Igneous rocks originate from deep within the Earth’s crust. Igneous rocks (from the Greek word for fire) are formed when magma crystallizes and solidifies. An increase in temperature, a change in the composition, or decrease in the pressure can cause melting of these rocks in the mantle which form igneous rocks. The melt begins deep below the surface of the Earth close to active plate boundaries. As the temperate increases the rocks rise toward the surface. Igneous rocks are divided into two categories, intrusive or extrusive, depending on where the magma solidifies (USGS, 2004). Intrusive or plutonic igneous rocks form when magma cools and solidifies beneath the surface of the Earth. These types of rocks cool very slowly and have mineral grains that can usually be seen with the naked eye. Intrusive rocks have a coarse grained texture. One example of an intrusive igneous rock would be granite. Extrusive, or volcanic, igneous rocks are formed when magma exits and cools outside of the Earth’s surface. These types of rocks are formed when lava flows from volcanoes. They solidify above the surface and have much shorter cooling times. Because lava cools and crystallizes quickly, it is a fine grain. The grains in extrusive rocks are quite small, so to classify them they have to be placed under a microscope to examine the thin sections to determine the mineral constituents. Given that igneous rocks form from a liquid state, their mineral grains are packed together very tightly (Geo.ua, n.d.).
One distinguishing characteristic between the two categories of rocks is that intrusive rocks are formed below the surface and extrusive rocks are formed above the surface. Another distinguishing characteristic is that intrusive rocks have a coarse grained texture and extrusive rocks have a fine grain texture. Intrusive rocks cool and solidify very slowly whereas extrusive rocks have a much shorter cooling time which means they solidify faster.
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