Rock texture describes the size, shape, and arrangement of the mineral grains or fragments that make up the rock. Grain size is a defining characteristic, classifying rocks as fine, medium, or coarse. A coarse-grained rock is one where the individual crystals or fragments are large enough to be easily seen without magnification, typically meaning the grains are larger than two millimeters in diameter. This visible texture provides geologists with immediate clues about the rock’s history.
Rocks Formed from Cooling Magma
The most abundant coarse rocks are created through the slow cooling of magma deep beneath the Earth’s surface, a process known as intrusive or plutonic formation. This slow cooling allows atoms to migrate and attach to existing crystal seeds, resulting in the growth of large, interlocking mineral crystals. Intrusive igneous rocks form the bulk of the continental crust, making them some of the most common coarse-grained rocks encountered globally.
Granite is the most widespread coarse-grained rock, forming huge batholiths that underlie mountain ranges and large parts of the continents. Its light coloration is due to its felsic composition, meaning it is rich in silica, aluminum, and alkali metals. The rock is primarily composed of quartz and feldspar, often including potassium feldspar which gives it a distinctive pink or reddish hue. It also contains smaller amounts of darker minerals like biotite mica or amphibole.
Gabbro is another common example, forming from magma with a mafic composition rich in iron and magnesium. Unlike Granite, Gabbro is a dark, dense rock, and is the intrusive equivalent of the volcanic rock Basalt. Its composition is dominated by calcium-rich plagioclase feldspar and pyroxene, sometimes containing olivine. Gabbro is a significant component of the lower oceanic crust.
Diorite represents an intermediate composition between Granite and Gabbro, containing a mix of light and dark minerals. This gives Diorite a characteristic “salt-and-pepper” appearance, as it consists mainly of plagioclase feldspar and dark amphibole or pyroxene minerals.
Rocks Formed from Cemented Fragments
Coarse texture in sedimentary rocks refers to the size of the rock fragments, or clasts, that have been cemented together, rather than crystal size. These clastic sedimentary rocks form from pieces of pre-existing rocks that have been weathered, eroded, and deposited. For a sedimentary rock to be classified as coarse-grained, the clasts must be larger than two millimeters in diameter, placing them in the gravel size category.
The two primary examples of coarse-grained sedimentary rocks are Conglomerate and Breccia, distinguished by the shape of their fragments. Conglomerate is characterized by rounded or sub-rounded clasts, indicating significant transport distance from their source. This prolonged transport, usually by water in fast-flowing rivers or coastlines, smooths the sharp edges of the clasts before they are deposited and lithified.
In contrast, Breccia is made up of angular clasts. The angular shape signifies that the rock fragments underwent minimal transport before being cemented into rock. Breccias often form in environments close to their source, such as rock falls at the base of steep cliffs, alluvial fans, or in fault zones. In both rock types, the large clasts are bound together by a finer-grained matrix of sand, silt, or a mineral cement like silica or calcite.
Rocks Formed from Intense Heat and Pressure
Metamorphic rocks develop a coarse texture through recrystallization, which occurs under high temperatures and pressures deep within the Earth’s crust. High-grade regional metamorphism causes the original mineral grains in the parent rock to grow larger as they chemically transform into new, stable mineral phases. This increase in grain size results in a coarse-grained rock where the individual crystals are easily visible.
Gneiss is the most prominent example of a coarse-grained metamorphic rock, often forming from the metamorphism of Granite or other igneous and sedimentary rocks. Its defining characteristic is gneissic banding, a type of foliation where minerals separate into alternating, discontinuous layers. The light bands are typically rich in quartz and feldspar, while the dark bands are composed of iron and magnesium-rich minerals like biotite and hornblende.
The intense pressure and heat experienced during the formation of Gneiss can lead to the growth of exceptionally large, eye-shaped crystals known as augen, often composed of potassium feldspar. These features are much larger than the fine, microscopic grains found in low-grade metamorphic rocks like Slate. The coarse texture and pronounced banding of Gneiss provide clear evidence of the extreme conditions present during its deep-crustal formation.