1. Classifying Rocks
Rocks are classified as either Igneous, Sedimentary, or Metamorphic.

2. Igneous Rocks
Igneous rocks are formed by volcanic activity. They are classified further by their origin, texture, and mineral composition.

3. Origins of Igneous Rocks
Intrusive Rock: These rocks form underground from magma.
Extrusive Rocks:
These rocks are formed above the ground when lava flows or explodes from a volcano.
Granite
Basalt
Obsidian
Gabbro

4. Texture of Igneous Rocks
Texture depends on the size and shape of the crystals when the rock forms. This is due to the cooling rate of the rocks.
Rapid Cooling:
Rapid cooling produces fine grains and small crystals.
This is associated with extrusive rocks.
Slow Cooling:
Slow cooling produces coarse grains with large crystals. This is associated with intrusive rocks.
Diorite
Andesite

5. Obsidian
Obsidian is formed above ground and cools quickly, but it forms no cyrstals and has a glassy texture.

6. Mineral Composite of Igneous Rocks
Low Silicates: Igneous rocks with low amounts of silica usually produce dark colored rocks.
High Silicates: Igneous rocks high in silica usually appear light in color.
Pumice
Peridotite

7. Uses of Igneous Rocks
For thousands of years people have used igneous rocks for building and tools.
Granite is still used today for constructing buildings.
Obsidian has been used for tool making by Native Americans. Some surgeons use obsidian instead of steel in their scalpels today because they cut better and leave a thinner scar.
Pumice is used for cleaning and for removing calluses from feet.

8. Sedimentary rocks are formed through a series of processes: erosion, deposition, compaction, and cementation.

9. Erosion
Erosion is the wearing away of rock due to wind, water or ice.

10. Deposition
Deposition is when the sediments from erosion settle out of the wind, water and ice and are left or deposited in a different place.

11. Compaction
Sediments build up over time and press down on the layers below. This squeezes the layers so that the particles or grains stick together.

12. Cementation
Minerals that are dissolved in water start to crystallize. These crystallized minerals “glue” the solids or grains together.

13. Types of Sedimentary Rocks
Geologist classify sedimentary rocks according to the type of sediments they have. There are three major groups of sedimentary rocks: clastic, organic, and chemical.

14. Clastic Rocks
shale
Clastic rocks are your basic sedimentary rock. They are formed from broken bits and pieces of other rocks that were eroded and deposited a long time ago. These rock pieces were squeezed together. The particles can be the size of sand or large and jagged.
Sandstone
Breccia

15. Organic Sedimentary Rocks
Organic sedimentary rocks are made from the remains of plants and animals. These get deposited, compacted, and cemented just like rock sediments. Coal and limestone are two examples. Sometimes limestone can have fossils in it.
Limestone
Coal

16. Chemical Rocks
When the minerals that are dissolved in a solution crystallize, chemical rocks are formed. One type of limestone can form this way when calcite is dissolved in lakes and becomes crystallized. Halite is another example of a chemical rock.
Limestone
Pink Halite

17. Metamorphic Rock
Metamorphic rock forms when pressure deep within the earth melts and changes existing rock. This is different than intrusive igneous rock, because you are starting with rock, not magma. The rocks melt only a little. There are two different types: foliated and non-foliated.

18. Foliated Metamorphic Rock
In foliated rock the grains are arranged in parallel layers or bands. These may appear as alternating layers of color or just as flat layers that break or cleave.
Gneiss
Muscovite Mica Schist

19. Nonfoliated Metamorphic Rock
In nonfoliated rock the grains are random. They do not split into layers.
Shale
Quartzite

20. All types of rock can change
Metamorphic rocks originally start out as different types of rocks. Because of pressure deep within the earth, they melt and change. Below are a few metamorphic changes.
Granite becomes Gneiss
Sandstone becomes Quartzite
Shale becomes Slate

21. The Rock Cycle
The earth recycles. Igneous, sedimentary and metamorphic rocks are constantly changing to form new rocks. Old rock is eroded and deposited somewhere else to become sedimentary rock. Tectonic plates subduct into the mantle to melt and form igneous rock. Intrusive igneous rock and sedimentary rock are buried deep in the earth and are subjected to high temperature and pressure to become metamorphic rock. This never-ending cycle has been going on for over 4 billion years. The earth is continually breaking down and rebuilding itself.