Metamorphism is the geological process of transforming one rock type into another, fundamentally altering its physical or chemical form without fully melting it. The initial rock, known as the protolith, can be igneous, sedimentary, or even a pre-existing metamorphic rock. This transformation happens deep within the Earth’s crust as the rock adjusts to new conditions of heat and pressure. The resulting metamorphic rock exhibits changes in mineral composition, texture, or both.
The Drivers of Metamorphism
The transformation of a protolith is driven by three primary agents: heat, pressure, and chemically active fluids. An increase in temperature causes minerals to recrystallize into new, more stable structures. This heat comes from deep burial or the proximity of a hot magma body intruding into the crust.
Pressure is classified into two types. Confining pressure is equal in all directions, caused by the weight of overlying rock, and produces a denser, more compact rock. Directed stress, or differential stress, is unequal pressure exerted in a specific direction, often resulting from tectonic forces like continental collision.
Chemically active fluids, primarily water containing dissolved ions, enhance the metamorphic process. These hot fluids circulate through the rock’s pores, acting as catalysts to speed up chemical reactions and transport dissolved minerals. The combination and intensity of these three factors determine the type of metamorphism and the final characteristics of the new rock.
Regional Metamorphism
Regional metamorphism is the most widespread form, affecting rock masses across vast geographical areas. This process is linked with major tectonic events, specifically the convergence of continental plates and associated mountain-building episodes (orogeny). The rock is simultaneously subjected to high temperatures from deep burial and intense directed pressure from the colliding plates.
The unequal forces of directed stress cause minerals to align themselves perpendicular to the applied stress. This preferential alignment creates a distinct layered or banded texture called foliation, a hallmark of regionally metamorphosed rocks. The intensity of the heat and pressure determines the metamorphic grade.
Common examples include slate, a low-grade change from shale, and schist, a medium-grade rock where platy minerals like mica are visible. At the highest grades, minerals separate into light and dark bands, forming the characteristic striped appearance of gneiss. These transformations occur at depths where temperatures range from approximately 200°C to over 750°C.
Contact Metamorphism
Contact metamorphism is a localized process primarily driven by intense heat from an igneous intrusion, such as a magma chamber, into cooler surrounding rock. Temperature is the dominant factor, while directed pressure is typically absent. The resulting metamorphic zone, called an aureole, is generally narrow and immediately surrounds the magma body.
The heat “bakes” the protolith, causing minerals to recrystallize into forms stable at high temperatures. Since the pressure is confining rather than directed, the resulting metamorphic rocks lack the layered structure of foliation. Instead, the texture is typically massive, with interlocking grains.
The specific rock formed depends on the composition of the original rock. Limestone recrystallizes into marble, and quartz-rich sandstone transforms into dense, hard quartzite. Mudstone may turn into a fine-grained, tough rock called hornfels.
Dynamic Metamorphism
Dynamic metamorphism, sometimes called cataclastic metamorphism, occurs in narrow zones along major fault lines or shear zones. This type is defined by high differential stress and mechanical deformation, with relatively low temperatures. The primary agent of change is the powerful shearing force as blocks of rock slide past one another.
The intense frictional forces mechanically crush and grind the rock, leading to a significant reduction in grain size. At shallower depths, this forms fault breccia, a collection of angular, shattered rock fragments. At greater depths, the high strain causes minerals to recrystallize dynamically.
The characteristic rock product is mylonite, a dense, fine-grained rock with a pronounced foliation created by the stretching and alignment of mineral grains. Mylonites develop in ductile shear zones and exhibit evidence of plastic flow and extreme shear strain.