Geological Rock Types


In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids. The Earth's outer solid layer, the lithosphere, is made of rock. Three majors groups of rocks are defined: igneous, sedimentary, and metamorphic. The scientific study of rocks is called petrology, which is an essential component of geology.

Balanced Rock

Image 1: Balanced Rock stands in Garden of the Gods park in Colorado Springs, CO

Rocks are generally classified by mineral and chemical composition, the texture of the constituent particles and the processes by which they formed.  These variables separate rocks into three types: igneous, sedimentary, and metamorphic.  They are further classified according to particle size.  The transformation of one rock type to another is described by the geological model called the rock cycle.

Igneous rocks are formed when molten magma cools and are divided into two main categories: plutonic and volcanic rock.  Plutonic or intrusive rocks result when magma cools and crystallizes slowly within the Earth's crust (i.e. granite), while volcanic or extrusive rocks result from magma reaching the surface either as lava or fragmental ejecta (i.e. pumice and basalt).

Sedimentary rocks are formed by deposition of either clastic sediments, organic matter, or chemical precipitates (evaporites), followed by compaction of the particulate matter and cementation during diagenesis.  Sedimentary rocks form at or near the Earth's surface. Mud rocks comprise 65% (i.e. mudstone, shale and siltstone); sandstones 20 to 25% and carbonate rocks 10 to 15% (i.e. limestone and dolostone).

Metamorphic rocks are formed by subjecting any rock type (including previously formed metamorphic rock) to different temperature and pressure conditions than those in which the original rock was formed.  These temperatures and pressures are always higher than those at the Earth's surface and must be sufficiently high so as to change the original minerals into other mineral types or else into other forms of the same minerals (i.e. by recrystallization).

The three classes of rocks—igneous, sedimentary and metamorphic—are subdivided into many groups. There are, however, no hard and fast boundaries between allied rocks.  By increases or decreases in the proportions of their constituent minerals they pass by every gradation into one another.  The distinctive structures also of one kind of rock may often be traced gradually merging into those of another.  Hence the definitions adopted in establishing rock nomenclature merely correspond to selected points (more or less arbitrary) in a continuously graduated series.

Image 1: Rock outcrop along a mountain creek near Orosi, Costa Rica

Image 2: Rock outcrop along a mountain creek near Orosi, Costa Rica

Igneous Rocks

Igneous rock (derived from the Latin word igneus meaning of fire, from ignis meaning fire) forms through the cooling and solidification of magma or lava.  Igneous rock may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.  This magma can be derived from partial melts of pre-existing rocks in either the planet's mantle or crust.  Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition.  Over 700 types of igneous rocks have been described, most of them having formed beneath the surface of Earth's crust. These have diverse properties depending on their composition and how they were formed.

Image 2: Igneous Rock - Gabbro Rock Creek

Image 3: Igneous Rock - Gabbro Rock Creek

Sedimentary Rocks

Sedimentary rocks are types of rock that are formed by the deposition of material on Earth's surface and within bodies of water.  Sedimentation is the collective name for processes that cause mineral and/or organic particles (detritus) to settle and accumulate or for minerals to precipitate from a solution.  Particles that form a sedimentary rock by accumulating are called sediment.  Before being deposited, sediment was formed by weathering and erosion in a source area, and then transported to the place of deposition by water, wind, ice, mass movement or glaciers which are called agents of denudation.

Image 3: Sedimentary Rock - Sandstone

Image 4: Sedimentary Rock - Sandstone

Metamorphic Rocks

Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form".  The original rock (protolith) is subjected to heat and pressure, (temperatures greater than 150 to 200 °C and pressures of 1500 bars) causing profound physical and/or chemical change.  The protolith may be sedimentary rock, igneous rock or another older metamorphic rock.

Image 4: Metamorphic Rock - Banded Gneiss

Image 5: Metamorphic Rock - Banded Gneiss