Types of Metamorphism
Geologists can duplicate metamorphic conditions in the laboratory and determine the precise combinations of pressure, temperature, and parent rock composition under which particular transformations might take place. But to understand when, where, and how such conditions came about in Earth’s interior, we must categorize metamorphic rocks on the basis of their geologic settings (Figure 6.4).
154
Regional Metamorphism
Regional metamorphism, the most widespread type of metamorphism, takes place where both high temperatures and high pressures are imposed over large parts of the crust. We use this term to distinguish this type of metamorphism from more localized transformations near igneous intrusions or faults. Regional metamorphism is a characteristic feature of convergent plate boundaries. It occurs in volcanic mountain belts, such as the Andes of South America, and in the cores of mountain chains produced by continent-continent collisions, such as the Himalaya of central Asia. These mountain chains are often linear features, so zones of regional metamorphism are often linear in their distribution. In fact, geologists usually interpret regionally extensive belts of metamorphic rocks as representing sites of former mountain chains that were eroded over millions of years, exposing the rocks at their core.
Some regional metamorphic belts are created by the high temperatures and moderate to high pressures near volcanic mountain belts formed where subducted plates sink deep into the mantle. Others are formed under the very high pressures and temperatures found deeper in the crust along boundaries where colliding continents deform rock and raise high mountain chains. In both cases, the metamorphosed rocks are typically transported to great depths in Earth’s crust, then eventually uplifted, exposed, and eroded at Earth’s surface. A full understanding of the patterns of regional metamorphism, including how rocks respond to systematic changes in temperature and pressure over time, depends on an understanding of the specific plate tectonic settings in which metamorphic rocks form. We will discuss that topic later in this chapter.
Contact Metamorphism
In contact metamorphism, the heat from an igneous intrusion metamorphoses the rock immediately surrounding it. This type of localized transformation normally affects only a thin zone of country rock along the zone of contact. In many contact metamorphic rocks, especially at the margins of shallow intrusions, the mineral and chemical transformations are largely related to the high temperature of the intruding magma. Pressure effects are important only where the magma is intruded at great depths. Here, the pressure results not from the intrusion forcing its way into the country rock, but from the presence of regional confining pressure. Contact metamorphism by volcanic deposits is limited to very thin zones because lavas cool quickly at Earth’s surface and their heat has little time to penetrate the surrounding rocks deeply and cause metamorphic changes. Contact metamorphism may also affect xenoliths that are not completely melted. Blocks of rock up to several meters wide may be torn off the sides of magma chambers and completely surrounded by hot magma. Heat projects into these xenoliths from all directions, and they may become completely metamorphosed.
Seafloor Metamorphism
Another type of metamorphism, a form of metasomatism called seafloor metamorphism, is often associated with mid-ocean ridges (see Chapter 4). Hot basaltic lava at a seafloor spreading center heats infiltrating seawater, which starts to circulate through the newly forming oceanic crust by convection. The increase in temperature promotes chemical reactions between the seawater and the rock, forming altered basalts whose chemical compositions differ from that of the original basalt. Metasomatism resulting from percolation of high-temperature fluids also takes place on continents when hydrothermal solutions circulating near igneous intrusions metamorphose the rocks they intrude.
Other Types of Metamorphism
There are several other types of metamorphism that produce smaller amounts of metamorphic rock. Some of these types are extremely important in helping geologists understand conditions deep within Earth’s crust.
Burial Metamorphism
Recall from Chapter 5 that sedimentary rocks are transformed by diagenesis as they are gradually buried. Diagenesis grades into burial metamorphism, low-grade metamorphism that is caused by the progressive increase in pressure exerted by the growing layers of overlying sediments and sedimentary rocks and by the increase in heat associated with increased depth of burial.
Depending on the local geothermal gradient, burial metamorphism typically begins at depths of 6 to 10 km, where temperatures range between 100°C and 200°C and pressures are less than 3 kbar. This fact is of great importance to the oil and gas industry, which defines its “economic basement” as the depth where low-grade metamorphism begins. Oil and gas wells are rarely drilled below this depth because temperatures above 150°C convert organic matter trapped in sedimentary rocks into carbon dioxide rather than crude oil and natural gas.
High-Pressure and Ultra-High-Pressure Metamorphism
Metamorphic rocks formed by high-pressure metamorphism (at 8 to 12 kbar) and ultra-high-pressure metamorphism (at pressures greater than 28 kbar) are rarely exposed at Earth’s surface for geologists to study. These rocks are rare because they form at such great depths that it takes a very long time for them to be recycled to the surface. Most high-pressure metamorphic rocks form in subduction zones as sediments scraped from subducting oceanic crust are plunged to depths of over 30 km, where they experience pressures of up to 12 kbar.
155
Unusual metamorphic rocks once located at the base of Earth’s crust can sometimes be found at Earth’s surface. These rocks, called eclogites (see Figure 3.27), may contain minerals such as coesite (a very dense, high-pressure form of quartz) that indicate pressures of greater than 28 kbar, suggesting depths of over 80 km. Such rocks form at moderate to high temperatures, ranging from 800°C to 1000°C. In a few cases, these rocks contain microscopic diamonds, indicative of pressures greater than 40 kbar and depths greater than 120 km! Surprisingly, outcrop exposures of these ultra-high-pressure metamorphic rocks may cover areas greater than 400 km by 200 km. The only other two rocks known to come from these depths are diatremes and kimberlites (see Chapter 12), igneous rocks that form narrow pipes just a few hundred meters wide. Geologists agree that these latter rock types form by volcanic eruption, albeit from very unusual depths. In contrast, the mechanisms required to bring eclogites to the surface are hotly debated. It appears that these rocks represent pieces of the leading edges of continents that were subducted during continent-continent collisions and subsequently rebounded (via some unknown mechanism) to the surface before they had time to recrystallize at lower pressures.
Shock Metamorphism
Shock metamorphism occurs when a meteorite collides with Earth. Upon impact, the energy represented by the meteorite’s mass and velocity is transformed into heat and shock waves that pass through the impacted country rock. The country rock can be shattered and even partially melted to produce tektites. The smallest tektites look like droplets of glass. In some cases, quartz is transformed into coesite and stishovite, two of its high-pressure forms.
Most large impacts on Earth have left no trace of a meteorite because these bodies are usually destroyed in the collision with Earth. The occurrence of coesite and craters with distinctive fringing fractures, however, provides evidence of these collisions. Earth’s dense atmosphere causes most meteorites to burn up before they strike its surface, so shock metamorphism is rare on Earth. On the surface of the Moon, however, shock metamorphism is pervasive. It is characterized by extremely high pressures of many tens to hundreds of kilobars.