The amount of heat and pressure is the main factor
that control mineral development. The length of time of metamorphism is
also a major factor on the eventual rock. The varying temperature and
pressures are found in different regions of the Earth, depending on its
position according to the plate tectonic model.
How do we know this? It took geologists a long
time to understand metamorphic rocks. They mapped the different area and
found that minerals were clues to the temperature and pressure an area
George Barrow, in the late 1900's was the first to
map zones in Scotland. He recognized six zones on the first appearance
of six minerals including chlorite, biotite, garnet, staurolite, kyanite,
and sillimanite. He did not know for sure if the zones had anything to
do with the physical conditions of metamorphism, because he could not
confirm his observations in a lab.
Simplified geologic map