Overview of Metamorphic Rocks
Agents of Metamorphism
Pressure
All rocks beneath Earth’s surface experience pressure due to the weight of the overlying sediment and rock layers, and with increasing depth there is a corresponding increase in pressure. This pressure—called lithostatic pressure—is equal in all directions. Lithostatic pressure can change the overall rock volume of a rock, such as by pushing the clasts in a sedimentary rock closer together and reducing the pore space between the clasts, but it will not cause a change in the shape of the rock. To change the shape, one part of the rock must be deformed more than another, and that means the forces acting on the rock cannot be equal in all directions.
In contrast to lithostatic pressure, differential pressure (sometimes called directed pressure) does not act equally in all directions, and thus can cause significant changes in the appearance of a rock. Figure 5.1 demonstrates how a mineral can change shape due to differential pressure, in this case with the greatest pressures applied from the top and bottom. Two initially rounded mineral grains (Figure 5.1A) within a sedimentary rock are experiencing the greatest amount of pressure at the contact between the grains (red arrows), and the bonds linking the atoms in this grain will break. The atoms will migrate into an area of lesser pressure and re-form a bond with other atoms in the mineral grain (Figure 5.1B). As a result, the grains will develop a flattened shape where the grain becomes wider in the horizontal direction, perpendicular to the up-and-down direction of greatest pressure (Figure 5.1C).
Figure 5.1 shows the deformation of only two grains; in reality, it’s happening to all of the grains in the rock, and in the end, the entire rock will have minerals that are aligned (meaning they’re longer in the same direction). The result is a rock with a metamorphic pattern called a foliation. Foliation causes the rock to have a layered appearance, and to break apart between layers. The layers can be fairly flat if the grains are smaller, or develop a wavy appearance when the grains are larger. You will see some examples of the different forms foliation can take later. Being able to recognize foliation in metamorphic rocks comes in handy for purposes of understanding how the rocks formed, because we can infer something about what forces were acting upon the rocks.
Heat
Increasing pressure can cause metamorphism, and often the increase in pressure is accompanied by an increase in temperature. Heating is also conducive to the presence of chemically reactive fluids, which can accelerate metamorphic changes.
Metamorphic rocks are classified broadly into low, medium and high grades of metamorphic intensity, primarily due to the effects of heat on mineral stability. Heat causes atoms to vibrate; the higher the temperature, the more vibration occurs, and the weaker the bonds between atoms become. Thus, at higher temperatures it’s easier to break the