The three main type of rocks are 1) Igneous, 2) Metamorphic, and 3) Sedimentary.    Igneous rocks are formed when molten rock cools and crystallizes into igneous minerals that make up the igneous rocks.  The formation of molten rock near the Earth’s surface or within the crust can best be explained with Plate Tectonics.

As magma rising into the crust, it metamorphoses the surrounding country rock and creates metamorphic rock.  Metamorphic rocks are composed of metamorphic minerals.  Metamorphic minerals are created from adding heat and/or pressure to pre-existing minerals and changing those minerals or creating new minerals without complete melting.

Sedimentary Rocks are created when sediment is lithified or cemented.  Sediment is created when pre-existing rocks (igneous, metamorphic, or sedimentary) are weathered and broken into rock fragments, eroded, transported, and deposited.  Wind, water, and ice are common transport agents of sediment.  If the sediment is deposited in a subsiding (sinking) basin, lithification (compaction and cementation) produces sedimentary rock.  Cementation occurs as deep groundwater with dissolved material (ions) circulates through the sediment and precipitates sedimentary minerals such as calcite or silica.  The majority of sedimentary basins on Earth can be explained through plate tectonic processes.

If the sedimentary rock continues to subside, increasing geothermal (inner heat of the earth) heat and pressure changes the sedimentary minerals or recrystallizes the minerals to metamorphic minerals.  The sedimentary rock becomes a metamorphic rock.  Common examples of sedimentary and metamorphic rock equivalents include limestone/marble, sandstone/quartzite, mudstone/slate.

The concept of the Rock Cycle provides a model to describe the transformation from one type of rock to another type of rock.  The rock cycle can also be placed into a Plate Tectonic framework.


Diagram of the Rock Cycle