Plutons are rock formations that form underground as intusions of magma. After cooling beneath the surface, this igneous rock is uplifted by regional tectonic forces and is slowly exposed as the overlying and surrounding sedimentary rocks are eroded away. As one might expect, plutons are found in areas of tectonic and volcanic activity.
Plutons exist in a variety of forms, including stocks, sills and laccoliths; batholiths are massive plutons that usually result from an amalgamation of smaller ones. Stocks are vertical columns of magma that intrude through a series of sedimentary layers; an excellent example is Devil's Tower, in eastern Wyoming, which formed about 60 million years ago as the nearby Black Hills were uplifted. Sills are vertical bands of magma which intrude between sedimentary layers; the most famous sill in North America is the Triassic Palisades Sill along the western side of the Hudson River Valley, which formed as Pangea began to rift apart.
Laccoliths are dome-shaped intrusions of magma which, like sills, develop between layers of sedimentary rock. Well known North American laccoliths include Mt. Katahdin, in Maine, Stone Mountain, near Atlanta, and the laccolithic mountains of eastern and southern Utah; the latter include the La Sal, Henry and Abajo Mountains and the isolated dome of Navaho Mountain; Sleeping Ute Mountain, in southwest Colorado, is also a laccolith. The Katahdin laccolith was intruded during the Acadian Orogeny, 360 million years ago, while Stone Mountain formed as the Blue Ridge was uplifted, 300-250 million years ago. The laccolithic mountains of the Colorado Plateau formed much later, during the regional Miocene Uplift. Finally, the massive Sierra Batholith, composed of three major and numerous smaller plutons, formed from the mid Jurassic through the Cretaceous Periods (see my blog of April 28, 2007).
