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 lac-
coliths are smaller intrusions and are usually grouped with batholiths in the cat-
egory “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 exam-
ple 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 igne-
ous 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.
Volcanic activity leads to the extrusion of lavas over Earth’s surface. This is
common around volcanoes of all types, but is common over larger areas as the
result of lava emerging from mantle plumes and midocean ridges in the absence
of volcanoes. These areas have rocks with small crystals indicating the relatively
quick cooling of Earth’s exterior. Basalt is the rock type that takes up the most
area. Great volumes of lava can be produced at various times resulting in thick358 Vulcanism
Fire
In Greek and other cultures of classical times, fire was thought to be one of the basic ele-
ments of the physical Earth. Although our knowledge of the physicalEarthhas become much
more complete, fire still has a basic importance to our planet. Fire is natural and needs a fuel
source in the presence of oxygen. If Earth’satmospherecontained more oxygen, fires
would be much more difficult to control. Natural fires have several causes: lightning, sponta-
neous combustion of organic material, sparks from falling rocks, and volcanic eruptions. Fire
has had a profound effect on the evolution of some plant and animal communities.Climate
and vegetation type combine to make somelandscapesmore fire prone than others. Some
life, like Sequoia trees (Sequoiadendron giganteum), has been tuned to resist damage by fire
and, indeed, the Sequoia’s dropped seed cones sprout after being heated by fire. Fire is dan-
gerous to all living things, but humans learned how to—more or less—control fire upwards
of 800,000 years ago. This was one of the most significant events in human history because
it allowed for the cooking of food and the ready clearing of land. There is no doubt that
humans are the most important starters of fire in today’s world.