Geological Glossary |
Igneous Rocks
Rocks are divided into three
basic types, igneous, sedimentary and metamorphic , depending upon how
they were formed. Plate tectonics provides an explanation for how rocks
are recycled from igneous to sedimentary to metamorphic and back to igneous
again.
Igneous rocksIgneous 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.
Extrusive igneous rockExtrusive , or volcanic, igneous rock is produced when magma exits and cools outside of, or very near the Earth’s surface. These are the rocks that form at erupting volcanoes and oozing fissures. The magma, called lava when molten rock erupts on the surface, cools and solidifies almost instantly when it is exposed to the relatively cool temperature of the atmosphere.
The cinder cone above and the close up at right are made of basalt . Intrusive igneous rock
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Quick
cooling means that mineral crystals don't have much time to grow, so
these rocks have a very fine-grained or even glassy texture. Hot gas
bubbles are often trapped in the quenched lava, forming a bubbly, vesicular
texture. Pumice, obsidian, and basalt are all extrusive 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. The image at right shows granite, an intrusive
igneous rock.>> Geological Glossary Home <<