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.
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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