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Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith ) is subjected to temperatures greater than 150 to 200 °C (300 to 400 °F) and, often, elevated pressure of 100 megapascals (1,000 bar ) or more, causing profound physical or chemical changes.
Greywacke or graywacke (German grauwacke, signifying a grey, earthy rock) is a variety of sandstone generally characterized by its hardness (6–7 on Mohs scale), dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or sand-size lithic fragments set in a compact, clay-fine matrix.
In between, the metamorphosed siltstone is visible as both the dark layer (~5 cm thick) and the pale layer below it. Contact metamorphism occurs typically around intrusive igneous rocks as a result of the temperature increase caused by the intrusion of magma into cooler country rock.
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains, cemented together by another mineral. Sandstones comprise about 20–25% of all sedimentary rocks. [1] Most sandstone is composed of quartz or feldspar, because they are the most resistant minerals to the weathering processes at the Earth's ...
Greywacke – Sandstone with angular grains in a clay-fine matrix; Gritstone – Hard, coarse-grained, siliceous sandstone; Itacolumite – Porous sandstone known for flexibility; Jaspillite – Banded mixture of hematite and quartz; Laterite – Product of rock weathering in wet tropical climates rich in iron and aluminium
Gneiss (/ n aɪ s / nice) is a common and widely distributed type of metamorphic rock.It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks.
Quartzite can have a grainy, glassy, sandpaper-like surface. Quartzite is a hard, non-foliated metamorphic rock which was originally pure quartz sandstone. [1] [2] Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts.
A sample of banded hornfels, formed by contact metamorphism of sandstones and shales by a granite intrusion. Hornfels is the group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. [1]