Vulcanism

Vulcanism is the term for the motion and landforming processes associated with molten rock. The term subsumes the more specific “volcanism” referring to shallow and external features and processes and “plutonic activity” when it is deep-seated. As covered in the article about Earth, the interior of our planet is ferociously hot. This interior energy reaches all the way out to the surface, builds some landscapes and destroys others. Earth’s crustal plates float on an upper mantle that is quite plastic and capable of deformation and flow. Concentrated flows far underground sometimes influence the surface on which we live. Other flows reach and spill out onto the surface. When this material is in Earth’s interior it is known as magma, and when it flows over the surface it is called lava.

When magma solidifies under the surface the spate of forms created is known as intrusive vulcanism. Large magma intrusions under the surface are known as batholiths. They have sizes greater than one hundred square kilometers and extend downward as far as 50 km below the surface. They are composed of rocks with large crystals (e.g., granite) formed by slowly cooling magma. Batholiths are often associated with the presence of major mountain ranges in that the batholithic intrusion has pushed overlying rock to great altitudes. Well-known examples reside under the Sierra Nevada of California, the Extremadura region of Spain, the Darling Range of Australia, and the Transantarctic Mountains. Stocks and laccoliths are smaller intrusions and are usually grouped with batholiths in the category “plutons.” The Black Hills of South Dakota, United States, the Tuscany region in Italy, and some of the High Himalayas are underlain by laccoliths. Minor intrusive forms include sills (horizontal) and dikes (vertical). A noteworthy example of the edge of a sill is the Palisades, which are a line of 100-m tall cliffs on the west bank of the Hudson River across from New York City. Dikes represent igneous intrusions that have been partially exhumed by erosion of the surrounding rocks. Dikes are typified by several 50-m tall walls radiating outward from the remains of a volcano at Shiprock in northern New Mexico, United States.