Metamorphism is the geological process where a pre-existing rock (protolith) transforms its mineral content or texture without undergoing complete melting. This transformation occurs as the rock adjusts to new physical or chemical conditions deep within the Earth’s crust. For metamorphism to occur, three primary environmental factors must increase: temperature, pressure, and the presence of chemically reactive fluids. These agents provide the necessary energy and medium for the rock’s internal structure to reorganize into a new, more stable configuration.
The Role of Elevated Temperature
An increase in temperature provides the energy to drive the chemical reactions that define metamorphism. This thermal energy allows atoms within the rock’s crystal lattices to vibrate more vigorously, facilitating their rearrangement to form new mineral structures. These mineralogical changes, called recrystallization, occur while the rock remains solid. Metamorphism typically begins at temperatures as low as \(200^\circ\text{C}\) and continues up to \(700^\circ\text{C}\) to \(1,100^\circ\text{C}\), beyond which the rock would begin to melt.
One source of increased temperature is the normal geothermal gradient, where temperature naturally rises with increasing depth into the Earth, often by \(15^\circ\text{C}\) to \(30^\circ\text{C}\) per kilometer. Burial alone can subject rocks to the necessary heat for metamorphic change, particularly in deep sedimentary basins. Another significant source is the intrusion of hot magma into cooler surrounding rock, which causes localized heating. This thermal-only effect, known as contact metamorphism, bakes the adjacent rock without introducing directional stress.
The heat promotes the growth of new mineral grains that are stable. In contact metamorphic environments, the rock produced is often fine-grained and lacks any linear fabric. For example, a shale protolith subjected to high heat might convert to a hard, dense rock called hornfels.
The Impact of Increased Pressure
Increased pressure acts in two distinct ways to promote metamorphic change, often in conjunction with rising temperature. The first is confining pressure, which is an equal force exerted on a rock from all directions, similar to the pressure felt underwater. This pressure is caused by the weight of the overlying rock column (lithostatic pressure), and it causes the rock’s volume to decrease, making it denser. Confining pressure is essential for forming new, more compact mineral phases that occupy less space.
The second form is differential stress, where the pressure is greater in one direction than in others. This unequal pressure is characteristic of tectonic plate collisions, where rocks are compressed and sheared. Differential stress physically changes the rock’s texture by forcing platy or elongate mineral grains, such as micas, to rotate and align perpendicular to the direction of the greatest stress.
This mineral alignment creates a distinct layered or planar fabric in the rock called foliation, which is a recognizable feature of many metamorphic rocks. For instance, differential stress transforms a fine-grained mudstone into slate. With further increases in pressure and temperature, it can develop into schist or gneiss.
Chemically Active Fluids
The presence of chemically active fluids increases the rate and extent of metamorphic reactions, even at lower temperatures and pressures. These fluids, primarily water containing dissolved ions and volatile compounds like carbon dioxide, become highly reactive under deep Earth conditions. They circulate through tiny cracks and pore spaces within the rock, acting as a transport medium for chemical components.
Fluids function as catalysts by facilitating the transfer of ions between mineral grains, allowing for the dissolution of unstable minerals and the precipitation of new, stable ones. This process enables mineral re-equilibration to occur much faster than it would through slow, solid-state diffusion alone. Without these fluids, many metamorphic reactions would take geologic eons to complete.
In certain cases, the fluid introduces or removes chemical components, fundamentally altering the rock’s bulk composition in a process known as metasomatism. For example, hot fluids released from a cooling magma body can carry dissolved elements that react with the surrounding limestone, creating a chemically distinct rock called skarn. This chemical exchange demonstrates that fluids can drive entirely new reactions by changing the overall elemental balance of the rock.