of one of the largest volcanoes on Earth. Long Valley Caldera, located in Eastern California, originally erupted predominantly basaltic lavas, but the magma evolved to be more silica rich and eventually erupted predominantly rhyolitic lavas. The evolution of the magma composition is reflective of the evolution of the tectonic settings controlling magma generation and movement in the Western U.S. during the late Tertiary 4 million years ago.
The most recent major eruption of Long Valley Caldera occurred 760,000 years ago when an impressive 600 cubic kilometers of material was ejected from the magma chamber. The chamber was so completely emptied out, that it caused a major collapse of the overlying crust, and formed the present day 17 km by 32 km oval depression of the caldera. Long Valley Caldera is still monitored for its activity and one of the pieces of evidence that it is still active is a tree kill zone located near the caldera, where carbon dioxide emissions are killing a group of trees.
Related Articles:
Subduction Zone Volcanoes of the United States
Sources:
USGS Cascades Volcano Observatory: Western USA Volcanic History and Historical Observations
USGS Long Valley Observatory: Geologic History of Long Valley Caldera and the Mono-Inyo Craters volcanic chain, California
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