Different types of Rocks – Sedimentary, Metamorphic and Igneous Chapter 2 – Sections 3-5 Different types of Rocks – Sedimentary, Metamorphic and Igneous

Igneous Rocks Igneous rocks are classified by their origin, texture, and mineral composition. Igneous rocks form beneath Earth’s surface. Extrusive rock is igneous rock from lava that erupted onto Earth’s surface. Intrusive rock is igneous rock that formed when magma hardened beneath the surface of Earth. Different igneous rocks may have similar mineral composition and yet have very different textures. The texture of an igneous rock depends on the size and shape of its mineral crystals.

Igneous Rocks Rapidly cooling lava forms fine-grained igneous rocks with small crystals or no crystals at all. Slowly cooling magma forms course-grained rocks, such as granite, with large crystals. Intrusive rocks have larger grains than extrusive rocks. Extrusive rocks have a fine grained or glassy structure. People throughout history have used igneous rock for tools and building materials.

More than 3,500 years ago, the Ancient Egyptians used granite to build statues. About 600 years ago, the Incas in Peru built fortresses out of great blocks of granite and other igneous rocks. Igneous rocks such as pumice and obsidian also have important uses.

Sedimentary Rock Most sedimentary rocks are formed through a sequence of processes: weathering, erosion, deposition, compaction, and cementation. Rock on Earth’s surface is constantly broken up by weathering – the effects of freezing and thawing, plant roots, acid, and other forces on rock. Erosion – the process of by which running water, wind or ice carry away bits of broken-up rock. Deposition – the process by which sediment settles out of the water or wind carrying it. Compaction – the process that presses sediments together. Cementation- the process in which dissolved minerals crystallize and glue particles of sediment together.

Sedimentary Rock The three major groups of sedimentary rock are clastic rocks, organic rocks, and chemical rocks. Different processes form each of these types of rocks. A clastic rock is a sedimentary rock formed when fragments are squeezed together. Organic rock forms where the remains of plants and animals are deposited in layers. The term “organic” refers to substances that once were part of living things or were made by living things. Chemical rock forms when minerals dissolved in a water solution crystallize. They can also form when mineral deposits are left when seas and lakes evaporate.

People have used sedimentary rocks throughout history for many different purposes, including for tools and building materials. Sedimentary rocks such as sandstone and limestone have been used in building materials for thousands of years. Arrowheads were made from sedimentary rock.

Metamorphic Rocks Any rock that forms from another rock as a result of changes in heat or pressure (or both heat and pressure) is a metamorphic rock. Metamorphic rock can form out of igneous, sedimentary, or other metamorphic rock. Many metamorphic rocks are found in mountains or near large masses of igneous rock. The heat that can change a rock into metamorphic rock can come from pockets of magma. Geologists classify metamorphic rocks according to the arrangement of the grains making up the rocks.