The Earth’s crust is composed of three major rock classes: igneous, sedimentary, and metamorphic. Igneous rocks form from cooling magma or lava, and sedimentary rocks are created from compacted fragments or chemical precipitates. The third class, formed by intense heat and pressure without complete melting, undergoes a profound transformation in the solid state. This process reworks the rock’s internal structure and mineral composition, resulting in a new material.
Naming the Resulting Rock Type
The rock type produced when a pre-existing rock is subjected to elevated temperatures and pressures is formally called a metamorphic rock. The term “metamorphism” is derived from Greek words meaning “change in form.” This process alters the original rock physically and chemically.
The original rock that undergoes this transformation is known as the protolith. A protolith can originate from any of the three main rock types: igneous, sedimentary, or even an older metamorphic rock. The chemical makeup of the protolith influences the types of new minerals that form during metamorphism.
The Role of Heat and Pressure in Transformation
Heat provides the energy necessary to break and reform chemical bonds within the rock’s minerals, driving metamorphism. Sources include the geothermal gradient, where temperatures increase with depth, and the intrusion of hot magma. This heating causes solid-state chemical reactions and facilitates recrystallization, where existing mineral grains reform into new, interlocking crystals.
Pressure affects the rock in two principal ways: confining pressure and differential stress. Confining pressure is equal in all directions, resulting from deep burial, which causes the rock to become denser as mineral grains are squeezed closer together. Differential stress is unequal pressure applied from specific directions, often associated with tectonic forces like mountain building or plate collision. This directed pressure changes the shape of mineral grains and rotates them into parallel alignments.
Textural Changes and Classification
Geologists classify metamorphic rocks primarily based on the resulting texture, which reflects the type of stress the rock endured. The two broad categories are foliated and non-foliated textures. Foliation refers to a planar, layered, or banded structure caused by the parallel alignment of platy or elongated minerals, such as micas or chlorite.
This alignment occurs when differential stress forces mineral grains to rotate or grow perpendicular to the maximum stress direction. Foliation ranges from the fine, slaty cleavage seen in low-grade rocks to the distinct, alternating light and dark mineral bands found in high-grade rocks. Non-foliated rocks lack this layered structure, typically forming under confining pressure or where the protolith contained equidimensional minerals, like quartz or calcite. In non-foliated rocks, the primary change is the growth of larger, interlocking crystals during recrystallization without preferred orientation.
Common Metamorphic Rock Examples
Common metamorphic rocks illustrate the transformation from a protolith and fall into these textural groups. Slate is a fine-grained, foliated rock that forms when shale is subjected to low-grade metamorphism. With increasing heat and pressure, the foliation becomes coarser, transforming into phyllite, then schist, and finally, the high-grade, banded gneiss, which often originates from granite.
Non-foliated rocks include marble, the result of limestone recrystallizing, and quartzite, created when quartz-rich sandstone is metamorphosed. In both cases, the new crystals grow larger and interlock tightly, making the rock significantly harder than its sedimentary parent. The resulting rock type provides clues about the pressure and temperature conditions experienced deep within the Earth’s crust.