Metamorphic rocks are created when a pre-existing rock, known as the protolith, is transformed by significant changes in temperature and pressure. This process, called metamorphism, occurs deep within the Earth’s crust without the rock melting entirely, causing profound physical and chemical alterations. These changes lead to the recrystallization of minerals, forming new textures and mineral assemblages stable under the new conditions.
Geologists classify these resulting rocks to systematically organize the vast diversity produced by metamorphism. Classification provides a framework for understanding the conditions of formation, including the intensity of heat and pressure involved, allowing researchers to interpret the geological history of the region.
The Foundational Split: Foliated Versus Non-Foliated
The initial step in classifying metamorphic rocks involves observing whether the rock possesses a layered or banded appearance. This structural characteristic is known as foliation, which describes the parallel alignment of mineral grains within the rock. Foliation forms when the rock is subjected to differential stress, meaning pressure is applied unequally from different directions, such as during continental collision.
Foliated rocks exhibit distinct planes of weakness due to this preferred orientation of minerals like mica or chlorite. These minerals become oriented perpendicular to the maximum stress, giving the rock a sheet-like quality.
In contrast, non-foliated rocks lack this layered structure, appearing massive, crystalline, or granular. These rocks typically form in environments where the pressure is uniform in all directions, often referred to as confining pressure. Non-foliated rocks may also form from parent rocks composed of minerals that are blocky and do not easily align, such as quartz or calcite.
The presence or absence of this mineral alignment is the primary textural criterion that divides metamorphic rocks into their two major categories. This split simplifies the initial identification process and directs further classification based on the specific details of texture or composition.
Classification Criteria for Foliated Rocks
Classification of foliated rocks depends on the type of foliation and the size of the mineral grains, both related to the intensity of metamorphism. As temperature and pressure increase, the rock moves from a low to a high metamorphic grade, resulting in a progressive change in texture. The lowest-grade foliated rock is slate, which develops a texture called slaty cleavage.
Slate forms from the metamorphism of shale, where microscopic clay and mica crystals are aligned perpendicular to the stress. This alignment allows the rock to split into thin, flat sheets, a property that makes it commercially desirable for roofing and floor tiles.
With slightly increased metamorphic intensity, the rock transitions into phyllite, which exhibits phyllitic foliation. The platy minerals, mostly mica, have grown larger than in slate but are still too small to be individually visible. The rock surface gains a distinct, silky or glossy sheen due to the reflection of light off these parallel mica crystals.
A further increase in temperature and pressure leads to the formation of schist, which is characterized by schistosity. In schist, the platy minerals, such as mica and chlorite, are coarse enough to be easily seen with the unaided eye. These visibly aligned crystals dominate the rock’s texture, giving it a sparkling, layered appearance.
Schists are often named using the most prominent visible index mineral, such as a garnet schist or a biotite schist. This use of mineralogy combined with the textural term schistosity provides a highly specific description. The highest-grade foliated rock is gneiss, which is defined by gneissic banding.
Gneiss forms under very high temperatures, causing the segregation of minerals into distinct alternating bands of light and dark colors. The light bands are composed of quartz and feldspar, while the dark bands contain minerals like biotite and amphibole. This textural separation is a result of the highest intensity of metamorphism before the rock begins to melt.
Classification Criteria for Non-Foliated Rocks
Since non-foliated rocks lack layered texture, their classification relies primarily on the rock’s mineral composition and the identity of the original parent rock. Metamorphism mainly involves recrystallization, where existing mineral grains grow larger and interlock without developing a preferred orientation. The resulting rock is massive and durable.
One common example is quartzite, formed from the metamorphism of quartz-rich sandstone. The classification is based solely on its dominant mineral, quartz. During metamorphism, the quartz grains fuse together, eliminating the original pore spaces and creating a rock so hard that fractures cut across the fused grains rather than following the grain boundaries.
Marble is another non-foliated rock classified by its mineral composition, which is predominantly calcite or dolomite. Its protolith is limestone or dolostone. Recrystallization causes the smaller calcite crystals in the parent rock to grow into a dense, crystalline mass, destroying any original sedimentary structures like fossils.
The rock is named for its composition, regardless of the metamorphic conditions that formed it. In some cases, classification is based on the rock’s fine-grained, interlocking texture and its association with a specific metamorphic process. Hornfels is an example, typically forming during contact metamorphism when a rock is “baked” by the heat of a nearby magma intrusion.
Hornfels is fine-grained and does not show mineral alignment because the metamorphism occurred primarily due to heat, not directed pressure. Therefore, the simple, granular texture and the mineral assemblage that reflects the original rock’s chemistry are the main classification criteria. For non-foliated rocks, composition is often a more reliable identifier than texture.