Factors Controlling Metamorphism

In most cases, the overall chemistry of the metamorphic rock is very similar to that of the parent rock. A quartz sandstone, for example, will metamorphose into a rock that contains a high percentage of silica. A calcite‐rich rock such as limestone can metamorphose only into a calcium‐rich metamorphic rock. A quartz sandstone cannot metamorphose into a calcium‐rich rock.

Temperature and pressure. Temperature and pressure are important factors in determining the new minerals that form in a metamorphic rock. Different minerals form under different pressure and temperature conditions. As pressures and temperatures change, a mineral reaches the edge of its stability field and breaks down to form new minerals that are stable in the new pressure‐temperature field. Higher‐temperature minerals tend to be less dense than lower‐temperature minerals. The higher temperatures also speed up the chemical reactions that take place during metamorphism.

Water. The amount of water available for metamorphic reactions and the length of time involved are important factors in how quickly and intensely metamorphism proceeds. Metamorphic textures and minerals are most likely formed over 10 to 20 million years or longer.

Geostatic pressure. The geostatic pressure, or confining pressure, is the pressure that is equally applied to all sides of a deeply buried mass of rock. Geostatic pressure increases with depth.

Differential stress. Differential stress is usually the result of tectonic forces applied to a body of rock from different directions. This stress “stretches out” the rock mass into an elongate shape (Figure ). Generally, the greater the differential stress, the greater the degree of stretching. Components of the rock, such as crystals, fragments, or pillow structures, will also be stretched out, often to the point where they are difficult to recognize.

Figure 1

Differential Stress

Compressive stress. In contrast, a compressive stress is applied from directly opposite directions and compresses and flattens the rock mass (Figure 2).

Figure 2

Compressive Stress

Shearing. Shearing is related to differential stress and forces parts of the rock mass (usually minerals) to align or grow along a shear plane. Shear planes become zones of weakness along which mineral grains are subjected to crushing or recrystallization. Water can enter rocks along shear planes, which speeds up the metamorphic chemical reactions.

Foliation. Prolonged compressive stress and differential stress and/or shearing forces the mineral grains in a metamorphic rock to form parallel layers or bands. This resulting alignment is called foliation. New metamorphic minerals crystallize along this foliation. The angle of the foliation is related to the direction of the stress and may cross‐cut the original bedding in the rock. A foliation can be so prominent that the original bedding is impossible to identify.

A rock has a slaty cleavage if it splits easily along abundant, parallel foliation planes. A schistose foliation is more massive and is identified by coarser‐grained minerals that have grown along the foliation planes. A schist can also be broken along foliation planes, but they are more widely spaced than those in a slate. A gneissic texture is common in intensely metamorphosed rocks where pressures and temperatures were so high that the rock became plastic, or soft, allowing new minerals to form distinctive light and dark bands.