Igneous Rocks 2/22/2019

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 2/22/2019

Igneous Rocks  On planet Earth where does magma cool slowly? Slowly cooling magma forms rock with large mineral crystals Ex. Granite  On planet Earth where does magma cool slowly? 2/22/2019

Igneous Rocks  On planet Earth, where does magma cool rapidly? Rapidly cooling magma forms rock with small mineral crystals Often too small to see Ex. basalt  On planet Earth, where does magma cool rapidly? 2/22/2019

Igneous Rocks  Where might obsidian form? When magma cools very fast crystals have no time to form Ex. Obsidian  Where might obsidian form? 2/22/2019

 Igneous Rock Formation: Crystal Size vs. Cooling Rate of Magma Draw a line! Rate of Cooling Fast Slow Tiny Large Crystal Size 2/22/2019

Intrusive Igneous Rocks Are formed deep underground Usually have large, well formed crystals Also called plutonic igneous rocks Example: granite 2/22/2019

Extrusive Igneous Rocks Are formed at or near the Earth’s surface Usually have small mineral crystals Also called volcanic igneous rocks Example: obsidian 2/22/2019

 Igneous Rock Formation: Crystal Size vs. Depth beneath Earth’s surface Draw a line! Depth below Earth’s surface Deep Shallow Tiny Large Crystal Size 2/22/2019

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 2/22/2019

 Describe a situation in which both small and large crystals would be found in the same igneous rock. 2/22/2019

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 color 2/22/2019

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Mafic Igneous Rocks Known as basaltic Dense Dark colored Make up Earth’s ocean floors 2/22/2019

Felsic Igneous Rocks Known as granitic Less dense than mafic igneous rocks Light colored Form Earth’s continents 2/22/2019

 2 examples:  2 examples: Cooling Magma • volcanic or extrusive Rapid cooling Slow cooling • volcanic or extrusive • crystal size is small - fine texture  2 examples: • Plutonic or intrusive • crystal size is large - coarse texture  2 examples: 2/22/2019

3 steps for identifying igneous rocks Determine texture (you have to look at the rock!) Determine color Determine what minerals are present, if you can see them 2/22/2019

Igneous Rock Identification--step 1 Determine the texture (crystal size): Can I see the crystals by eye? yes no fine textured coarse textured 2/22/2019

Igneous Rock Identification--step 2 Determine the relative color: light colored (felsic)  Example: In-between  Example: dark colored (mafic)  Example: 2/22/2019

Igneous Rock Identification--step 3 Determine mineral composition - what minerals are in the rock?  Use physical characteristics and special properties of minerals such as: 2/22/2019

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