GEOL 104 Lecture Notes - Lecture 2: Igneous Rock, Brazos River, Igneous Differentiation
7 Jan 2018
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Document Summary
Igneous rocks can be divided into two broad textural classes: coarse-grained rocks, which are intrusive and therefore cooled slowly; and fine- grained rocks, which are extrusive and cooled rapidly. Igneous rocks can also be classified on the basis of their sil- ica content using a scale that runs from felsic (rich in silica) to ultramafic (poor in silica). Magmas form at places in the lower crust and mantle where temperatures are high enough for partial melting of rock. Because the miner- als within a rock melt at different temperatures, the com- position of magmas varies with temperature. Pressure raises the melting temperature of rock, and the presence of water lowers it. Because melted rock is less dense than solid rock, magma rises through the surrounding rock, and drops of magma come together to form magma chambers. Because different minerals crystallize at different temperatures, the composition of magma changes as it cools and various minerals are with- drawn by crystallization.
