Regional metamorphic rocks occur where rocks are altered by high temperatures and / or high pressures usually deep within the Earth.
Regional metamorphism can affect large volumes of the crust and typically happens beneath new mountain ranges.
When rocks are metamorphosed at high temperatures and high pressures they may be converted to gneiss in which light and dark minerals separate into bands (foliation).
The red-brown blobs in this rock are crystals of the mineral garnet which have grown during metamorphism and were then deformed.
These rocks were likely heated to around 600 degrees Celsius deep within the crust.
The original rocks were likely fine-grained sedimentary strata but the there is no indication of the original layers.
∗ Gneiss and Amphibolite
This outcrop near Albany in Western Australia shows high-grade gneiss (light coloured rock with grey bands) that was probably originally granite.
The dark material is a block of amphibolite which is metamorphosed dolerite. The amphibolite was likely an intrusion of dolerite in the granite.
The layering in the gneiss is foliation that was produced during initial metamorphism. The foliation is clearly bent and twisted (folded) by later compression as are the light coloured bands in the amphibolite which were layers of melted rock.
These rocks were heated to temperatures above 600 degrees Celsius.
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