Metamorphic rocks are stones transformed from pre-existing igneous, sedimentary, or other metamorphic rock types by intense heat and pressure deep within the Earth’s crust. The transformation occurs in the solid state, reorganizing the rock’s mineral structure and texture without melting it. Geologists classify these rocks primarily to understand the specific pressure and temperature conditions of their formation, which reveals a great deal about the region’s history. Classification relies on two main characteristics: the physical structure of the mineral grains (texture) and the specific chemical makeup (mineral content).
Textural Characteristics: Foliation and Grain Arrangement
Texture, in the context of metamorphic rocks, refers to the size, shape, and arrangement of the mineral grains within the stone. This physical characteristic is often the first and most visible feature geologists use to categorize a specimen. Metamorphic rocks are broadly divided into two textural groups: foliated and non-foliated.
Foliation is a fabric-like feature where mineral grains are aligned parallel to one another, giving the rock a layered or banded appearance. This parallel arrangement is caused by directional stress during metamorphism, such as the immense pressure from colliding tectonic plates. The degree of foliation increases with the intensity of metamorphism, leading to distinct textural types.
The least intense form is slaty cleavage, found in low-grade rocks like slate, where microscopic mica and chlorite crystals are aligned, allowing the rock to split into thin, flat sheets. As temperature and pressure increase, the mineral grains grow larger, leading to schistosity, which is characterized by visible, platy minerals like mica that give the rock a wavy, shimmering look. The highest grade of foliation is gneissic banding, where light-colored granular minerals (quartz and feldspar) separate into alternating stripes with dark-colored minerals (biotite and amphibole).
Non-foliated rocks form when the pressure is uniform from all directions, or when the parent rock is composed of minerals that are not easily flattened, such as quartz or calcite. These rocks typically have a massive, interlocking grain structure, often described as granoblastic. Examples include quartzite, which forms from quartz-rich sandstone, and marble, which forms from limestone.
Mineral Assemblage and Index Minerals
Geologists classify metamorphic rocks based on their specific mineral content, known as the mineral assemblage. The types of minerals present are a direct consequence of the temperature and pressure conditions the rock experienced. The bulk chemical composition of the original rock also controls which minerals can form, even if the pressure and temperature conditions are identical.
Certain minerals, called index minerals, are useful because they are stable only within specific, narrow ranges of temperature and pressure. The first appearance of an index mineral marks an “isograd,” which helps geologists map the intensity of metamorphism across a region. Index minerals act as geological thermometers and barometers for the rock.
For example, starting with a mudrock, the sequence of index minerals that appear with increasing temperature includes chlorite, then biotite, followed by garnet, staurolite, and finally kyanite or sillimanite. The presence of kyanite suggests high pressure and relatively low temperature, while the appearance of sillimanite indicates very high temperatures. The entire collection of minerals that are stable under a similar pressure and temperature range is called a metamorphic facies.
Metamorphic Grade and Parent Rock
Classification involves integrating textural and mineral data to assign a metamorphic grade and identify the parent rock. Metamorphic grade describes the overall intensity of the transformation in terms of temperature and pressure. Low-grade metamorphism occurs at relatively low temperatures, generally below 400°C, and is characterized by very fine-grained, foliated rocks like slate.
High-grade metamorphism involves significantly higher temperatures and pressures, causing extensive recrystallization and the formation of coarse-grained rocks like gneiss. The grade is an important indicator of the depth and tectonic setting in which the rock formed, with high-grade rocks originating from deep burial in mountain belts.
The parent rock, or protolith, is the original rock type before metamorphism began. The protolith strongly influences the final rock name by determining the chemical elements available to form new minerals. For instance, the metamorphism of shale creates foliated rocks like slate and schist, while the metamorphism of limestone yields non-foliated marble. The final name often combines these elements, such as a “garnet-biotite schist,” which describes the rock’s texture, prominent index minerals, and links it to the original parent rock.