Igneous Rocks

Formation of Igneous Rocks Igneous comes from the

Latin word ignis, which means fire

Intrusive igneous rocks are formed when magma hardens beneath Earth’s surface

Extrusive igneous rocks are formed when lava hardens on Earth’s surface

Classification of Igneous Rocks

Texture and composition are two characteristics used to classify igneous rocks

Texture describes the appearance of an igneous rock based on its size, shape, and arrangement of its interlocking crystals

Composition of igneous rocks are based on the proportions of light and dark minerals in the rock

Texture

Coarse-grained texture is caused by slow cooling resulting in larger crystals

Fine-grained texture is caused by rapid cooling resulting in smaller, interconnected mineral grains

Gabbro

Andesite

Texture continued

Glassy texture is caused by very rapid cooling

Porphyritic texture is caused by different rates of cooling resulting in varied sized minerals

Obsidian (compact glass)

Classification of Igneous Rocks

Composition of igneous rocks are based on the proportions of light and dark minerals in the rock

There are four types of composition Granitic Andesitic Basaltic Ultramafic

Igneous Rock Summary

Igneous rocks form when magma or lava cools and hardens

Intrusive rocks form from magma deep within Earth

Extrusive rocks form from lava at Earth’s surface

Igneous rocks can be classified according to texture and composition

Table 1 on page 74