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