Rocks

How Geologists Classify Rocks When studying a rock sample, geologists observe the rock’s color and texture and determine its mineral composition. Geologists can then identify a rock’s origin, or where and how it formed.

Texture Most rocks are made up of particles of minerals or other rocks, called grains. A rock’s texture is the size, shape, and pattern of the rock’s grains. Some rocks are smooth and glassy, others are rough or chalky.

Grains Texture due to the grains can vary depending on the grain: Size (fine/coarse) Shape (rounded/jagged) Pattern (Banded/Nonbanded) Visible/Invisible

Three Major Groups of Rocks

Igneous Rock Any rock that forms from magma or lava. Extrusive rock is igneous rock formed from lava that erupted onto Earth’s surface. Example: Basalt Intrusive rock is igneous rock that formed when magma hardened beneath the Earth’s surface. Example: Granite

Igneous Rock Igneous rocks are classified according to their origin, texture, and mineral composition. Origin: Intrusive/Extrusive Texture: fine/coarse grained, porphyritic, etc… Mineral Composition: silica content

Sedimentary Rocks Sedimentary rocks form from particles (sediments) deposited by water and wind. Sediment is small, solid pieces of material that come from rocks or living things.

Sedimentary Rock Cycle Sediment undergoes the following process that will change it into sedimentary rock. 1) Erosion 2) Deposition 3) Compaction 4) Cementation

Erosion and Deposition Erosion occurs when running water or wind loosens and carries away fragments of rock. Deposition is when sediment settles out of the wind or water carrying it. After the sediments have been deposited, they begin to compact.

Compaction and Cementation Compaction is the process of pressing the sediments together. While compaction is taking place, minerals dissolve and seep in between the spaces. Cementation occurs when the minerals crystallize and “glue” the sediment together.

Types of Sedimentary Rocks There are three major groups of sedimentary rocks: Clastic, Organic, and Chemical rocks. Clastic: Rock fragments squeezed together. Organic: Plant and animal remains. Chemical: Minerals dissolved in a solution crystallize.

Rock Examples Clastic Rocks: Shale: particles of clay Sandstone: sand on beaches. Conglomerate: rock fragments of different sizes. Organic Rocks: Coal: swamp plant remains Limestone: made from shells. Chemical Rocks: Rock Salt: mineral evaporation.

Metamorphic Rocks Metamorphic comes from the Greek word meta that mean “change” and morphosis that means “form” HEAT AND PRESSURE deep beneath the Earth’s surface can change ANY rock into metamorphic rock. Forces can push rock back toward the heat of the mantle and change rock to metamorphic rock.

Classifying Metamorphic Rocks Geologists classify metamorphic rocks by the arrangement of the grains that make up the rocks. Grains arranged in parallel layers or bands are said to be foliated. Some metamorphic rocks are nonfoliated and the grains are arranged randomly.