Chapter 5. Metamorphic Rocks
Overview of Metamorphic Rocks
Adapted by Lyndsay R. Hauber & Joyce M. McBeth (2018) University of Saskatchewan from Deline B, Harris R & Tefend K. (2015) “Laboratory Manual for Introductory Geology”. First Edition. Chapter 11 “Metamorphic Rocks” by Karen Tefend, CC BY-SA 4.0. View source. Last edited: 28 Dec 2019
5.2 AGENTS OF METAMORPHISM
5.2.1 Pressure
All rocks beneath the surface of the earth experience an increase in pressure due to the weight of the overlying sediment and rock layers, and with increasing depth there is a corresponding increase in pressure. This increased pressure does not necessarily cause a rock to become metamorphic, because this particular pressure is typically equal in all directions and is known as lithostatic pressure. Lithostatic pressure on rocks below the earth’s surface may have a change in overall rock volume, but will not cause a change in the shape. An example of decreasing volume due to lithostatic pressure would be a closer packing of clasts and reduction of pore space within a clastic sedimentary rock. Differential pressure occurs when the pressure on a rock is unequal, and it can result in a significant change 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 from the top and bottom. Two initially rounded mineral grains (e.g. Figure 5.1A) within a sedimentary rock are experiencing the greatest amount of pressure at the contact between the grains (see red arrows in the figure), and the bonds linking the atoms in this grain will break. The atoms will migrate into the area of lesser pressure and reform a bond with other atoms in the mineral grain (e.g. Figure 5.1B). As a result, the grains have a flattened shape that is perpendicular to the direction of greatest pressure (e.g. Figure 5.1C).
Figure 5.1 only shows the deformation of two grains; imagine that this is happening to all of the grains in the sedimentary rock, or to all of the phenocrysts (crystals) in an igneous rock. In that case, you will end up with the entire rock having minerals aligned in a certain direction, all by the breaking of bonds between atoms in a mineral, and reforming (recrystallizing) in the lower pressure areas among the grains or crystals in the rock. The end result is a rock with a metamorphic pattern called a foliation. Metamorphic foliations are the patterns seen in a rock that has experienced differential pressure; these foliations may be fairly flat, or have a wavy appearance possibly due to more than one direction of greatest pressure. Some rocks may also develop what is called a lineation, which can be formed by an elongation of minerals that form a linear feature through the rock. To understand the difference between a foliation and a lineation, let us use some food analogies: a stack of pancakes demonstrates a foliation in your breakfast food, with each pancake layer representing flattened m