Introduction
Metamorphic rocks are formed through the transformation of pre-existing rocks in a process known as metamorphism (meaning “change in form”). The original rock, or protolith, is subjected to heat and pressure which cause physical, chemical and mineralogical changes to the rock. Protoliths may be igneous, sedimentary or pre-existing metamorphic rocks.
Metamorphic rocks are formed within the Earth’s crust. Changing temperature and pressure conditions may result in changes to the mineral assemblage of the protolith. Metamorphic rocks are eventually exposed at the surface by uplift and erosion of the overlying rock.
There are two main types of metamorphism: regional metamorphism and contact, or thermal, metamorphism. Metamorphic rocks are categorised by texture and mineralogy. Specific zones of temperature and pressure define different metamorphic facies. Rocks containing minerals diagnostic of each facies can be linked to the temperature and pressure that characterise that facies. For example, blueschist and eclogite facies rocks form under low to medium temperatures and high pressures. Such conditions as usually associated with subduction zones. Hornfels facies rocks typically occur under the medium to high temperature and low pressure conditions associated with contact aureoles.
Each metamorphic facies is associated with a group of index minerals from which it can be identified. The exact set of indicator minerals from each group that is present in a particular instance depends on the chemical composition of the protolith. Information about the mineralogy of each metamorphic facies may be found in Grotzinger and Jordan.
Metamorphic facies. Greenschist, amphibolite and granulite conditions are typically associated with regional metamorphism. Blueschist and eclogite are associated with the type of regional metamorphism that occurs at subduction zones. Hornfels and sanidinite are typically associated with contact metamorphism. (Image from Wikipedia Commons.)
Regional Metamorphism
Regional metamorphism usually results in the formation of rocks that are strongly foliated, such as slates, schists, and gneisses (see below). The differential stress needed to produce such foliated rocks usually results from tectonic forces that produce compressional stresses in the rocks such as occur when two continental masses collide. Thus, regionally metamorphosed rocks are found in the cores of fold/thrust mountain belts or in eroded mountain ranges. Compressive stresses result in folding of rock and thickening of the crust, which results in some of the rocks being pushed to deeper levels where they are subjected to higher temperatures and pressures.
The pressure and heat which drive metamorphism come from three sources: the internal heat of the Earth; the weight of the overlying rock; and the horizontal pressures developed as rocks become deformed.
Temperature within the crust increases with increasing depth from the surface by 20 to 60°C per kilometre of depth; 30°C per km is considered to be the average. For example, at a depth of 15km the temperature will be about 450°C. The pressure at 15km depth is derived from the weight of the overlying rock, and is about 4000 times the pressure at the Earth’s surface.