1.34: Igneous Rocks
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• The term igneous applies to rocks or minerals that have solidified from molten material.
• Molten material underground is call magma; when it erupts and flows on the surface it is called lava.
* When molten material cools, it crystallizes into rock.
• When magma intrudes other rocks underground and cools it forms intrusive igneous rocks (examples include granite, diorite, and gabbro). Slower cooling times underground result in bigger mineral crystals. These rocks typically have acrystalline texture from interlocking crystal grains.
• Lava that extrudes on the surface as a volcanic eruption cools quickly, forming extrusive igneous rocks (examples include rhyolite, andesite, and basalt).
Igneous rocks are generally classified by their color and size of their crystals, and more specifically classified by their mineral composition. Study of igneous rocks has played an important role in deciphering the origin of rocks beneath and around ocean basins (discussed in Chapters 3 and 4).