1. Rocks Igneous Rocks

2. Classifying Igneous Rocks  The name igneous comes from the latin word ignis, meaning fire.  Igneous rocks are classified according to their origin, texture, and mineral composition.

3. Origin  Igneous rocks may form on or beneath Earth's surface.  Rocks that formed from lava that cooled on the surface are called extrusive rock.  Igneous rock that formed when magma hardened beneath Earth’s surface is called intrusive rock.

4. Texture  The texture of an igneous rock depends on the size and shape of its mineral crystals.  Igneous rocks may have similar composition, but there textures can be different.  The rate in which the igneous rock cools determines if the rock will be fine grained or large grained.

5. Texture 2  Extrusive rock will have fine grained texture because it cools rapidly on the surface.  Intrusive rock will have large grained texture because it cools slowly underneath the earth's surface.

6. Mineral Composition  Most of the Earth’s minerals contain silica.  Lava that is low in silica usually form dark-colored rocks such as Basalt.

7. Mineral Composition 2  Magma that is high in silica usually forms light-colored rocks, such as granite.

8. Uses of Igneous Rocks  People throughout history have used igneous rock for tools and building materials.

9. Building Materials  Granite has been used by humans for over 3,500 years. Egyptians used granite to make buildings and statues. Today we use granite on our floors and kitchen counter tops.  Basalt is crushed to make gravel.

10. Other Uses  Pumice has a rough surface which makes it great at cleaning.  Obsidian has a smooth surface that makes it great for polishing.  Native Americans used obsidian to sharpen their tools and weapons.  Perlite formed from heating of obsidian, is often mixed with soil for starting vegetable seeds.