Tomichi Dome rises north of

U.S. Highway 50 U.S. Route 50 or U.S. Highway 50 (US 50) is a major east–west route of the U.S. Highway system, stretching from Interstate 80 (I-80) in West Sacramento, California, to Maryland Route 528 (MD 528) in Ocean City, Maryland, on the Atlantic ...

west of Hot Springs Creek and south of Waunita Hot Springs Reservoir in the southeast quarter of

Gunnison County, Colorado Gunnison County is a county located in the U.S. state of Colorado. As of the 2020 census, the population was 16,918. The county seat is Gunnison. The county was named for John W. Gunnison, a United States Army officer and captain in the Army ...

. It is situated within the

Gunnison National Forest The Gunnison National Forest is a U.S. National Forest covering 1,672,136 acres (2,612.71 sq mi, or 6,766.89 km²) igneous Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...

mountain formed during the

Tertiary Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago. The period began with the demise of the non-avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...

. The predominant rock is rhyolite, but microgranite,

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...

, and

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...

have been cited as present. Surrounding the mountain, on the valley floor, is

Mancos Shale The Mancos Shale or Mancos Group is a Late Cretaceous (Upper Cretaceous) geologic formation of the Western United States. The Mancos Shale was first described by Cross and Purington in 1899 and was named for exposures near the town of Mancos, ...

, a

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

. Tomchi Dome has been described as an intrusion and also as an

extrusive Extrusive rock refers to the mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out (extrudes) onto the surface as lava or explodes violently into the atmosphere to fall back as pyroclastics or tuff. In contra ...

volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates a ...

feature. As an intrusion, the pluton may have formed when

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

was intruded into the Mancos Shale resulting in a

laccolith A laccolith is a body of intrusive rock with a dome-shaped upper surface and a level base, fed by a conduit from below. A laccolith forms when magma (molten rock) rising through the Earth's crust begins to spread out horizontally, prying apar ...

. Given the fine-grained texture of the rock, the laccolith is assumed to have been at a shallow depth and cooled rapidly. Venting may have been associated with the intrusion resulting in the rhyolytic breccia and tuff documented in the vicinity. Shallow rhyolitic plutons southwest of Tomichi Dome, in the

San Juan Mountains The San Juan Mountains is a high and rugged mountain range in the Rocky Mountains in southwestern Colorado and northwestern New Mexico. The area is highly mineralized (the Colorado Mineral Belt) and figured in the gold and silver mining industry ...

near Lake City, have also been described as laccoliths. Alternatively, Tomichi Dome has been described as an extrusive, volcanic feature where the initial eruptions deposited breccia and tuff that is over thick. A flow or dome of topaz rhyolite overlies the tuff. Whether plutonic or volcanic, Tomichi Dome is possibly one of a string of igneous structures associated with the

Rio Grande Rift The Rio Grande rift is a north-trending continental rift zone. It separates the Colorado Plateau in the west from the interior of the North American craton on the east. The rift extends from central Colorado in the north to the state of Chihuahu ...

, which stretches from Mexico to Colorado.

Historical names

*Tomichi Dome – 1907 *Tumitche Dome

Notes

*Maps: **''Gunnison Basin Public Lands''. Forest Service Series Map. United States Department of Agriculture. Colorado, 2008.

External links

Mountains of Colorado Mountains of Gunnison County, Colorado North American 3000 m summits Geologic domes Rock formations of Colorado Volcanoes of Colorado {{Colorado-geo-stub

Historical names

See also

Notes

External links