3.2 – Igneous Rocks

The Formation of Magma magma: liquid rock that forms deep underground The three factors that affect whether a rock will melt include: ~ temperature ~ pressure ~ presence of fluids in the rock

lava: liquid rock at the surface magma igneous rock: rock that forms when magma (underground) or lava (on the surface) hardens into a solid

igneous rocks are classified on their texture and composition texture of igneous rock is determined by the size of the crystals in the rock size of the crystals is determined by the cooling rate of the magma/lava cooling rate of the magma/lava is determined by where the material is found

Textures of Igneous Rocks intrusive igneous rock: igneous rock formed inside the Earth from magma magma will cool very, very slowly which allows large mineral crystals to form coarse-grained texture: igneous rocks composed of large, well-developed mineral crystals

extrusive igneous rock: igneous rock formed on Earth’s surface (exterior) from lava lava will cool rapidly which will only allow small mineral crystals to form fine-grained texture: igneous rocks that are composed of small crystals

sometimes lava will cool immediately so no crystals will form because there isn’t any time for that process still considered an extrusive igneous rock because it forms on the surface (exterior) glassy texture: igneous rock with no crystals

Chapter 6

large bodies of magma take thousands and thousands of years to cool some minerals crystalize out sooner than others due to their composition this causes some rocks to have a mixed texture porphyritic texture: igneous rock that has large crystals embedded within a mass of smaller crystals

Composition of Igneous Rock mineral composition of an igneous rock is determined by the chemical composition of the magma granitic: lighter colored rocks which are made mostly of quartz and feldspar these make up most of the continental crust of Earth

includes rocks such as granite, rhyolite, obsidian, and pumice

basaltic: darker colored rocks which are made mostly of iron-rich minerals like hornblende, olivine and feldspar make up most of the oceanic crust includes the rocks basalt and gabbro.