Igneous Rock

V

Extrusive Bodies

Many different types of extrusive bodies occur throughout the world. The physical characteristics of these bodies depend on their chemical composition and on how the magma from which they formed erupted. The chemical composition of the parent magma affects its viscosity, or its resistance to flow, which in turn affects how the magma erupts. Felsic magma tends to be thick and viscous, while mafic magma tends to be fluid. (See also Volcano)

Flood basalts are the most common type of extrusive rock. They form when highly fluid basaltic lava erupts from long fissures and many vents. The lava coalesces and floods large areas to considerable depths (up to 100 m/300 ft). Repeated eruptions can result in accumulated deposits up to 5 km (3 mi) thick. Typical examples are the Columbia River basalts in Washington and the Deccan trap of western India; the latter covers an area of more than 500,000 sq km (200,000 sq mi).

When basalt erupts underwater, the rapid cooling causes it to form a characteristic texture known as pillow basalt. Pillow basalts are lava flows made up of interconnected pillow-shaped and pillow-sized rocks. Much of the ocean floor is made up of pillow basalt.

Extrusive rocks that erupt from a main central vent form volcanoes, and these are classified according to their physical form and the type of volcanic activity. Mafic, or basaltic, lava is highly fluid and erupts nonexplosively. The fluid lava quickly spreads out, forming large volcanoes with shallow slopes called shield volcanoes. Mauna Loa (Hawaii) is the best-known example. Intermediate, or andesitic, magmas have a higher viscosity and so they erupt more explosively. They form steep-sided composite volcanoes. A composite volcano, or stratovolcano, is made up of layers of lava and volcanic ash. Well-known examples of composite volcanoes include Mount Rainier (Washington), Mount Vesuvius (Italy), and Mount Fuji (Japan).

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Felsic (rhyolitic) magmas are so viscous that they do not flow very far at all; instead, they form a dome above their central vent. This dome can give rise to very explosive eruptions when pressure builds up in a blocked vent, as happened with Mount Saint Helens (Washington) in 1983, Krakatau (Indonesia) in 1883, and Vesuvius (Italy) in AD 79. This type of explosive behavior can eject enormous amounts of ash and rock fragments, referred to as pyroclastic material, which form pyroclastic deposits (See also Pyroclastic Flow)

VI

Plate Tectonics and Igneous Rocks

The advent of the theory of plate tectonics in the 1960s provided a theoretical framework for understanding the worldwide distribution of different types of igneous rocks. According to the theory of plate tectonics, the surface of the earth is covered by about a dozen large plates. Some of these plates are composed primarily of basalt and are called oceanic plates, since most of the ocean floor is covered with basalt. Other plates, called continental plates because they contain the continents, are composed of a wide range of rocks, including sedimentary and metamorphic rocks, and large amounts of granite.

Where two plates diverge (move apart), such as along a mid-ocean ridge, magma rises from the mantle to fill the gap. This material is mafic in composition and forms basalt. Where this divergence occurs on land, such as in Iceland, flood basalts are formed.

When an oceanic plate collides with a continental plate, the heavier oceanic plate subducts, or slides, under the lighter continental plate. Some of the subducted material melts and rises. As it travels through the overriding continental plate, it melts and mixes with the continental material. Since continental material, on average, is more felsic than the mafic basalt of the oceanic plate, this mixing causes the composition of the magma to become more mafic. The magma may become intermediate in composition and form andesitic volcanoes. The Andes Mountains of South America are a long chain of andesitic volcanoes formed from the subduction of the Pacific Plate under the South American plate. If the magma becomes mafic, it may form rhyolitic volcanoes like Mount Saint Helens. Magma that is too viscous to rise to the surface may instead form granitic batholiths.

VII

Economic Importance of Igneous Rocks

Many types of igneous rocks are used as building stone, facing stone, and decorative material, such as that used for tabletops, cutting boards, and carved figures. For example, polished granite facing stone is exported all over the world from countries such as Italy, Brazil, and India.

Igneous rocks may also contain many important ores as accessory or trace minerals. Certain mafic intrusives are sources of chromium, titanium, platinum, and palladium. Some felsic rocks, called granitic pegmatites, contain a wealth of rare elements, such as lithium, tantalum, tin, and niobium, which are of economic importance. Kimberlites, formed from magmas from deep within the earth, are the primary source of diamonds. Many magmas release large amounts of metal-rich hot fluids that migrate through nearby rock, forming veins rich in metallic ores. Newly formed igneous rocks are also hot and can be an important source of geothermal energy.

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