2 METAMORPHIC CHANGES

䊳 8.2 METAMORPHIC CHANGES

Metamorphism commonly alters both the texture and mineral content of a rock.

TEXTURAL CHANGES As a rock undergoes metamorphism, some mineral grains

grow larger and others shrink. The shapes of the grains may also change. For example, fossils give fossiliferous limestone its texture (Fig. 8–1). Both the fossils and the cement between them are made of small calcite crystals. If the limestone is buried and heated, some of the calcite

Figure 8–2 Metamorphism has destroyed the fossiliferous grains grow larger at the expense of others. In the process,

texture of the limestone in Figure 8–1 and replaced it with the the fossiliferous texture is destroyed.

large, interlocking calcite grains of marble.

Metamorphic Changes 127

Figure 8–3 Shale is a very fine-grained sedimentary rock, Figure 8–4 Hornfels forms by metamorphism of shale. containing clay, quartz, and feldspar.

The white spots are metamorphic minerals. (Geoffrey Sutton)

Micas are common metamorphic minerals; they form MINERALOGICAL CHANGES

as many different parent rocks undergo metamorphism. As a general rule, when a parent rock (the original rock)

Recall from Chapter 3 that micas are shaped like pie contains only one mineral, metamorphism transforms the

plates. When metamorphism occurs without deforma- rock into one composed of the same mineral but with a

tion, the micas grow with random orientations, like pie coarser texture. The mineral content does not change be-

plates flying through the air (Fig. 8–5). However, when cause no other chemical components are available dur-

metamorphism and deformation occur together, the ing metamorphism. Limestone converting to marble is one example of this generalization. Another is the meta- morphism of quartz sandstone to quartzite, a rock com-

Unfoliated

posed of recrystallized quartz grains.

metamorphic rock

In contrast, metamorphism of a parent rock contain- ing several minerals usually forms a rock with new and different minerals and a new texture. For example, a typ- ical shale contains large amounts of clay, as well as quartz and feldspar (Fig. 8–3). When heated, some of those minerals decompose, and their atoms recombine to form new minerals such as mica, garnet, and a different kind of feldspar. Figure 8–4 shows a rock called horn- fels that formed when metamorphism altered both the texture and minerals of shale.

If migrating fluids change the chemical composition of a rock, new minerals invariably form. These effects are discussed further in Section 8.3.

DEFORMATION AND FOLIATION Changes in temperature, pressure, or the chemical envi-

ronment alter a rock’s texture during metamorphism. But another factor also causes profound textural changes. Metamorphic rocks commonly form in large regions of the Earth’s crust near a subduction zone, where two tec- tonic plates converge. The tectonic forces crush, break, and bend rocks in this environment as the rocks are un- dergoing metamorphism. This combination of metamor-

Figure 8–5 When metamorphism occurs without deforma- phism and deformation creates layering in the rocks.

tion, platy micas grow with random orientations.

128 CHAPTER 8 M E TA M O R P H I C RO C K S

metamorphic rock

Old sedimentary layers

Figure 8–7 Horizontal compression formed this tight fold in interbedded shale and sandstone. Slaty cleavage developed in the shale but not in the sandstone. (Karl Mueller)

METAMORPHIC GRADE Metamorphic grade expresses the intensity of meta-

morphism that affected a rock. Because temperature is the most important factor in metamorphism, metamor- phic grade closely reflects the highest temperature at- tained during metamorphism. Geologists can interpret

Figure 8–6 When deformation accompanies metamor- the metamorphic grade of most rocks because many phism, platy micas orient in a parallel manner to produce

metamorphic layering called foliation. metamorphic minerals form only within certain temper-

ature ranges.

The temperature in shallow parts of the Earth’s crust rises by an average of 30ºC for each kilometer of depth. It continues to rise in deeper parts of the crust and in the

micas develop a parallel orientation. This parallel align- mantle, but at a lesser rate. The rate at which tempera- ment of micas (and other minerals) produces the meta-

ture increases with depth is called the geothermal gra- morphic layering called foliation (Fig. 8–6). The layers

dient. Consequently, the metamorphic grade of many range from a fraction of a millimeter to a meter or more

rocks is related to the depth to which they were buried in thickness. Metamorphic foliation can resemble sedi-

(Fig. 8–8). Low-grade metamorphism occurs at shallow mentary bedding but is different in origin.

depths, less than 10 to 12 kilometers beneath the surface, Micas and other platy minerals orient at right angles

where temperature is below 350ºC. High-grade condi- to the tectonic force squeezing the rocks. Pencil-shaped

tions are found deep within continental crust and in the minerals such as amphiboles align in a similar manner.

upper mantle, 40 or more kilometers below the Earth’s When horizontal forces deform shale into folds during

surface. The temperature in these regions is 600ºC or metamorphism, the clays decompose and micas grow

hotter and is near the melting point of rock. High-grade with their flat surfaces perpendicular to the direction of

metamorphism can occur at shallower depths, where squeezing. As a result, the rock develops vertical folia-

magma rises to a shallow level of the Earth’s crust. tion—perpendicular to the horizontal force. Many meta- morphic rocks break easily along the foliation planes.

THE RATE OF METAMORPHISM This parallel fracture pattern is called slaty cleavage

(Fig. 8–7). In most cases, slaty cleavage cuts across the

A rule of thumb among laboratory chemists is that the original sedimentary bedding.

speed of a chemical reaction doubles with every 10ºC

rise in temperature. Thus, reactions occur slowly in a cold environment, but rapidly in a hot one. For the same reason, metamorphic changes occur slowly at low tem- perature, but much faster at high temperature. For exam- ple, clay minerals in 20-million-year-old shale buried to

a depth of 2 to 3 kilometers in the Mississippi River delta show mineralogical changes at 50ºC, about the tempera- ture of a cup of hot coffee. Elsewhere, similar clays at the same temperature, but only 1 million years old, show no changes. Thus, metamorphism can occur at tempera- tures as low as 50ºC, but the reactions require millions

of years. 1 In contrast, geologists routinely produce meta- morphic reactions in the laboratory at temperatures above 500ºC in a few days.

The upper limit of metamorphism is the point at which rocks melt to form magma. That temperature varies depending on rock composition, pressure, and amount of water present, but it is between 600° and 1200°C for most rocks. A rock heated to its melting point creates magma, which forms igneous rocks when it solidifies.

Metamorphism refers only to changes that occur without melting.