The word “metamorphic” comes from the ancient Greek words meta (“after”) and morph (“form”), translating directly to “change in form.” In geology, metamorphism is the transformation of a pre-existing rock, known as the protolith, into a new rock with a different mineral composition or texture. This process is a fundamental part of the rock cycle, allowing igneous, sedimentary, or even older metamorphic rocks to be altered deep within the Earth’s crust. Crucially, the alteration occurs in the solid state, meaning the rock never fully melts, which distinguishes it from the formation of igneous rocks.
The Driving Forces of Metamorphism
The transformation of a protolith into a metamorphic rock is driven by three main agents: heat, pressure, and chemically active fluids. Heat accelerates the chemical reactions that cause minerals to recrystallize into new, more stable forms. This thermal energy originates primarily from the geothermal gradient, the natural increase in temperature with depth, which averages about 25°C per kilometer in the crust. Additional heat can be supplied by the intrusion of hot magma bodies into cooler surrounding rock via conduction.
Pressure is categorized into two types based on its directionality. Confining pressure, or lithostatic pressure, is uniform, exerted equally in all directions by the weight of the overlying rock column. This uniform stress reduces the rock’s volume and density, often leading to the formation of new, compact mineral structures. Differential stress, conversely, is a directed pressure that is greater in one direction than others, typically resulting from tectonic plate movement.
Directed pressure deforms the rock and aligns mineral grains perpendicular to the maximum stress, creating a layered texture. Chemically active fluids, primarily hot water rich in dissolved ions, enhance the movement of material. These fluids facilitate the transfer of atoms between mineral grains, speeding up chemical reactions and enabling the growth of new minerals. Metamorphism generally begins when temperatures exceed approximately 150°C to 200°C.
Environments of Rock Transformation
Metamorphism occurs across a range of geological settings, with the specific environment determining the scale and characteristics of the resulting rock. The most widespread type is regional metamorphism, which affects enormous volumes of rock across areas measuring thousands of square kilometers. This process is intimately linked to mountain building events, such as those that occur at convergent plate boundaries where tectonic plates collide. The intense compression and deep burial associated with these collisions subject the rock to high temperatures and directed differential stress over long periods.
Contact metamorphism is a localized type driven predominantly by heat and low pressure. This transformation happens when a body of magma intrudes into the cooler surrounding host rock, causing a thermal alteration. The affected area, known as the metamorphic aureole, is typically small, ranging from a few meters to a few kilometers wide, with the degree of change decreasing rapidly away from the heat source. Rocks formed here, like hornfels, are often fine-grained and do not exhibit the layered textures seen in regionally metamorphosed rocks because the pressure was not directed.
Dynamic metamorphism is focused within fault zones. Here, rocks are subjected to intense, concentrated shear stress as rock masses slide past one another. Mechanical deformation is the dominant process, grinding and crushing the rock into fine-grained material like mylonite, typically under relatively low temperatures.
How Metamorphic Rocks Are Classified
Metamorphic rocks are broadly classified based on the texture that develops in response to the pressure conditions during their formation. The first group is foliated rocks, which possess a layered or banded appearance known as foliation. This texture results from the alignment of platy or elongated mineral grains, such as mica, perpendicular to the directed differential stress. The intensity of foliation tracks the metamorphic grade, which increases with temperature and pressure.
Low-grade examples include slate, a very fine-grained rock derived from shale that exhibits perfect flat cleavage. With increasing grade, slate transforms into phyllite, which has a glossy sheen due to the slight increase in mica crystal size. Further heating and pressure produce schist, where the mineral grains are large enough to be visible to the naked eye. The highest grade of foliation is found in gneiss, characterized by distinct light and dark bands of segregated minerals, such as quartz and feldspar alternating with layers of dark silicates.
The second group is non-foliated rocks, which lack a layered appearance because they formed under uniform confining pressure or are composed of minerals that are not platy. Even if pressure was applied, blocky minerals like quartz and calcite do not align into sheets. Common examples include marble, which is the metamorphosed equivalent of limestone, and quartzite, which forms from the recrystallization of quartz-rich sandstone. Hornfels is another non-foliated example, typically formed during contact metamorphism where heat was the primary transforming agent without significant directed pressure.