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Igneous Rocks 5/24/2017 Igneous Rocks • Rock heated to liquid (molten) state is called magma. • When magma cools and hardens it becomes Igneous rock. –Different minerals solidify (or melt) at different temperatures • Usually 600 - 1000 degrees Celsius 5/24/2017 Igneous Rocks • Slowly cooling magma forms rock with large mineral crystals –Ex. Granite On planet Earth where does magma cool slowly? 5/24/2017 Igneous Rocks • Rapidly cooling magma forms rock with small mineral crystals –Often too small to see • Ex. basalt On planet Earth, where does magma cool rapidly? 5/24/2017 Igneous Rocks • When magma cools very fast crystals have no time to form –Ex. Obsidian Where might obsidian form? 5/24/2017 Igneous Rock Formation: Crystal Size vs. Cooling Rate of Magma Draw a line! Tiny Large Crystal Size 5/24/2017 Intrusive Igneous Rocks • Are formed deep underground • Usually have large, well formed crystals –Also called plutonic igneous rocks –Example: granite 5/24/2017 Extrusive Igneous Rocks • Are formed at or near the Earth’s surface • Usually have small mineral crystals –Also called volcanic igneous rocks –Example: obsidian 5/24/2017 Igneous Rock Formation: Crystal Size vs. Depth beneath Earth’s surface Draw a line! Tiny Large Crystal Size 5/24/2017 Igneous Rock Characteristics • Do not usually contain layers • Varied in color – Color depends on minerals present • Color and crystal size are used to identify igneous rocks 5/24/2017 Describe a situation in which both small and large crystals would be found in the same igneous rock. 5/24/2017 Igneous Rocks - Color • Light colored igneous rocks are usually rich in feldspar (fel) and silica (s) – Called felsic • Dark colored igneous rocks are usually rich in magnesium (Ma) and iron (Fe) – called Mafic igneous rocks • Exceptions – Obsidian is felsic but appears dark in 5/24/2017 color 5/24/2017 5/24/2017 Mafic Igneous Rocks • Known as basaltic • Dense • Dark colored • Make up Earth’s ocean floors 5/24/2017 Felsic Igneous Rocks • Known as granitic • Less dense than mafic igneous rocks • Light colored • Form Earth’s continents 5/24/2017 Cooling Magma • volcanic or extrusive • crystal size is small - fine texture • Plutonic or intrusive • crystal size is large - coarse texture 2 examples: 2 examples: 5/24/2017 3 steps for identifying igneous rocks 1. Determine texture (you have to look at the rock!) 2. Determine color 3. Determine what minerals are present, if you can see them 5/24/2017 Igneous Rock Identification--step 1 Determine the texture (crystal size): Can I see the crystals by eye? coarse textured 5/24/2017 fine textured Igneous Rock Identification--step 2 Determine the relative color: light colored (felsic) Example: 5/24/2017 In-between Example: dark colored (mafic) Example: Igneous Rock Identification--step 3 Determine mineral composition - what minerals are in the rock? Use physical characteristics and special properties of minerals such as: 5/24/2017 5/24/2017