2. The Changing Earth

3. Chapter Thirteen: Formation of Rocks • 13.1 The Composition of Rocks • 13.2 Igneous Rocks • 13.3 How Rocks Change

4. Investigation 13B Igneous rocks • How are igneous rocks classified?

5. 13.2 Igneous Rocks • Igneous rocks begin to form when rock melts in Earth’s mantle. • A good way to describe hot, solid mantle rock is that it is like stiff putty that takes millions of years to move. • Decreased pressure and the addition of water lower the melting temperature of mantle rock so that it melts.

7. 13.2 Crystals in igneous rocks • Minerals in magma or lava form crystals as the melted rock cools. • Crystallizationis the process by which crystals form and grow in size.

8. 13.2 Comparing igneous rocks • Crystal size can tell us a lot about how a rock formed. • Basalt and gabbro are made from the same low silica magma. • Basalt has crystals too small to see, and gabbro has large crystals.

9. 13.2 Comparing igneous rocks • Granitemakes up continental plates. • It is less dense than basalt, is made of high-silica magma, and has large crystals. • Obsidian is so smooth that it is called volcanic glass. • Obsidian contains almost no crystals because it cools very quickly.

10. 13.2 Comparing igneous rocks • An igneous rock that forms above Earth’s surface is called an extrusive rock. • An igneous rock that forms within Earth’s crust is called an intrusive rock. Which rocks are more likely to have large crystals?