Igneous Rocks Igneous rock begins as magma mixture

Igneous Rocks • Igneous rock begins as magma (mixture of many minerals) • Magma can form: • When rock is heated • When pressure is released • When rock changes composition • Magma freezes between 700 °C and 1, 250 °C http: //www. fi. edu/fellows/payton/rocks/create/igneous. htm

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.

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.

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.

Comparing igneous rocks • Extrusive Rocks: forms when magma erupts onto the Earth’s surface (lava), cools quickly with very small or no crystals formed • An igneous rock that forms within Earth’s crust is called an intrusive rock, magma pushes into surrounding rock below the Earth’s surface Which rocks are more likely to have large crystals?

Igneous Rocks • Felsic: light colored rocks that are rich in elements such as aluminum, potassium, silicon, and sodium • Mafic: dark colored rocks that are rich in calcium, iron, and magnesium, poor in silicon • Coarse-grained: takes longer to cool, giving mineral crystals more time to grow • Fine-grained: cools quickly with little to no crystals

Igneous Rocks Coarse-Grained Fine-Grained Felsic Granite Rhyolite Mafic Gabbro Basalt

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.

Igneous Rocks Obsidian is a dark-colored volcanic glass that forms from the very rapid cooling of molten rock material. It cools so rapidly that crystals do not form. Is this rock Felsic or Mafic? Is it fine-grained or coarse-grained? Is this rock Intrusive or Extrusive? Mafic, fine grained, extrusive