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Igneous Rocks Igneous Rocks Lava is melt that has erupted from a volcano at the Earth’s surface. When the melt is underground, it is called magma. Igneous rocks are classified according to texture and composition. Igneous Rocks Magma forms when rock in the Earth partially melts. This can occur during the following conditions: a. Pressure decreases b. Volatiles are added c. Heat is transferred into the crust by rising magma Igneous Rocks Magma can have various compositions. These include felsic (silicic), intermediate, mafic, and ultramafic. Composition of the magma is a reflection of the composition of the original rock from which it forms and the way the magma forms. Igneous rocks Magma rises because of its buoyancy and pressure caused by overlying rock. The viscosity of magma depends on composition. Felsic magma is more viscous than mafic magma. Igneous Rock Intrusive magma cools at a rate that depends on depth, size, and shape of magma body, and whether groundwater is present. How quickly the magma cools has an effect on the texture of the igneous rock. Igneous Rocks Extrusive igneous rocks form from lava. Intrusive igneous rocks form from magma. Lava may solidify to form flows, or it may explode to form ash. Igneous Rocks Intrusive igneous rocks form when magma intrudes into preexisting rock below the Earth’s surface. Plutons are blob-shaped intrusions. Tabular intrusions cut across layering. These layers can be called dikes. Huge intrusions are batholiths, and they are often made of many plutons.