Tectonic plates do not always crash head-on into each other. In some places, the edge of one plate slides underneath the other in a process called subduction. The bottom plate in the subduction zone gets squeezed under enormous pressures, many thousands of times as strong as the atmosphere's pressure at sea level. Such pressures push atoms and molecules closer together into compact crystalline structures. However, the temperature of the lower plate remains relatively low - only 200-500 degrees C (400-900 degrees F). Igneous basalt rocks pulled into the subduction zone initially change into dark-colored, medium-grade rocks called blueschist. After further high-pressure metamorphism, blueschist changes into a dense, colorful metamorphic rock called eclogite, containing tiny red garnets mixed into green pyroxene, a silicate mineral.

Fabric is one way geologists describe how the small-scale components of a rock are arranged with respect to each other. Scientists look at grain size, grain shape, and the distribution of grain sizes within rocks to determine the fabric of those rocks. Sometimes the grains may be oriented randomly with respect to each other. The randomness may indicate that the rock may have formed at high temperatures (a rock called hornfels, for example) or from simple materials (marble, which results from the metamorphism of limestone, a sedimentary rock). Other times, the grains may be aligned in one direction at right angles to the direction of the pressure that helped to change (or metamorphose) the rock. Fabric is one of the many clues that help geologists identify rocks and learn about the history of those rocks.