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

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

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

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

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

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

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.

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

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

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.

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.

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

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.

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

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

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

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.

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

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

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

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.