1. Igneous Rocks
Chapter 3, section 2

2. Characteristics of Igneous Rock
Most igneous rocks are made of mineral crystals
Igneous rocks are classified according to their origin, texture, and mineral composition

3. Origin
Geologists classify igneous rocks according to where they formed:
Extrusive rock formed from lava that erupted onto Earth’s surface
Intrusive rock is igneous rock that formed when magma cooled and hardened beneath the Earth’s surface

4. Intrusive Rock Intrusive rock forms below the surface of Earth
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5. Extrusive Rock Extrusive rock forms on top of the surface of Earth
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6. Erosion exposes an Intrusive Rock formation
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7. Texture
The texture of an igneous rock depends on the size and shape of its mineral crystals
Texture can be fine-grained, course-grained, glassy, or porphyritic
*Rapid cooling lava forms fine-grained rocks with small crystals
*Slow cooling magma forms coarse-grained rock with large crystals

8. Texture
Extrusive rock cools quickly on Earth’s surface and the crystals grow for a short time and are small
Intrusive rock cools slowly under Earth’s surface and the crystals grow for a long time and are large

9. Texture Glassy rock may form if extrusive rock cools very quickly
Porphyritic texture is a mixed texture that has large crystals scattered on a background of much smaller crystals
Porphyritic texture forms if an intrusive rock cools in two stages. Some magma cools slowly then the surrounding magma cools quickly

10. Texture Course-grained granite Fine-grained basalt Volcanic glass
Porphyritic texture

11. Mineral Composition
Silica is a material made of oxygen and silicon that is found in most minerals. Silica content affects the color of rocks
Lava that is low in silica tends to form dark-colored basalt while magma that is high in silica forms light-colored granite

12. Basalt image from http://www.es.ucl.ac.uk/schools/Glossary/basalt.jpg
Obsidian image from
Granite image from
Porphyritic texture image from