There are 3 large classifications of rocks - igneous, metamorphic, and sedimentary. Each type of rock is formed differently and can change from one type to another over time. The way rocks are formed determines how we classify them.
- Forms when molten rock (magma or lava) cools and hardens.
- If cooling takes place slowly beneath Earth's surface, the igneous rock is called intrusive.
- If the cooling takes place rapidly on Earth's surface, the igneous rock is called extrusive.
- Forms when rocks are changed into different kinds of rocks by great heat and/or pressure - they are heated, squeezed, folded, or chemically changed by contact with hot fluids and/or tectonic forces.
- When heat and pressure reach the rock's melting point, it melts into magma.
- Forms from the compaction and/or cementation of rock pieces, mineral grains, or shell fragments called sediments.
- Sediments are formed through the processes of weathering and erosion of rocks exposed at Earth's surface.
- Sedimentary rocks can also form from the chemical depositing of materials that were once dissolved in water.