CH 3 2 Igneous Rocks IGNEOUS ROCK Igneous

CH. 3. 2 Igneous Rocks

IGNEOUS ROCK Igneous rock forms when magma or lava cools and hardens. There are different types of igneous rock depending on where it cools and what it is made of.

INTRUSIVE IGNEOUS ROCKS Rocks that form when magma hardens beneath the Earth’s surface. We would not see these rocks unless the Earth above is eroded away. Magma is typically made of silicon and oxygen, plus aluminum, iron, calcium, sodium, potassium, and magnesium. These form together to make minerals and then masses that are rock. Common type of intrusive igneous rock is granite.

EXTRUSIVE IGNEOUS ROCKS Lava does not have gases that magma has. When lava hardens it forms extrusive igneous rocks (extrusive think exit). Rhyolite is an example of extrusive igneous rock.

CLASSIFICATION OF IGNEOUS ROCKS Two characteristics of igneous rocks are texture and composition. Texture describes the appearance of the rock based on size, shape, and arrangement of interlocking crystals. Composition is based on the amount of light and dark minerals in the rock.

COARSEGRAINED TEXTURE The rate of cooling (fast or slow) affects the texture of igneous rock. If magma cools slowly there is very little crystal growth in the middle. Slow cooling forms large crystals. Large crystals give a coarse-grained texture.

FINE-GRAINED TEXTURE Rapid cooling causes the materials to combine. This gives many tiny crystals. So rapid cooling magma or lava gives small, interconnected mineral grains. These rocks have a fine-grained texture.

GLASSY TEXTURE When lava comes out of the Earth there is not always time for crystals to form. These rocks then have a glassy texture. Obsidian and pumice both have glassy textures.

PORPHYRITIC TEXTURE These are extremely slow forming rock. They are formed from large amounts of magma that take along time to harden. That means that crystals form at different rates. It may have very large crystals called phenocrysts and then fine-grained minerals. This is called porphyritic texture.

GRANITE COMPOSITION These are made of light-colored silicate minerals quartz and feldspar. They also have about 10% dark silicate minerals. These rocks make up most of the continental crust. Rhyolite is an example of granite rock.

BASALTIC COMPOSITION Have many dark silicate minerals. They are also rich in magnesium and iron. These are darker and denser than granite rocks. An example is basalt.

OTHER COMPOSITIONAL GROUPS If a rock has a composition between granite and basaltic then it is andesitic composition. These are at least 25% dark silicate minerals. An example is andesite. Another type of igneous rock is periodite. This has different minerals and almost all dark silicate minerals. These rocks are also called ultramafic. This rock makes a lot of the upper mantle. Check out pg. 74 to compare types of igneous rock.