The outcome of metamorphism depends on pressure, temperature, and the abundance of fluid involved, and there are a great many settings with unique combinations of these factors. Some types of metamorphism are characteristic of specific plate tectonic settings, but others are not.
Burial metamorphism occurs when sediments are buried deeply enough that the heat and pressure cause minerals to begin to recrystallize and new minerals to grow, but does not leave the rock with a foliated appearance. As far as metamorphic processes go, burial metamorphism takes place at relatively low temperatures (above 300 °C) and pressures (100s of metres depth). One rock that can form in this setting is metaconglomerate (Figure 10.17). It looks like a regular conglomerate, except the clasts have become elongated.
Regional metamorphism refers to the large-scale metamorphism that happens to continental crust along convergent tectonic margins (where plates collide). The collisions result in the formation of long mountain ranges, like those along the western coast of North America. The force of the collision causes rocks to be folded, and broken and stacked on each other, so not only is there the squeezing force from the collision, but the lithostatic pressure from the weight of rocks being stacked on top of each other. The deeper rocks are within the stack, the higher the pressures and temperatures, and the higher the grade of metamorphism that occurs. Rocks that form from regional metamorphism are likely to be foliated because of the strong directional pressure of converging plates.
The Himalaya range is an example of where regional metamorphism is happening because two continents are colliding (Figure 10.18). Sedimentary rocks have been both thrust up to great heights (nearly 9,000 m above sea level) and also buried to great depths. Considering that the normal geothermal gradient (the rate of increase in temperature with depth) is around 30°C per kilometre in the crust, rock buried to 9 km below sea level in this situation could be close to 18 km below the surface of the ground, and it is reasonable to expect temperatures up to 500°C. In Figure 10.17 the dashed lines are isotherms– lines of equal temperature– resulting from the geothermal gradient. Notice the sequence of rocks that from, beginning with slate higher up where pressures and temperatures are lower, and ending in migmatite at the bottom where temperatures are so high that some of the minerals start to melt. These rocks are all foliated, and that is to be expected because of the strong compressing force of the converging plates.
Seafloor (Hydrothermal) Metamorphism
At an oceanic spreading ridge, recently formed oceanic crust of gabbro and basalt is slowly moving away from the plate boundary (Figure 10.19). Water within the crust is forced to rise in the area close to the source of volcanic heat, and this draws more water in from farther out, which eventually creates a convective system where cold seawater is drawn into the crust, heated to 200° to 300°C as it passes through the ocean crust, and released again onto the sea floor near the ridge.
The passage of this water through the oceanic crust at 200° to 300°C promotes metamorphic reactions that change the original pyroxene in the rock to chlorite ((Mg5Al)(AlSi3)O10(OH)8) and serpentine ((Mg, Fe)3Si2O5(OH)4). The low-grade metamorphism occuring at these relatively low pressures and temperatures can turn mafic igneous rocks in ocean crust into greenstone (Figure 10.20), a non-foliated metamorphic rock.
Chlorite and serpentine are both hydrated minerals, containing water in the form of OH in their crystal structures. When metamorphosed ocean crust is later subducted, the chlorite and serpentine are converted into new non-hydrous minerals (e.g., garnet and pyroxene) and the water that is released migrates into the overlying mantle, where it contributes to flux melting (Chapter 7, section 7.1).
Subduction Zone Metamorphism
At subuction zones, where oceanic crust is forced down into the hot mantle, there is a unique combination of relatively low temperatures and very high pressures. The high pressures are to be expected, given the force of collision between tectonic plates, and the increasing lithostatic pressure as the subducting slab is forced further and further into the mantle. The lower temperatures exist because even though the mantle is very hot, the ocean crust is relatively cool, and doesn’t take in heat from the mantle rapidly. That means it can be several hundreds of degrees cooler than the surrounding mantle. In Figure 10.21, notice that the isotherms (dotted lines) plunge deep into the mantle along with the subducting slab. This means that regions of relatively low temperature exist deeper in the mantle.
A special type of metamorphism takes place under these very high-pressure but relatively low-temperature conditions, producing an amphibole mineral known as glaucophane (Na2(Mg3Al2)Si8O22(OH)2). Glaucophane is blue in colour, and the major component of a rock known as blueschist (Figure 10.22).
If you’ve never seen or even heard of blueschist, that not surprising. What is surprising is that anyone has seen it! Most of the blueschist that forms in subduction zones continues to be subducted. It turns into eclogite at about 35 km depth, and then eventually sinks deep into the mantle, never to be seen again. In only a few places in the world, the subduction process was interrupted, and partially subducted blueschist rock returned to the surface. One such place is the area around San Francisco. The blueschist is part of a set of rocks known as the Franciscan Complex (Figure 10.22).
Contact metamorphism happens when a body of magma intrudes into the upper part of the crust. Heat is important in contact metamorphism, but pressure is not a key factor, so contact metamorphism produces non-foliated metamorphic rocks such as hornfels, marble, and quartzite.
Any type of magma body can lead to contact metamorphism, from a thin dyke to a large stock. The type and intensity of the metamorphism, and width of the metamorphic aureole will depend on a number of factors, including the type of country rock, the temperature of the intruding body and the size of the body (Figure 10.23). A large intrusion will contain more thermal energy and will cool much more slowly than a small one, and therefore will provide a longer time and more heat for metamorphism. That will allow the heat to extend farther into the country rock, creating a larger aureole.
Contact metamorphic aureoles are typically quite small, from just a few centimetres around small dykes and sills, to as much as 100 m around a large stock. Contact metamorphism can take place over a wide range of temperatures — from around 300° to over 800°C. Different minerals will form depending on the exact temperature and the nature of the country rock.
Although bodies of magma can form in a variety of settings, one place magma is produced in abundance, and where contact metamorphism can take place, is along convergent boundaries with subduction zones, where volcanic arcs form (Figure 10.24). Regional metamorphism also takes place in this setting, and because of the extra heat associated with the volcanism, the geothermal gradient is typically a little steeper in these settings (somewhere between 40° and 50°C/km). That means higher grades of metamorphism can take place closer to surface than is the case in other areas (note the foliated metamorphic rocks listed on the right-hand side of the diagram).
When extraterrestrial objects such as meteorites and asteroids hit the Earth, the result is a shock wave. Where the object hits, pressures and temperatures become very high in a fraction of a second. A “gentle” impact can hit with 40 GPa and raise temperatures to up to 500 °C. Pressures in the lower mantle start at 24 GPa (giga Pascals), or and climb to 136 GPa at the core-mantle boundary, so the impact is like plunging the rock deep into the mantle and releasing it again within seconds. The sudden change associated with shock metamorphism makes it very different from other types of metamorphism which can develop over hundreds of millions of years, starting and stopping as tectonic conditions change.
Two features of shock metamorphism are shocked quartz, and shatter cones. Shocked quartz (Figure 10.25 left) refers to quartz crystals which display damage that shows up as parallel lines throughout the crystal. The quartz crystal in Figure 10.25 has two sets of these lines. The lines are small amounts of glassy material within the quartz. Shatter cones are cone-shaped fractures within the rocks (Figure 10.25 right). The fractures are nested together like a stack of ice-cream cones, and point in the direction of the impact.