Metamorphic rock is the Earth's memory, preserving the intense history of heat and pressure that once buried it deep within the planet. This rock arises from the transformation of existing rock into new types through a process called metamorphism when the original rock is subjected to temperatures greater than 100 degrees Celsius and often elevated pressures of 1000 bars or more.
The importance of heating in the formation of metamorphic rock was first noted by the pioneering Scottish naturalist James Hutton in 1795. James Hutton wrote that some rock beds of the Scottish Highlands had originally been sedimentary rock but had been transformed by great heat.
Index minerals such as sillimanite, kyanite, staurolite, andalusite, and some garnet indicate the approximate temperatures and pressures at which the rock underwent metamorphism. For instance, the mineral kyanite transforms to andalusite at a temperature of about 500 degrees Celsius and andalusite transforms to sillimanite when the temperature reaches about 600 degrees Celsius.
Recrystallization changes the particle size of metamorphic rock by transforming small calcite crystals in limestone into larger crystals in marble. High temperatures allow atoms and ions in solid crystals to migrate and reorganize the crystals while high pressures cause solution of the crystals within the rock at their point of contact.
Foliation is a distinctive layering in metamorphic rock derived from the Latin word folia meaning leaves that develops when a rock is being shortened along one axis during recrystallization. This causes crystals of platy minerals such as mica and chlorite to become rotated such that their short axes are parallel to the direction of shortening.
On the 17th of August 1959, a magnitude 7.2 earthquake destabilized a mountain slope near Hebgen Lake in Montana composed of schist. This caused a massive landslide that killed 26 people camping in the area.