Introduction to Igneous Forms
Igneous rocks are formed by the solidification of magma or molten rock material on or within the
surface of the earth. Igneous rocks are classified either intrusive, (formed beneath the surface of
the earth), or extrusive (formed on the earth’s surface).
Intrusive Igneous Rocks (Granitic Materials)- Intrusive igneous rocks were solidified from
molten rock material beneath the surface of the earth as plutons, regardless of size, shape, or
composition. The crystalline structure of igneous plutonic rocks is well developed owing to their
slow process of solidification. Plutonic rocks underlie all rock types, forming a platform or
basement supporting the surface rocks. Exposure takes place if the overriding materials are
weathered or eroded away. Plutonic rocks occur on only 15 percent of the earth’s surface.
Extrusive Igneous Rocks (Basaltic Materials)- Extrusive igneous rocks are of two types. One
type is formed by volcanic eruptions which pour molten lava onto the earth’s surface, where it
solidifies. The other type includes fragmental rocks of all sizes which have solidified at the
surface of the earth. Volcanic magma does not develop a large crystalline structure, for the
cooling of the material is rapid and the resulting crystalline texture is so fine that it is not
apparent without magnification. Most extrusive rocks are dense and glassy in appearance. But
they can be filled with gas bubbles or even frothy. Extrusive rocks occur throughout the world,
but account for only about 3 percent of the total exposed continental land surface.
Distribution- Granite is a predominant igneous intrusive rock form providing the foundation for
most of the continental masses and the central core of many mountainous structures. Basaltic and
volcanic forms occur scattered across most of the continents in small deposits.
 North America, US, and Canada- The New England states of Massachusetts, Vermont,
New Hampshire, and Maine contain massive granitic forms. Other granitic areas include
the Adirondack Mountains of northern New York, a large batholith in central Idaho, the
Black Hills of South Dakota, the Sierra Nevada region of California, and the northern
Cascades in Washington.
 Most of the eastern half of Canada has exposed granitic rock. Basaltic formations are
found in southcentral British Columbia.
 South and Central America- Igneous granitic intrusions are found in southern Venezuela,
southern Guinea, Surinam, French Guinea, northern Brazil, the SE coast of Brazil, and
prominent through most of Central America.
 Africa- Igneous granitic formations occupy large areas in Africa including Madagascar.
The broadest coverage extends across central Africa, then southward along eastern
coastal sections. A large basaltic region extends across central Ethiopia into western
Kenya.
 Europe- Norway, Sweden, Finland, and bordering Russian territories consist mainly of
granitic materials. Igneous intrusions occupy N Scotland, central France, and NW Spain.
 Asia- Exposed granitic intrusions occur over most of India, Ceylon, the northern portion
of the Mongolian Republic, northern Manchuria, and the bordering regions of Siberia.
 Australia- Most of Western Australia is granitic, there are fewer outcrops in the Northern
Territory and Southern Australia. No significant basaltic formations are found in
Australia.

Pacific and Caribbean Regions- No significant regional granitic deposits are found in the
Pacific island region. Most of the islands of the Pacific central basin are basaltic,
including the Hawaiian Islands, the Aleutians, and the foundations of many lagoonforming islands.
 Small scattered granitic outcrops exist throughout Puerto Rico and the Virgin Islands.
The Windward Islands consist of basaltic materials, and scattered deposits are found in
the Dominican Republic.
Source: Terrain analysis procedural guide for surface configuration, US Army engineer
topographic laboratories, p. 7-97