Metamorphic rocks are one of the three fundamental rock types, alongside igneous and sedimentary rocks. The name, derived from Greek words, means “change of form,” which describes their origin. These rocks begin as a pre-existing rock—igneous, sedimentary, or even another metamorphic rock—that is altered. This transformation occurs deep within the Earth’s crust under extreme conditions.
The Transformative Process
Metamorphism is the geological process that changes a rock’s texture and mineral composition in the solid state, without the rock ever fully melting. This process is driven by three main agents: heat, pressure, and chemically active fluids. Heat, which can come from deep burial or nearby magma, causes the rock’s minerals to recrystallize into new forms stable under the high-temperature environment.
Pressure, often generated by tectonic plate collisions, compresses the rock, making it denser. Fluids, primarily water laced with dissolved ions, circulate through the rock and act as catalysts, promoting the breakdown of old minerals and the growth of new ones. The resulting metamorphic rock depends on the initial composition of the “parent rock,” or protolith, which determines the available chemical components for the transformation.
Category One: Foliated Rocks
The classification of metamorphic rocks hinges on the presence or absence of a layered texture known as foliation, a term derived from the Latin word folium, meaning “leaf.” Foliated rocks are characterized by repetitive layering or banding that results from the application of unequal stress, or directed pressure. This pressure forces the rock’s mineral grains to flatten and align themselves perpendicular to the applied force.
The degree of heat and pressure controls the resulting texture, leading to different grades of foliation. Low-grade metamorphism of shale produces slate, which exhibits microscopic alignment of mica and clay minerals, creating slaty cleavage that allows it to split into thin sheets. With increased temperature and pressure, the grain size grows, resulting in schist, which features a coarser, wavy layering called schistosity due to the alignment of visible, platy minerals like mica.
The highest grade of foliation is found in gneiss, which displays a pattern called gneissic banding. This banding consists of alternating layers of light-colored, granular minerals (such as quartz and feldspar) and dark-colored, flattened minerals (like biotite or hornblende). The segregation of these mineral components into stripes is a hallmark of intense metamorphic conditions.
Category Two: Non-Foliated Rocks
Non-foliated metamorphic rocks lack the parallel, layered texture characteristic of foliated rocks. This absence of alignment is due to two factors: the rock was subjected to uniform pressure, known as confining pressure, or the protolith was composed of minerals not prone to alignment. Confining pressure is equal in all directions and does not cause minerals to flatten or organize into layers.
Many non-foliated rocks form through contact metamorphism, where heat from an intrusive magma body “bakes” the surrounding rock without significant directed pressure. The resulting texture is interlocking and crystalline, rather than layered. This texture is exemplified by the complete recrystallization of the parent rock’s mineral grains into a mosaic of larger crystals.
A common example is marble, which forms when the sedimentary rock limestone (composed of calcite) is metamorphosed. The original calcite crystals recrystallize into larger, tightly interlocking grains, creating a dense, crystalline rock that lacks any planar fabric. Similarly, quartzite is the metamorphic equivalent of quartz-rich sandstone, where the quartz grains weld together under heat to form an extremely hard rock.