Rocks

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.