Igneous Rock Igneous Rock n Igneous rocks form

Igneous Rock

Igneous Rock n Igneous rocks form when magma (beneath the surface) or lava (reaches the surface) cools and hardens n Igneous comes from the Latin word ignis, which means fire n There are two types of igneous rock, intrusive and extrusive

Intrusive Igneous Rocks n Intrusive igneous rocks form when magma hardens beneath Earth’s surface n As magma rises, it cools, allowing elements to combine and form minerals n Minerals grow in size, forming a solid mass of interlocking crystals Granite is a common intrusive igneous rock

Extrusive Igneous Rocks n Extrusive igneous rocks form when lava hardens at the Earth’s surface n Lava is similar to magma, except that most of the gases have escaped in lava Rhyolite Basalt Obsidian

Classification of Igneous Rocks n Texture and composition are two characteristics used to classify igneous rocks n Texture describes the appearance of an igneous rock based on its size, shape, and arrangement of its interlocking crystals n Composition of igneous rocks are based on the proportions of light and dark minerals in the rock

Classification of Igneous Rocks n Texture describes the appearance of an igneous rock based on its size, shape, and arrangement of its interlocking crystals n There are four types of texture: Coarse-grained n Fine-grained n Glassy n Porphyritic n

Coarse-grained Texture n Slow cooling of magma allows ions, charged atoms, to move large distances within the magma n Slow cooling results in the formation of large crystals n Igneous rocks with large crystals show coarse-grained texture Gabbro Diorite

Fine-grained Texture n Rapid cooling of magma or lava causes ions to lose their motion and quickly combine n Rapid cooling results in the formation of many small, interconnected mineral grains n Igneous rocks with small grains show finegrained texture Rhyolite Andesite

Glassy Texture n When lava spews onto Earth’s surface, there may not be time for the ions to arrange themselves into crystals n The ions are randomly distributed causing a glassy texture in the igneous rock Obsidian (compact glass) Pumice (frothy glass)

Porphyritic Texture n Minerals that crystallize from magma do not form at the same rate or the same time n Some crystals will become large while others even start to form n Igneous rocks with large crystals and fine-grained minerals show a porphyritic texture Andesite

Classification of Igneous Rocks n Composition of igneous rocks are based on the proportions of light and dark minerals in the rock n There are four types of composition Granitic n Basaltic n Other Compositional Groups n

Granitic Composition n Made almost entirely of the light- colored silicate minerals, quartz and feldspar n Most granitic rocks contain 10% dark silicate minerals, often biotite and amphibole

Basaltic Composition n Made almost entirely of the dark-colored silicate minerals and plagioclase feldspar n Basaltic rocks are rich in the elements magnesium and iron

Other Compositional Groups n Rocks with a composition n Rocks composed entirely of between granitic and basaltic rocks have an andesitic composition n Named after the common volcanic rock andesite n Contains at least 25% dark silicate minerals, iron and magnesium, is referred to as ultramafic n Ultramafic rocks are rare at Earth’s surface because they make up the mantle. Peridotite

Igneous Rock Summary n Igneous rocks form when magma or lava cools and hardens n Intrusive rocks form from magma deep within Earth n Extrusive rocks form from lava at Earth’s surface n Igneous rocks can be classified according to texture and composition

Table 1 on page 74