Earth Science Chapter 3 Section 2 Igneous Rock Earth Science Chapter 3 Section 2

Introduction Igneous rock is sometimes called volcanic rock Igneous rock forms when magma cools and hardens (crystallizes)

Formation of Igneous Rock Most igneous rock forms from magma deep below the earths surface. Igneous rocks are classified as either intrusive or extrusive depending on where they form. Intrusive igneous rock forms below the surface Extrusive igneous rock form on the surface

Composition of Magma Magma is composed of … silicon oxygen aluminum iron calcium sodium potassium magnesium Magma also contains gases which makes it less dense than the surrounding rock. This is why magma rises.

Intrusive Igneous Rock (Plutonic) Intrusive igneous rock tends to have large crystals because the rock formed slowly below the earth’s surface When an igneous rock forms slowly the mineral crystals have more time to form Intrusive igneous rock is only visible at the surface when weathering and erosion wear away the rock above. The most common example on the earth’s surface is granite.

Extrusive Igneous Rock (Volcanic) Magma that reaches the earth’s surface is called lava. Most of the gases escape when magma reaches the surface. Extrusive igneous rock are commonly found around volcanoes. Extrusive igneous rocks tend to have small crystals because the rock formed quickly The temperature at the surface is much cooler than inside the earth When an igneous rock forms quickly the mineral crystals do not have time to form Basalt, pumice, and obsidian are examples of extrusive igneous rock.

Classification of Igneous Rock Texture Intrusive Cools slowly below the earth’s surface Have larger crystals Coarse grained granite Extrusive Cools quickly at the earth’s surface Have smaller crystals Fine grained May have a glassy appearance when no crystals form when the rock formed very quickly Obsidian, pumice, basalt

Porphyritic Contain a combination of large and small crystals Form because some minerals crystallize faster than others Andesite

Magma Composition Magma and lava that are rich in potassium, sodium, and calcium form light colored silicates such as quartz, mica, and feldspar Magma and lava that contain iron and/or magnesium form dark colored silicates such as olivine and pyroxene.

Granitic Composition magma Light color Mostly quartz and feldspar Much of the continental crust Granitic rocks are sometimes called felsic which is a combination of the words feldspar and silica

Basaltic composition magma Contain mostly magnesium and iron rich dark silicates Darker in color Basalt is the most common and makes up most of the ocean floor Sometimes called mafic which is a combination of the words magnesium and ferrum which is a term for iron.

Andesitic composition magma Igneous rocks with an intermediate composition which is a combination of light and dark minerals Named for the common volcanic rock andesite