What are igneous rocks?
Igneous rocks (from the Greek word for fire) form from when hot, molten rock (magma) crystallizes and solidifies. The melt originates deep within the Earth near active plate boundaries or hot spots, then rises toward the surface. Igneous rocks are divided into two groups, intrusive or extrusive, depending upon where the molten rock solidifies. Intrusive Igneous Rocks: Intrusive, or plutonic igneous rock forms when magma is trapped deep inside the Earth. Great globs of molten rock rise toward the surface. Some of the magma may feed volcanoes on the Earth’s surface, but most remains trapped below, where it cools very slowly over many thousands or millions of years until it solidifies. Slow cooling means the individual mineral grains have a very long time to grow, so they grow to a relatively large size. Intrusive rocks have a coarse grained texture. Extrusive Igneous Rocks: Extrusive, or volcanic, igneous rock is produced when magma exits and cools outside of, or very near the Earth’s su
Early study of igneous rocks was shrouded in controversy. In an attempt to organize rocks into a simple easily understood system, the 18th century german mineralogist Abraham Werner proposed that all rocks were precipitated in layers from a universal sea. Active volcanoes were explained by burning of subterranean coal beds. Because he was highly respected by his peers, Werner’s theory, called Neptunism, gained wide acceptance and was not questioned. Neptunism, however, had a number of problems: e.g., the volume of rock assumed to have been precipitated was much greater than could have been in solution. Eventually Werner’s theory was disproven by studies of volcanic rocks. The geological community came to accept Plutonic theory, the belief that igneous rocks originate as molten rock material deep in the Earth. The new theory got its name from the Greek god of the underworld, Pluto.
