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 a great number of years to move (plastic.) • Decreased pressure and the addition of water lower the melting temperature of mantle rock so that it melts. 13.2 Crystals in igneous rocks • Crystallization is the process by which crystals form and grow in size. Minerals in magma or lava form crystals as the melted rock cools. 13.2 Comparing igneous rocks • • Crystal size can tell us a lot about how a rock formed. Larger crystals take longer to form, so rock with larger crystals cooled slowly. 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 has large crystals because it cools slowly beneath the surface of the earth. • Obsidian is so smooth that it is called volcanic glass. It contains almost no crystals because it cools very quickly on the surface of the earth. 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?