Metamorphism is the process by which a rock is transformed into a new type of rock without melting. This change occurs when the original rock, known as the protolith, is subjected to conditions significantly different from those under which it initially formed. When sedimentary rocks, such as shale or limestone, are buried deep within the Earth’s crust, they become protoliths for metamorphic transformation. This process involves extensive physical and chemical restructuring, turning the layered, porous sedimentary material into a denser, harder metamorphic rock.
The Forces That Drive Metamorphism
The transformation of a sedimentary rock is driven by three primary agents: heat, pressure, and chemically active fluids. Heat is a major catalyst for change, causing the atoms within existing minerals to vibrate and become unstable. This heat can come from the geothermal gradient, or from the nearby intrusion of hot magma bodies.
Pressure acts in two distinct ways to restructure the rock. Confining pressure is the equal force exerted by the weight of overlying rocks, which compacts the protolith and eliminates pore space. Directed stress, or differential pressure, is an unequal force resulting from tectonic activity, which flattens and shears the rock in specific directions. These forces push the rock toward a more compact and stable state.
Chemically active fluids, typically rich in water or carbon dioxide, also play a significant part. These fluids move through the rock’s tiny fractures and grain boundaries, facilitating the transport of ions between mineral grains. The fluid presence dramatically increases the speed at which chemical reactions occur, enabling the large-scale reorganization of the rock’s components.
Textural Transformations in the Rock
One of the most visible effects of metamorphism is the physical transformation of the rock’s texture. Existing mineral grains reorganize themselves through recrystallization, growing larger and interlocking more tightly than in the original sedimentary rock. This process reduces the empty space, or porosity, within the rock, resulting in a much denser final product.
When directed stress is present, the rock develops a characteristic fabric known as foliation, which is the parallel alignment of mineral grains. Platy minerals, such as clay and mica, rotate and grow perpendicular to the greatest applied stress. This alignment creates distinct planar structures, such as slaty cleavage or the more pronounced banding of schistosity.
Rocks composed of minerals that are not platy, such as quartz or calcite, tend to develop a non-foliated texture. The primary change is the growth of equigranular crystals that evenly interlock, creating a massive texture. This recrystallization effectively welds the grains together into a highly cohesive rock.
Mineralogical Changes and New Compositions
Beyond physical rearrangement, metamorphism causes profound chemical changes as original minerals become unstable under new temperature and pressure conditions. The minerals within the sedimentary protolith react with one another and with circulating fluids to form entirely new mineral species, a process called neocrystallization. For example, clay minerals common in shale protoliths break down and recombine to form micas and chlorite at lower metamorphic grades.
As temperature and pressure continue to increase, the mineral assemblage changes further. New, denser minerals begin to grow, often consuming the earlier metamorphic products. This chemical recombination can also lead to the loss of volatile components, such as water and carbon dioxide, which were locked within the crystal structures.
Geologists use index minerals to determine the specific pressure and temperature conditions a rock has experienced. Minerals like garnet, staurolite, and kyanite are highly diagnostic because they only form within specific, narrow pressure and temperature ranges. Their appearance serves as a clear marker for the metamorphic grade achieved by the rock.
Examples of Transformed Sedimentary Rocks
Several common sedimentary rocks yield predictable metamorphic products. When shale is subjected to increasing heat and directed stress, its clay minerals transform into mica and align to create slate, a strongly foliated rock. With further metamorphism, slate progresses into schist, characterized by coarse, visible flakes of mica and a more pronounced foliation.
Limestone, primarily composed of the mineral calcite, transforms into marble when metamorphosed. Marble is non-foliated because calcite crystals are not platy. The transformation involves massive recrystallization, where small calcite grains grow into a tight mosaic of larger, interlocking crystals.
Quartz sandstone transforms into quartzite. This change is driven by recrystallization, where the original quartz grains fuse together along their boundaries. The resulting rock is so cohesive that when struck, it breaks across the original grain boundaries rather than around them.