Rocks are classified by their mineral composition and their color. Ex. Basalt is usually a dark-colored rock with a low silica concentration.

Geologists observe the shape and color of crystals in a rock to identify the minerals that the rock contains.

Rocks are also classified by their texture, the look and feel of their surface. Geologists look at grain size, shape, and pattern.

Geologists classify rocks into three major groups: igneous rock, sedimentary rock, and metamorphic rock.

Igneous rock forms from the cooling of magma or lava.

Sedimentary rock forms when particles of other rocks or the remains of plants and animals are pressed and cemented together.

Metamorphic rock forms when an existing rock is changed by heat, pressure, or chemical reaction.

Extrusive rock is an igneous rock that formed from lava. Basalt is the most common extrusive rock found in oceanic crust.

Intrusive rock is an igneous rock that formed from magma. Granite is the most common intrusive rock found in continental crust.

Basalt

Granite

People throughout history have used igneous rocks for tools and building materials.

Sediment is small, solid pieces of material that come from rocks or living things.

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

Erosion is the destructive process in which water, wind, or gravity loosens and carries away fragments of rock.

Deposition is the process by which sediment settles out of the water or wind carrying it.

The process that presses sediments together is compaction. Layers of sediments build up over time and often remain visible, like the Badlands of SD.

Cementation is the process in which dissolved minerals crystallize and glue particles of sediment together.

A clastic rock is a sedimentary rock that forms when rock fragments are squeezed together. EX. Shale, sandstone, and breccia.

Organic rock forms where the remains of plants and animals are deposited in thick layers. EX. Coal and limestone.

Shale

Sandstone

When minerals that are dissolved in a solution crystallize, chemical rocks form. EX. Limestone and rock salt.

Coal

Rock Salt

Geologists classify metamorphic rocks according to the arrangement of the grains that make up the rocks.

Metamorphic rocks that have their grains arranged in parallel layers or bands are said to be foliated.

Examples of foliated rocks: slate, schist, and gneiss.

Mineral grains in nonfoliated rocks are arranged randomly and do not split into layers. EX. Marble & quartzite.

Gneiss

Quartzite

Igneous

Heat & Pressure

Metamorphic

& foliated

Gneiss

Sandstone

sedimentary

Heat & Pressure Heat &

Pressure

Quartzite

Metamorphic & nonfoliated

Shale

sedimentary

Metamorphic & foliated

Certain metamorphic rocks are important materials for building and sculpture (marble & slate).

Marble & slate come in a variety of colors.

Lincoln Memorial in Washington D.C.