3.1 Magma, the Parent Material of Igneous Rock

Igneous rocks form as molten rock cools and solidifies. Abundant evidence supports the fact that the parent material for igneous rocks, called magma, is formed by a process called partial melting. Partial melting occurs at various levels within Earth’s crust and upper mantle to depths of about 150 miles.

Once formed, a magma body buoyantly rises toward the surface because it is less dense than the surrounding rocks. Occasionally molten rock breaks through, producing a spectacular volcanic eruption. Magma that reaches earth’s surface is called lava. Sometimes lava is emitted as fountains that are produced when escaping gases propel molten rock from a magma chamber. On other occasions, magma is explosively ejected from a vent, producing a catastrophic eruption. However, not all eruptions are violent. Many volcanoes emit quiet outpourings of very fluid lava.

Playlist of Geology and Earth Science Videos from Snow Mountain
https://www.youtube.com/watch?v=owlPSr39Nn8&list=PL6taRb3I0WLhYc8g1cuBk5crqUtLs3oXZ

Igneous rocks that form when molten rock solidifies at the surface are classified as extrusive or volcanic. Extrusive igneous rocks are abundant in western portions of the Americas, including the volcanic cones of the Cascade Range and the extensive lava flows of the Columbia Plateau. In addition, many oceanic Islands, typified by the Hawaiian chain, are composed almost entirely of volcanic igneous rocks.

Magma that loses its mobility before reaching the surface eventually crystallizes at depth. Igneous rocks that form at depth are termed intrusive or plutonic. Intrusive igneous rocks would never outcrop at the surface if portions of the crust were not uplifted and the overlying rocks stripped away by erosion. When a mass of crustal rock is exposed, not covered with soil, it is called an outcrop. Exposures of intrusive igneous rocks occur in many places, including Mount Washington, New Hampshire, Stone Mountain, Georgia, the Black Hills of South Dakota, and Yosemite National Park, California.