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Facts about Igneous Rocks for Kids

Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock.Igneous rock is formed through the cooling and solidification of magma or lava. Igneous rock may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.

  • This magma can be derived from partial melts of pre-existing rocks in either a planet’s mantle or crust.
  • Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition.
  • Over 700 types of igneous rocks have been described, most of them having formed beneath the surface of Earth’s crust.
  • Igneous and metamorphic rocks make up 90-95% of the top 16 km of the Earth’s crust by volume.
  • Surrounded by pre-existing rock (called country rock), the magma cools slowly, and as a result these rocks are coarse grained.
  • Intrusive rocks can also be classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes.
  • Felsic and intermediate magmas that erupt often do so violently, with explosions driven by release of dissolved gases — typically water but also carbon dioxide.
  • If the cooling has been so rapid as to prevent the formation of even small crystals after extrusion, the resulting rock may be mostly glass.
  • Because the minerals are mostly fine-grained, it is much more difficult to distinguish between the different types of extrusive igneous rocksthan between different types of intrusive igneous rocks.
  • Generally, the mineral constituents of fine-grained extrusive igneous rocks can only be determined by examination of thin sections of the rock under a microscope, so only an approximate classification can usually be made in the field.
  • These are formed due to cooling and resultant solidification of rising magma just beneath the earth surface.
  • Igneous rocks are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body.
  • In a simplified classification, igneous rock types are separated on the basis of the type of feldspar present, the presence or absence of quartz, and in rocks with no feldspar or quartz, the type of iron or magnesium minerals present.
  • Rocks with feldspathoids are silica-undersaturated, because feldspathoids cannot coexist in a stable association with quartz.
  • The Earth’s crust averages about 35 kilometers thick under the continents, but averages only some 7-10 kilometers beneath the oceans.
  • The continental crust is composed primarily of sedimentary rocks resting on crystalline basement formed of a great variety of metamorphic and igneous rocks including granulite and granite.
  • Both continental and oceanic crust rest on peridotite of the mantle.
  • Rocks may melt in response to a decrease in pressure, to a change in composition such as an addition of water, to an increase in temperature, or to a combination of these processes.
  • Peridotite at depth in the Earth’s mantle may be hotter than its solidus temperature at some shallower level.