Definition and Formation
Metamorphic rocks form when existing rocks, either igneous or sedimentary, undergo transformation due to intense heat, pressure, or chemically active fluids deep within the Earth's crust. This process, called metamorphism, alters the mineral structure and texture without melting the rock, resulting in new materials like marble from limestone or slate from shale.
Foliation and Layered Texture
A primary characteristic is foliation, the alignment of minerals into parallel layers or bands caused by directed pressure. This gives rocks like schist and gneiss a banded appearance. Non-foliated rocks, such as quartzite or soapstone, lack this layering and form under uniform pressure, resulting in a more massive, granular texture.
Mineral Composition and Hardness Changes
Metamorphic rocks often feature recrystallized minerals like mica, garnet, or talc, which can increase hardness or durability. For example, limestone turns into hard marble through calcite recrystallization, while shale becomes durable slate. These changes enhance resistance to weathering compared to parent rocks.
Applications and Geological Importance
These rocks are crucial for understanding tectonic processes, as they indicate past burial depths and mountain-building events. Practically, they are used in construction—marble for sculptures, slate for roofing—and in identifying ore deposits, highlighting their economic and scientific value in geology.