A body of rock formed from magma migrating and solidifying deep in the subsurface is called a pluton or an igneous intrusion.

Huge intrusions, covering areas greater than one hudred square kilometers (40 square miles) are called a batholith. Batholiths typically contain many separate intrusions that form over a relatively long period of time.

A laccolith is a blister-shaped intrusion. See "Dome Mountain".

Dome Mountains:
Dome Mountains are formed from hot molten material (magma) rising from the Earth's mantle into the crust that pushes overlying sedimentary rock layers upward to form a "dome" shape. Unlike a volcano, the magma typically does not reach the Earth's surface. Instead, the magma cools underneath the surface and forms the core of the mountains. Dome mountains in Utah include Navajo Mountain, and the La Sal, Abajo, and Henry Mountains in the southeastern part of the state.

Stocks, Dikes, Sills:
Other types of intrusions typically form at shallower crustal depths; these include stocks, dikes, and sills A stock is smaller than a batholith and typically represents the subsurface passage that fed molten material to a volcano or field of volcanoes over time. Sills and dikes are layers of igneous rock that typically form along fault zones, fractures, or between and parallel to sedimentary layers.

A sheetlike body of igneous rock that cuts across layering or contacts in the rock into which it intrudes. Dikes form when magma rises into an existing fracture, or creates a new crack by forcing its way through existing rock, and then solidifies. Hundreds of dikes can invade the cone and inner core of a volcano, sometimes preferentially along zones of structural weakness.

A tabular body of intrusive igneous rock, parallel to the layering of the rocks into which it intrudes.

Past Igneous Activity:
Stocks, sills, dikes, laccoliths and other intrusions are remnants of past igneous activity and are exposed at the surface long after erosion has stripped away any ancient volcanoes and other overlying rocks and sediments that may have existed in an area.

-- Excerpts from:
Philip Stoffer, 2002, Rocks and Geology in the San Francisco Bay Region, USGS Bulletin 2195, Volcano World Website, 2003, USGS Volcano Hazards Programs Photoglossary, 2003, and Utah Geological Survey Website, 2003

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