Metamorphic rocks are fundamentally changed by intense heat and high pressure deep beneath the Earth’s surface. The transformation process is called metamorphism, which translates from Greek as “change of form.” This geological process occurs when an existing rock, known as the protolith, is subjected to conditions different from those under which it originally formed. The protolith can be igneous, sedimentary, or even an earlier metamorphic rock. It undergoes physical and chemical alteration while remaining mostly in a solid state, resulting in a denser, more compact rock with a new texture and mineral composition.
Understanding Metamorphism
The transformation that creates metamorphic rocks is driven by three primary mechanisms: heat, pressure, and chemically active fluids. Heat is a major catalyst, often sourced from the geothermal gradient as rocks are buried deeper in the crust, or from the intrusion of hot magma bodies. This increased temperature, which can range between 200°C and 850°C, causes the original minerals to become unstable and recrystallize, changing their internal structure without melting the rock entirely. If the rock were to melt, the process would become igneous.
Pressure acts on the rock in two distinct ways. Confining pressure is the equal force exerted on a rock from all directions, typically from the weight of overlying layers, which causes the rock to become more compact and dense. Directed pressure, or differential stress, involves unequal forces, such as those found where tectonic plates collide. This directed force is responsible for deforming the rock and physically aligning mineral grains perpendicular to the greatest applied stress.
Chemically active fluids, primarily hot water containing dissolved ions, accelerate the metamorphic reactions. These fluids move through the rock’s pore spaces, transporting chemical components and facilitating the growth of new minerals. This chemical alteration, known as metasomatism, changes the overall chemical makeup of the rock by introducing or removing elements.
Classifying Metamorphic Rocks
Geologists primarily classify metamorphic rocks based on their texture, which is a direct consequence of the temperature and pressure conditions they experienced. The most obvious textural distinction is between foliated and non-foliated rocks. Foliated rocks possess a layered or banded appearance caused by the parallel alignment of platy mineral grains, such as mica.
This distinct foliation texture develops under conditions of directed pressure, where the unequal stress causes the minerals to rotate and regrow in planes perpendicular to the force. The degree of foliation can be used to determine the intensity of the metamorphism, progressing from fine-grained slate to coarsely banded gneiss.
Non-foliated rocks lack this layered texture, instead showing a massive, crystalline structure. This absence of layering often occurs when the metamorphism is dominated by heat, such as near a magma intrusion, or when the pressure is confining and equal in all directions. Another factor is the composition of the protolith, as rocks composed of minerals that are not easily flattened, like quartz or calcite, typically form non-foliated rocks even under directed pressure.
Specific Examples of Transformation
The metamorphic process creates a wide variety of rocks, depending on the composition of the original rock and the conditions of the transformation. Shale, a common sedimentary rock composed of fine clay minerals, undergoes low-grade metamorphism to become slate. With increasing heat and pressure, the clay minerals in slate change into mica, transforming the rock into schist and eventually into the coarsely banded gneiss, which often forms from the metamorphism of granite.
Limestone, a sedimentary rock dominated by the mineral calcite, transforms into marble when subjected to metamorphism. The original fine calcite crystals recrystallize and grow larger, interlocking to form a dense, non-foliated rock. Similarly, quartz-rich sandstone is transformed into the durable, non-foliated rock called quartzite. During this change, the individual quartz grains weld together and enlarge, resulting in a rock so hard that a break will cut right through the original grains instead of separating them.