Metamorphic rocks are one of the three fundamental rock types, alongside igneous and sedimentary rocks. The term “metamorphism” is derived from Greek words meaning “change of form,” which describes the process these rocks undergo. They begin as a pre-existing rock—either igneous, sedimentary, or even another metamorphic rock—and are fundamentally altered without melting. This transformation is a solid-state change where the rock remains solid while its internal structure, texture, and mineral composition are reorganized. This process records the immense forces and heat deep within the Earth’s crust, providing geologists with a history of tectonic activity.
The Forces That Create Metamorphic Rocks
The transformation of a parent rock, or protolith, is driven by high heat and intense pressure. Heat is a primary engine of change, accelerating chemical reactions that cause minerals to recrystallize into new, more stable forms. Temperatures typically range from 150°C to 800°C, which is hot enough to drive profound change but cool enough to prevent the rock from melting entirely.
Pressure is applied in two distinct ways. Confining pressure, or lithostatic pressure, is the equal stress exerted on all sides of a rock due to the weight of overlying material. This pressure compacts the rock, making it denser, and works with heat to initiate chemical changes. Directed pressure, or differential stress, occurs when pressure is unevenly applied, usually during tectonic events like continental collisions. This directed force deforms the rock and aligns its mineral grains.
These forces act in two major environments. Regional metamorphism affects vast areas and is associated with the large-scale movements of tectonic plates, particularly at convergent boundaries where rocks are subjected to both high temperatures and high directed pressures. In contrast, contact metamorphism is a local phenomenon that occurs when hot magma intrudes into cooler surrounding rock, subjecting the protolith to very high temperatures but relatively low pressure.
Textural Classification
Geologists categorize metamorphic rocks based on their texture, which is a direct consequence of the pressure and mineral composition of the protolith. The primary distinction is between foliated and non-foliated rocks. Foliation describes a layered or banded appearance that results from the parallel alignment of platy or elongate mineral grains, such as micas, under directed pressure.
The degree of foliation indicates the intensity of metamorphism, often referred to as the metamorphic grade. For example, shale first transforms into slate, which exhibits a fine-grained, planar layering called slaty cleavage. With increasing temperature and pressure, slate can transform into schist, where the mineral grains become large enough to be visible and create a wavy, lustrous texture. The highest grade of foliation is found in gneiss, characterized by distinct, alternating bands of light and dark minerals that separate during the most intense conditions.
Non-foliated rocks lack this layered appearance because they formed without significant directed pressure or because their mineral composition prevents alignment. They typically have an interlocking, granular texture, described as granoblastic. Minerals like quartz and calcite are not flat or elongated, so they simply recrystallize into larger, more compact crystals without arranging themselves into planes.
Common Metamorphic Rock Types
Marble is a non-foliated rock that forms when the sedimentary rock limestone undergoes metamorphism. The original calcite crystals recrystallize and grow into a mosaic of larger, tightly interlocked calcite grains, making marble highly valued for sculpture and architecture.
Another non-foliated example is quartzite, derived from quartz-rich sandstone. During its formation, the original quartz sand grains merge and fuse together, creating an exceptionally hard and durable rock used frequently in construction. Representing the foliated category, slate is the lowest-grade metamorphic rock formed from shale, and its ability to split cleanly into thin sheets has made it a historical favorite for roofing tiles and flooring.
At the high end of the foliated scale is gneiss, a coarse-grained rock recognizable by its striking bands. Gneiss can form from a variety of protoliths, including granite or shale, and its presence indicates that the area experienced the extreme pressures and temperatures of deep-seated regional metamorphism.