Types of Rock

There are 3 rock types: igneous, sedimentary, and metamorphic.

Rock Types - Igneous Sedimentary Metamorphic
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Igneous

Igneous rock is the primary rock type, or the base material for all others.

Formation – magma from the Earth’s mantle erupts as lava and cools.

Classification – based on texture, size and arrangement of texture grains, and other physical conditions of the matter. If the lava cools slowly in deeper parts of the Earth’s crust, mineral grains may be very large. Quick cooling on the crust surface creates small, smooth grains.

Examples Basaltbrecciagabbrogranite, and tuff are some examples.

 Sedimentary

All Earth rocks are subject to the forces of erosion, which breaks them down into pieces and particles of varying size. These particles become the building blocks for sedimentary rock.

Formation – Eroded rock is compacted, turning it back into rock. This is called lithification. Some particles may retain their original characteristics, even after lithification. This is why differing layers can be viewed in their strata.

Classification – There are three classes of sedimentary rock formations: Mechanically formed, Organically formed, and Chemically formed.

Examples

Metamorphic

Metamorphic means change of form. Metamorphic rocks are formed in the Earth with immense pressure, volume, and temperature changes.

Formation – Rocks are forced to lower layers of the Earth’s crust due to plate tectonic movement, or the rising of magma from the mantle through the crust, or lower level rocks receive significant pressure from higher level rocks. This process causes the impacted rock to recrystallize and reorganize within the original rock. This happens through one of five different types of metamorphism:

Dynamic metamorphism Mechanical disruption from breaking without chemical changes.

Thermal metamorphism – Chemical alteration and recrystallization of rocks. There are two types of this metamorphism:

Contact – hot intruding magma touches rock, causing recrystallization from the high temperature.

Regional – recrystallization from deformation happens by tectonic shearing combined with high temperature and/or pressure.

Foliation (lineation) – at the point of recrystallization, minerals are arranged in layers or lines

Banding – rearrangement of minerals and materials of different groups into alternating layers of varying thickness and shades.

Classification – Metamorphic rocks are classified by composition, source, and context.

ExamplesGranite, marble, quartzite, slate, and schist.