Sedimentary rocks form from sediments—fragments of pre-existing rocks, organic matter, or chemical precipitates—that accumulate and undergo compaction and cementation. Metamorphic rocks, in contrast, form when existing rocks, including sedimentary rocks, undergo profound physical or chemical changes without completely melting. This transformation occurs due to elevated temperatures, increased pressures, or interactions with hot, chemically active fluids. This article explores how sedimentary rocks transform into metamorphic rocks, detailing the driving forces, internal alterations, and geological settings.
Forces That Drive Metamorphism
The transformation of sedimentary rocks into metamorphic rocks is driven by three geological forces: heat, pressure, and chemically active fluids. Heat triggers chemical reactions and recrystallization within the rock. This heat originates from deep burial within Earth’s crust, where geothermal gradients increase temperatures with depth, or from the intrusion of hot molten rock (magma) into cooler surrounding rocks. Friction from tectonic plate movements can also contribute to localized temperature increases.
Pressure, a driving force, includes confining and directed pressure. Confining pressure results from the weight of overlying rocks, applying uniform stress from all directions, compacting the rock and reducing its pore space. Directed pressure, or differential stress, arises from tectonic forces at plate boundaries, leading to non-uniform stress that can flatten mineral grains and induce specific textures. Both types of pressure contribute to the physical rearrangement of minerals.
Chemically active fluids, often water with dissolved ions, also contribute to metamorphism. These hot fluids circulate through the rock, dissolving existing minerals and facilitating the growth of new ones. They act as catalysts, speeding up chemical reactions and enabling new mineral assemblages to form that are stable under altered conditions, often without adding or subtracting bulk material from the rock.
How Rocks Physically and Chemically Transform
The internal changes within a sedimentary rock during metamorphism involve both physical and chemical alterations. Recrystallization is a fundamental physical change where existing mineral grains grow larger or change shape without altering their chemical composition. For instance, tiny quartz grains in sandstone can merge to form larger, interlocking quartz crystals, resulting in a denser, more cohesive rock.
New mineral formation is a significant chemical transformation. Under elevated temperatures and pressures, atoms within original minerals rearrange or react with surrounding fluids to create entirely new mineral assemblages. For example, clay minerals in shale can transform into micas (like muscovite or biotite) or chlorite as temperature and pressure increase. These new minerals are stable under metamorphic conditions, signifying a fundamental change in the rock’s composition.
Directed pressure often leads to foliation, a prominent planar texture in many metamorphic rocks. Foliation occurs as platy or elongated mineral grains, such as micas, align perpendicular to the direction of maximum stress. This alignment can manifest as slaty cleavage, where the rock breaks into thin sheets, or as schistosity, where visible mica flakes are aligned, giving the rock a shimmering appearance. At higher metamorphic grades, minerals may segregate into distinct light and dark bands, forming gneissic banding.
Beyond mineralogical changes, the rock’s overall texture and structure are modified. The rock becomes denser and harder as pore spaces are eliminated and mineral grains interlock. This transformation results in a compact, crystalline metamorphic rock with properties distinct from its sedimentary parent.
Examples of Sedimentary Rocks Becoming Metamorphic
Common examples illustrate the metamorphic journey of sedimentary rocks. Shale, a fine-grained sedimentary rock composed mainly of clay minerals, undergoes a series of changes with increasing metamorphism. At lower grades, shale transforms into slate, characterized by slaty cleavage, allowing it to split into thin sheets. With further increases in temperature and pressure, slate evolves into schist, where platy minerals like mica grow larger and become visibly aligned, giving the rock a sparkling, foliated texture. At highest metamorphic grades, schist transforms into gneiss, a coarse-grained rock with distinct light and dark banding from mineral segregation.
Limestone, a sedimentary rock primarily composed of calcite (calcium carbonate), transforms into marble through recrystallization. Here, fine-grained calcite crystals grow larger and interlock, forming a dense, crystalline rock. Original sedimentary features, like fossils or bedding planes, are typically obliterated during this recrystallization. The resulting marble is known for its hardness and appearance.
Sandstone, a clastic sedimentary rock dominated by quartz grains, metamorphoses into quartzite. Here, individual quartz grains recrystallize and fuse. Pore spaces are eliminated, and the rock becomes extremely hard and durable. Unlike foliated metamorphic rocks, quartzite is typically non-foliated because quartz grains are not elongated or platy and do not align under directed pressure.
Geological Settings for Metamorphism
Sedimentary rock metamorphism occurs in various geological settings, defined by specific conditions of heat, pressure, and fluid activity. Regional metamorphism is the most widespread type, affecting vast areas of Earth’s crust. It is associated with large-scale tectonic processes like continental collisions and mountain building, where sedimentary rocks are deeply buried and subjected to significant directed pressure and moderate to high temperatures. This setting commonly produces foliated metamorphic rocks such as slate, schist, and gneiss.
Contact metamorphism is a localized phenomenon occurring when hot magma intrudes into cooler surrounding sedimentary rocks. Magma’s heat “bakes” adjacent rocks, causing recrystallization and new mineral formation. This type is characterized by high temperatures but relatively low pressures, often resulting in non-foliated metamorphic rocks like hornfels. Intensity decreases with distance from the igneous intrusion.
Burial metamorphism takes place in sedimentary basins where thick accumulations of sediments are buried to depths sufficient to experience elevated temperatures and pressures. While temperatures and pressures are generally lower than in regional metamorphism, they are sufficient to cause significant diagenetic changes and initial metamorphism. This process leads to changes in porosity and density, and the formation of low-grade metamorphic minerals, without the strong directed pressures that produce pervasive foliation.