Regional or Barrovian metamorphism covers large areas of continental crust typically associated with mountain ranges. At 10 km depth, the temperature is about 300°C and at 20 km it’s about 600°C. When rocks are buried deep in the crust, regional metamorphism occurs. Most regional metamorphism takes place within the continental crust. Studies linking tectonic environments to types of metamorphic rocks, with key examples from the Pacific Rim and Alpine regions, were published as plate tectonic theory became widely accepted (e.g., Miyashiro, 1967, 1973; Ernst, 1971). On the other hand, most clay minerals are only stable up to about 150° or 200°C; above that, they transform into micas. At 10 to 15 kilometres, we are in the greenschist zone (where chlorite would form in mafic volcanic rock) and very fine micas form in mudrock, to produce phyllite. Large geological processes such as mountain-building cause regional metamorphism. For this reason, it is very difficult to study metamorphic processes in a lab. the transformation of a parent rock into a new rock as a result of heat and pressure that leads to the formation of new minerals, or recrystallization of existing minerals, without melting, the original, un-metamorphosed parent rock from which a given metamorphic rock is formed. Considering that the normal geothermal gradient (the rate of increase in temperature with depth) is around 30°C per kilometre, rock buried to 9 kilometres below sea level in this situation could be close to 18 kilometres below the surface of the ground, and it is reasonable to expect temperatures up to 500°C. In only a few places in the world, where the subduction process has been interrupted by some tectonic process, has partially subducted blueschist rock returned to the surface. Because burial to 10 km to 20 km is required, the areas affected tend to be large. As we learned in the context of igneous rocks, mineral stability is a function of temperature, pressure, and the presence of fluids (especially water). This is commonly associated with convergent plate boundaries and the formation of mountain ranges. Pressure is important in metamorphic processes for two main reasons. belts at convergent plate boundaries Hikaru Iwamori Department of Earth and Planetary Sciences, University of Tokyo, Tokyo, Japan Received 2 February 2002; revised 31 December 2002; accepted 25 February 2003; published 28 June 2003. Because burial is required from 10 km to 20 km, the affected areas tend to be large. By way of example, if we look at regional metamorphism in areas with typical geothermal gradients, we can see that burial in the 5 km to 10 km range puts us in the zeolite[1] and clay mineral zone (see Figure 7.20), which is equivalent to the formation of slate. Are certain types of metamorphic rocks indicative of particular plate boundaries or tectonic settings? Most regional metamorphism takes place within continental crust. The movement of tectonic plates transports sediment and rocks into different geologic setting—these changes can result in metamorphism, particularly in zones where tectonic plates are converging, as in a subduction zone or where continental plates converge, pushing up high mountain ranges while material below the mountains are pushed down under increasing temperature and pressure condition. Metamorphic rocks formed there are likely to be foliated because of the strong directional pressure of converging plates. Because this metamorphism takes place at temperatures well below the temperature at which the rock originally formed (~1200°C), it is known as retrograde metamorphism. Metamorphism and Plate Tectonics Metamorphic rocks result from the forces active during plate tectonic processes.