Introduction

The agents of metamorphism are the "ingredients" needed to make a metamorphic rock and include pressure, temperature, and chemically reactive fluids.

Pressure

All rocks beneath the earth's surface experience increased pressure due to the weight of the overlying sediment and rock layers. Therefore, with increasing depth, there is a corresponding increase in pressure. Typically, this lithostatic (confining) pressure is equal in all directions and will not necessarily cause a rock to become metamorphic (Figure 13.3A). Have you ever dived deep into a pool, lake, or ocean? As you descend, your eardrums experience hydrostatic pressure, similar to what sediments and rocks experience under lithostatic pressure. Lithostatic pressure may alter the overall rock or sediment volume but will not cause a change in overall shape. For example, lithostatic pressure can cause clasts to become more closely packed or reduce pore space within a clastic sedimentary rock.

Right block is subjected to greater pressure in one direction, flattening it. — **Figure 13.3:** Confining pressure (left) and differential pressure (right). (CC-BY 4.0; Emily Haddad, own work)

What happens if the pressure on a rock is unequal, and the rocks become squeezed in one direction more than another direction? This is known as differential (directional) pressure and can result in a significant change in the appearance of a rock (Figure 13.3B). Imagine deformation happening to all the grains in the sedimentary rock or to all the crystals in an igneous rock. The resulting rock will have minerals aligned in a certain direction through bond breaking and recrystallization. The 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. Foliations may be flat or have a wavy appearance, possibly due to more than one direction of greatest pressure (Figure 13.4). 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.

Organized black and white pattern. — **Figure 13.4:** Foliation formed by the realignment of micas and clays under differential stresses. (CC-BY 4.0; Chloe Branciforte, own work)

Temperature

Metamorphism often involves both an increase in temperature and pressure changes; however, the broad classification for metamorphism into low-, medium-, and high-grades of metamorphic change exists mainly due to temperature conditions. Higher temperatures increase the vibrational energy between the bonds linking atoms in the mineral structure, making it easier for bonds to be broken and minerals to recrystallize into new crystal shapes; high temperatures can also sometimes result in the development of foliation and lineation outlined above, even without differential pressure conditions. When temperatures are increased, there can be a corresponding increase in mineral sizes as initially small minerals become fused into larger crystals. This fusing of numerous smaller mineral sizes into fewer, yet larger, is also an example of recrystallization (Figure 13.5).

Top, a sandstone with rounded grains. Bottom, a crystalline metamorphic rock. — **Figure 13.5:** An example of the recrystallization of crystals into larger sizes due to increased temperature: A) a sedimentary rock with rounded quartz grains, and pore spaces between the grains. B) a metamorphic rock with larger, interlocking quartz crystals due to increased temperature conditions. (CC-BY-SA 3.0; Karen Tefend via LibreText)

Why do increasing temperatures lead to increased grain sizes? Generally, a mineral grain or crystal is most stable when it has a low surface area to volume ratio, therefore large grains are more stable than small grains. Increasing the grain size results in a greater increase in volume as opposed to a smaller increase in the surface area. Why does stability matter? Rocks become unstable when their environment changes, and through a recrystallization process (metamorphism), they can return to a stable form once again. Figure 13.5 demonstrates the recrystallization process in response to elevated temperature. In this example, the original grains are smaller and rounded, but recrystallization resulted in larger grains that become interlocking; the pore spaces are gone, and instead larger crystals exist.

Index mineral chart, showing possible minerals that can form from a shale protolith. — **Figure 13.6:** Different index minerals form depending on the amount of heat and pressure the protolith undergoes. (CC-BY 4.0; Chloe Branciforte, own work)

In addition to increased grain size with increased temperature, a new mineral will occasionally form during metamorphism. These new minerals form at certain temperatures and are called index minerals, which can be used to determine the temperature of metamorphism (Figure 13.6).

Chemically Reactive Fluids

Higher temperatures are sometimes associated with metamorphism due to chemically reactive fluids. The phrase “chemically reactive” refers to the dissolved ions in a fluid phase that may react with minerals in a rock. These ions may replace some of the atoms in the mineral’s structure, which can significantly change a rock's chemical composition. Sometimes these fluids are quite hot, especially if they are fluids released from a nearby magma body that is crystallizing while cooling. Metamorphism due to such fluids is known as hydrothermal metamorphism. California’s state rock, serpentinite, is formed during this process!