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What are Igneous Rocks?
Igneous rocks are formed when magma -- rock heated to a molten state under the Earth's surface -- crystallizes and solidifies. The majority of the heat that melts rock originates from the Earth's core. This heat moves from the core to the Earth's solid outer crust through convection and conduction processes. Magma is forced from Earth's lower mantle to the upper mantle by convection, producing both intrusive and extrusive igneous rocks on the surface of the Earth. The friction caused by the movement of tectonic plates can also generate enough heat and pressure to melt rock, often resulting in volcanoes on plate boundaries.
Intrusive
When magma gets trapped far below the surface of the Earth, intrusive igneous rock is formed; this is also known as plutonic igneous rock. These molten rock masses rise; although some of this magma escapes through volcanoes, the majority of it remains trapped, eventually solidifying after a cooling process that could potentially take millions of years. The coarse grains within intrusive igneous rocks form because each mineral grain can reach a large size over the length of time that it takes it to grow. An example of an intrusive igneous rock is diorite; this mineral contains large, coarse grains of pyroxene, feldspar, hornblende and quartz.
Extrusive
When magma cools close to or above the Earth's surface, extrusive igneous rock is formed. This is also sometimes referred to as volcanic igneous rock, as it often forms as a result of volcanic eruptions. When magma -- called lava once it is above the surface -- is exposed to the temperature of the atmosphere, it cools very quickly. These rocks contain much smaller grains than intrusive igneous rocks, as they have little time to grow during the short time that it takes them to cool; some even develop a glassy appearance. The fast cooling process can also trap gas bubbles, forming rocks such as pumice.
Igneous Rock Composition
Three factors influence the type of igneous rock that is formed by cooling magma. These are the solidification temperature, the cooling rate and the magma's chemical composition. The chemical composition of an igneous rock is classified as felsic, intermediate, mafic or ultramafic, depending on the minerals that they contain. Felsic magma has large amounts of aluminium, sodium and potassium and is over 65 percent silica. Igneous rocks from mafic magma have high levels of iron, magnesium and calcium and contain between 45-54 percent silica. Rocks with a composition between felsic and mafic are classified as intermediate igneous rocks. Ultramafic rocks have less than 45 percent silica but large amounts of the minerals calcium, pyroxene and olivine. Each of these classifications of igneous rock can either be intrusive or extrusive, producing a different form of the rock. For example, granite is felsic intrusive rock with the same mineral composition as the felsic extrusive rock known as rhyolite.