So far, we've discussed the nature of changes that occur during metamorphism, the agents of metamorphism (heat, pressure, compression and shear, and hydrothermal fluids), the rock types that form as a result of metamorphism, and the concepts of metamorphic grade and metamorphic facies. With this background, let’s now examine the geologic settings on Earth where metamorphism takes place, as viewed from the perspective of plate tectonics theory.
Because of the wide range of possible metamorphic environments, metamorphism occurs at a wide range of conditions in the Earth. You will see that the conditions under which metamorphism occurs are not the same in all geologic settings. That’s because the geothermal gradient (the relation between temperature and depth), the extent to which rocks endure compression and shear during metamorphism, and the extent to which rocks interact with hydrothermal fluids all depend on the geologic environment.
Thermal or Contact Metamorphism
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| Geologic settings of metamorphism. |
Imagine a hot magma that rises from great depth beneath the Earth’s surface and intrudes into cooler rock at a shallow depth. Heat flows from the magma into the wall rock, for heat always flows from hotter to colder materials. As a consequence, the magma cools and solidifies while the wall rock heats up. In addition, hydrothermal fluids circulate through both the intrusion and the wall rock. As a consequence of the heat and hydrothermal fluids, the wall rock undergoes metamorphism, with the highest-grade rocks forming immediately adjacent to the pluton, where the temperatures were highest, and progressively lower-grade rocks forming farther away. The distinct belt of metamorphic rock that forms around an igneous intrusion is called a metamorphic aureole or contact aureole (figure above a). The width of an aureole depends on the size and shape of the intrusion, and on the amount of hydrothermal circulation larger intrusions produce wider aureoles.
The local metamorphism caused by igneous intrusion can be called either thermal metamorphism (see Pottery Making—An Analog for Thermal Metamorphism), to emphasize that it develops in response to heat without a change in pressure and without differential stress, or contact metamorphism, to emphasize that it develops adjacent to the contact of an intrusion with its wall rock. Because this metamorphism takes place without application of compression or shear, aureoles contain hornfels, a nonfoliated metamorphic rock.
Contact metamorphism occurs anywhere that the intrusion of plutons occurs. In the context of plate tectonics theory, plutons intrude into the crust at convergent plate boundaries, in rifts, and during the mountain building that takes place where continents collide.
Burial Metamorphism
As sediment gets buried in a subsiding sedimentary basin, the pressure increases due to the weight of overburden, and the temperature increases due to the geothermal gradient. At depths greater than about 8 to 15 km, depending on the geothermal gradient, temperatures may be great enough for metamorphic reactions to begin, and low-grade metamorphic rocks form. Metamorphism due only to the consequences of very deep burial is called burial metamorphism.
Faults are surfaces on which one piece of crust slides, or shears, past another. Near the Earth’s surface (in the upper 10 to 15 km) this movement can fracture rock, breaking it into angular fragments or even crushing it to a powder. But at greater depths, rock is so warm that it behaves like soft plastic as shear along the fault takes place. During this process, the minerals in the rock recrystallize. We call this process dynamic metamorphism, because it occurs as a consequence of shearing alone under metamorphic conditions, without requiring a change in temperature or pressure. The resulting rock, a mylonite, has a foliation that roughly parallels the fault (figure above b). Mylonites are very fine-grained, due to processes during dynamic metamorphism that replace larger crystals with a mass of very tiny ones. Dynamic metamorphism takes place anywhere that faulting occurs at depth in the crust. Thus, mylonites can be found at all plate boundaries, in rifts, and in collision zones.
During the development of mountain ranges, in response to either convergent-margin tectonics or continental collision, regions of crust are squeezed and large slices of continental crust slip along faults and move up and over other portions of the crust. As a consequence, rock that was once near the Earth’s surface along the margin of a continent ends up at great depth beneath the mountain range (figure above c). In this environment, three changes happen to the protolith: (1) it heats up because of the geothermal gradient and because of igneous activity; (2) it endures greater pressure because of the weight of overburden; and (3) it undergoes compression and shearing. As a result of these changes, the protolith transforms into foliated metamorphic rock. The type of foliated rock that forms depends on the grade of metamorphism slate forms at shallower depths, whereas schist and gneiss form at greater depths. Since the metamorphism we've just described involves not only heat but also compression and shearing, we can call it dynamothermal metamorphism. Typically, such metamorphism affects a large region, so geologists also call it regional metamorphism.
Erosion eventually removes the mountains, exposing a belt of metamorphic rock that once lay at depth. Such belts may be hundreds of kilometres wide and thousands of kilometres long.
Hydrothermal Metamorphism at Mid-Ocean Ridges
Hot magma rises beneath the axis of mid-ocean ridges, so when cold seawater sinks through cracks down into the oceanic crust along ridges, it heats up and transforms into hydrothermal fluid. This fluid then rises through the crust, near the ridge, causing hydrothermal metamorphism of ocean-floor basalt (figure above d). Eventually, the fluid escapes through vents back into the sea; these vents are called black smokers.
A brick for the wall of an adobe house, an earthenware pot, a stoneware bowl, or a translucent porcelain teacup may all be formed from the same lump of soft clay, scooped from the surface of the Earth and shaped by human hands. This pliable and slimy muck is a mixture of very fine clay minerals and quartz grains formed during the chemical weathering of rock and water. Fine potter’s clay for making white china contains a particular clay mineral called kaolinite, named after the locality in China (called Kauling, meaning high ridge) where it was originally discovered.
People in arid climates make adobe bricks by forming damp clay into blocks, which they then dry in the sun. Such bricks can be used for construction only in arid climates, because if it rains heavily, the bricks will rehydrate and turn back into sticky muck drying clay in the sun does not change the structure of the clay minerals.
To make a more durable material, brick makers place clay blocks in a kiln and bake (“fire”) them at high temperatures. This process makes the bricks hard and impervious to water. Potters use the same process to make jugs. In fact, fired clay jugs that were used for storing wine and olive oil have been found intact in sunken Greek and Phoenician ships that have rested on the floor of the Mediterranean Sea for thousands of years! Clearly, the firing of a clay pot fundamentally and permanently changes clay in a way that makes it physically different (see 1st figure a). In other words, firing causes a thermal metamorphic change in the mineral assemblage that composes pottery. The extent of the transformation depends on the kiln temperature, just as the grade of metamorphic rock depends on temperature. Potters usually fire earthenware at about
1100C and stoneware (which is harder than a knife or fork) at about 1250C. To produce porcelain fine china the clay must partially melt at even higher temperatures up to 1400C. Just as it begins to melt, the potter cools it relatively quickly. Such cooling of the melt creates glass, which gives porcelain its translucent, vitreous (glassy) appearance.
Metamorphism in Subduction Zones
Blueschist is a relatively rare rock that contains an unusual blue-coloured amphibole. Laboratory experiments indicate that formation of this mineral requires very high pressure but relatively low temperature. Such conditions do not develop in continental crust usually, at the high pressure needed to produce blue amphibole, temperature in continental crust is also high. So to figure out where blueschist forms, we must determine where high pressure can develop at relatively low temperature.
Plate tectonics theory provides the answer to this puzzle. Researchers found that blueschist occurs only in the accretionary prisms that form at subduction zones. They realized that because prisms grow to be over 20 km thick, rock at the base of the prism feels high pressure (due to the weight of overburden). But because the subducted oceanic lithosphere beneath the prism is cool, temperatures at the base of the prism remain relatively low.
Shock Metamorphism
When large meteorites slam into the Earth, a vast amount of kinetic energy instantly transforms into heat, and a pulse of extreme compression (a shock wave) propagates into the Earth. The heat may be sufficient to melt or even vaporize rock at the impact site, and the extreme compression of the shock wave causes quartz in rocks below the impact site to undergo a phase change and become a more compact mineral called coesite. The changes in rock due to the passage of a shock wave are called shock metamorphism.
Where Do You Find Metamorphic Rocks?
When you stand on an outcrop of metamorphic rock, you are standing on material that once lay many kilometers beneath the surface of the Earth. How does metamorphic rock return to the Earth’s surface? Geologists refer to the overall process by which deeply buried rocks end up back at the surface as exhumation.
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| Processes that bring metamorphic rock back to Earth’s surface. Three phenomena contribute to exhumation of rocks at depth. Here, the red dot (representing metamorphic rocks formed at the base of a mountain range) gets progressively closer to the surface over time. |
To see how exhumation works, let’s look at the specific processes that contribute to bringing high-grade metamorphic rocks from below a collisional mountain range back to the surface (figure above). First, as two continents progressively push together, the rock caught between them squeezes upward, much like dough pressed in a vise; the upward movement takes place by slip on faults and by plastic-like flow of rock. Second, as the mountain range grows, the crust at depth beneath it warms up and becomes softer and weaker. Eventually, the range starts to collapse under its own weight, much like a block of soft cheese placed in the hot sun. As a result of this collapse, the upper crust spreads out laterally. Horizontal stretching of the upper part of the crust causes it to become thinner in the vertical direction, and as the upper part of the crust becomes thinner, the deeper crust ends up closer to the surface. Third, erosion takes place at the surface; weathering, landslides, river flow, and glacial flow together play the role of a giant rasp, stripping away rock at the surface and exposing rock that was once below the surface.
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| Examples of rock exposures consisting of Precambrian metamorphic rocks. |
Keeping in mind the processes that form metamorphic rock and cause exhumation, let’s ask the question, “Where are metamorphic rocks presently exposed?” You can start your quest to find metamorphic rock outcrops by hiking into a mountain range. As we've seen, the process of mountain building produces and eventually exhumes metamorphic rocks. The towering cliffs in the interior of a mountain range typically reveal schist, gneiss, and quartzite (figure above a). Even after the peaks have eroded away, the record of mountain building remains in the form of a belt of metamorphic rock at the ground surface.
Vast expanses of metamorphic rock crop out in continental shields. A shield is a broad region of long-lived, stable continental crust where Phanerozoic sedimentary cover either was not deposited or has been eroded away so that Precambrian rocks are exposed (figure above b, c). These rocks were metamorphosed during a succession of Precambrian mountain-building events that led to the original growth of continents.
Credits: Stephen Marshak (Essentials of Geology)
