1. Igneous Rocks

2. 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

3. 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

4. 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

5. 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)

6. 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

7. 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

8. Table 1 on page 74