Metamorphism is the geological process of changing the form of existing rocks without melting them entirely. This solid-state transformation converts an original rock, known as the protolith, into a new metamorphic rock. These changes affect the rock’s texture, mineralogy, or chemical composition. Metamorphism occurs when a rock is subjected to physical and chemical conditions—specifically temperature, pressure, and fluids—that are significantly different from those under which the rock first formed.
The Agents of Metamorphism: Heat, Pressure, and Fluids
Heat is a fundamental agent that provides the energy necessary to drive chemical reactions and cause minerals to recrystallize. This heat is supplied either by the geothermal gradient, where temperatures increase with depth, or by heat released from a nearby magma intrusion. Metamorphic changes primarily occur in a temperature range between approximately 200°C and 850°C.
Pressure is categorized into two main types: confining pressure and differential stress. Confining pressure, caused by the weight of overlying rocks, is equal in all directions and reduces the rock’s volume without changing its shape. Differential stress is an unequal pressure that acts more strongly in certain directions, causing the rock to shear, flatten, or deform.
Chemically active fluids are the third significant agent, typically composed of water, carbon dioxide, and other volatile compounds. These fluids reside in the rock’s pore spaces and fractures, accelerating the metamorphic process by dissolving, transporting, and precipitating ions. When these fluids introduce new elements or remove existing ones, the process is called metasomatism, which changes the rock’s overall chemical composition.
The Four Primary Types of Metamorphism
The specific combination and dominance of these three agents define the four primary types of metamorphism, each occurring in distinct geological settings.
Regional Metamorphism
Regional metamorphism affects vast areas of the Earth’s crust, often covering hundreds to thousands of square kilometers. This type is primarily associated with major tectonic events, such as the collision of continental plates and the formation of mountain ranges. The rocks are deeply buried and subjected to both high temperature and high differential stress from the intense compression.
The strong directional pressures cause the rock material to be squeezed, folded, and stacked. This process results in the formation of strongly foliated rocks, meaning their mineral grains are aligned in parallel layers or bands. Regional metamorphism creates widespread metamorphic rock types such as slate, schist, and gneiss.
Contact Metamorphism
Contact metamorphism occurs on a localized scale, typically surrounding an igneous intrusion like a magma chamber or a dike. The defining characteristic is the dominance of high heat with low to moderate pressure. The country rock immediately adjacent to the hot magma is “baked,” causing the minerals to recrystallize.
This localized area of alteration is called a metamorphic aureole, which can range from a few centimeters to several kilometers wide. Because the pressure is usually confining rather than differential, the resulting rocks often lack the layered texture of foliation. Common results include fine-grained rocks like hornfels, and the recrystallization of limestone into marble.
Dynamic (or Cataclastic) Metamorphism
Dynamic metamorphism (or cataclastic metamorphism) is characterized by intense, localized shear stress and crushing. This type is restricted to narrow zones, primarily along active fault lines where tectonic plates grind past one another. The transformation is driven mainly by mechanical deformation rather than significant temperature change, although frictional heat may be generated.
The immense physical force acts to crush, pulverize, and grind the rock, leading to a reduction in the size of the mineral grains. Rocks formed at shallower depths are coarse-grained fault breccias. Those formed deeper under higher pressure are fine-grained, stretched rocks known as mylonites.
Hydrothermal Metamorphism
Hydrothermal metamorphism is driven by the pervasive movement of hot, chemically charged fluids through the rock. This process typically occurs at relatively low temperatures and pressures. It is particularly active along mid-ocean ridges, where cold seawater infiltrates the oceanic crust, is heated by underlying magma, and circulates through fractures.
The hot water becomes highly reactive, dissolving minerals and depositing new ones, which is a form of metasomatism. This chemical alteration changes the rock’s mineral composition and forms metal-laden deposits, such as the sulfide deposits near deep-sea hydrothermal vents. The fluids can also originate from cooling magma or from rocks undergoing other types of metamorphism.
Textural Outcomes: Foliated Versus Non-Foliated Rocks
Metamorphic rocks are broadly classified into two groups based on their final texture: foliated and non-foliated. This texture is a direct record of the pressure conditions under which the rock formed. The key difference lies in whether the rock experienced differential stress or only confining pressure during its transformation.
Foliated rocks exhibit a layered or banded appearance caused by the parallel alignment of platy or elongate mineral grains, such as mica and chlorite. This foliation is a direct result of differential stress, which forces the minerals to rotate and recrystallize perpendicular to the direction of maximum stress. Examples include slate, which has a fine foliation, and gneiss, which displays distinct bands of light and dark minerals.
Non-foliated rocks lack a layered structure, featuring a texture where the mineral grains are randomly oriented and interlocking. They typically form in environments where confining pressure or heat is the dominant factor, such as in contact metamorphism. Non-foliated rocks can also result from a protolith composed of minerals that are not naturally platy, like quartz or calcite. Common examples include marble, which forms from limestone, and quartzite, which forms from sandstone.