The Changing Earth

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

Investigation 13B Igneous rocks How are igneous rocks classified?

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.

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

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.

13.2 Comparing igneous rocks Granite makes 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.

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?