Overview

The rock cycle is a circular diagram demonstrating how rocks are continually recycled throughout geologic time. All rocks can be classified into one of three groups: igneous, sedimentary, or metamorphic. This rock classification is based on the origin of each rock type, or the processes that form the rock. The Rock Cycle (Figure 13.1) represents the processes that produce each rock type. Igneous rocks are produced through the melting of rock and the eventual cooling of lava or magma; as cooling occurs, crystals may form. Sedimentary rocks are produced via the processes of weathering (the chemical and physical breakdown of material at Earth’s surface that produces sediments), erosion (the movement and deposition of sediments), and lithification (the compaction and cementation of sediments). Now, we're ready to learn about metamorphic rocks, which are produced via the agents of change, primarily heat and pressure, that typically result in the recrystallization of minerals. This is depicted in the Rock Cycle below, where the arrows pointing at "metamorphic rock" from igneous and sedimentary rocks are labeled heat and pressure.

The Rock Cycle

The rock cycle. — **Figure 13.1:** The processes of the rock cycle. (CC-BY 4.0, Emily Haddad, own work)

Introduction

All rocks form at certain temperatures and pressures and are most stable in the conditions under which they form. If the rock is subjected to significant increases in temperature and/or pressure minerals in the rock will change to be stable under these new conditions. This is metamorphism. Metamorphism means to change form. A metamorphic change can also occur if the rock’s composition is altered by hot, chemically reactive fluids, causing a change in the mineral content of the rock. So, pressure, temperature, and chemically reactive fluids are the "agents of metamorphism" - the mechanisms needed to make a metamorphic rock.

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 13.2A). 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 13.2**: 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 13.3:** 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 13.4).

Top, a sandstone with rounded grains. Bottom, a crystalline metamorphic rock. — **Figure 13.4:** 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 13.4 demonstrates the recrystallization process in response to elevated temperature.

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 13.5). For example, if the metamorphic rock schist contains garnet crystals we can assume the rock reached a temperature of at least 500^oC.

Index mineral chart, showing possible minerals that can form from a shale protolith. — **Figure 13.5:** Different index minerals form depending on the amount of heat and pressure the protolith undergoes. (CC-BY 4.0; Chloe Branciforte, own work)

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!

Metamorphic Grade

Metamorphic grade refers to the general temperature and pressure conditions that prevailed during metamorphism. As the pressure and temperature increase, rocks undergo metamorphism at higher metamorphic grade (Figure 13.7).

Chart illustrating the metamorphic grades from diagenesis through partial melting of wet Granites. — **Figure 13.6:** Metamorphic grades are categorized by the amount of heat and pressure. (CC-BY 4.0; Chloe Branciforte)

Very Low and Low-grade metamorphism occurs at approximately 150–300°C and relatively low pressure. This is not far beyond the conditions in which sediments are lithified into sedimentary rocks, and it is common for a low-grade metamorphic rock to look similar to its protolith. Low-grade metamorphic rocks tend to be characterized by an abundance of hydrous minerals, minerals that contain water within their crystal structure. Examples of low-grade hydrous minerals include clay, serpentine, and chlorite. Under low-grade metamorphism many of the metamorphic minerals will not grow large enough to be seen without a microscope.

Intermediate-grade metamorphism takes place at approximately 300–450°C and at moderate pressures. Low-grade hydrous minerals are replaced by micas such as biotite and muscovite, and non-hydrous minerals such as garnet may now grow. Garnet is an example of a mineral which may form porphyroblasts, metamorphic mineral grains that are larger in size and more equant in shape (about the same diameter in all directions), thus standing out among the smaller, flatter, or more elongate minerals.

High-grade metamorphism takes place at temperatures above about 450°C. Micas tend to break down. New minerals such as hornblende will form, which is stable at higher temperatures. As metamorphic grade increases to even higher grade, however, all hydrous minerals, including hornblende, may break down and be replaced by other, higher-temperature, non-hydrous minerals such as pyroxene.

Metamorphic rocks form by the physical and sometimes chemical alteration of a pre-existing rock. To distinguish between the pre-existing rock and the new metamorphic one, the term protolith (proto- meaning “original” and -lith meaning “rock”), or parent rock, is used to describe the pre-existing rock (Figure 13.2). All metamorphic rocks have at least one protolith that was altered during metamorphism. Igneous, sedimentary, and even metamorphic rocks can be altered into completely new or different metamorphic rocks.

A close-up of the sedimentary rock shale (left) and slate (right). — **Figure 13.7:** Heat and pressure alter shale, the protolith, to slate. (*Left:* CC-BY 2.0; James St. John via Flickr; *Right:* CC-BY 4.0; Chloe Branciforte, own work)