Metamorphism refers to a set of processes that result in changes in mineralogy and texture accompanying changes in temperature and pressure.
In other words, transformation of existing rock, usually beneath the Earth's surface, as a consequence of one (or a combination) of three agents: heat, pressure, and fluids.
The boundary between diagenesis (sedimentary process) and the onset of metamorphism is one of semantics.
Contact Metamorphism - nearby heat source - an intrusive igneous body is injected into a colder, older rock - heat flows from the intrusive body into the country rock
May also involve fluid flow or exchange
Regional Metamorphism - no obvious, local heat source - increasing depth of burial plus deformation results in an increase in temperature and pressure
Remember the geothermal gradient
Dynamic Metamorphism - variable pressure at relatively low temperatures - often associated with fault zones
Commonly reduces grain size
High Pressure - Low Temperature Metamorphism - associated with subduction zones
Characterized by unusual minerals - blueschists
Increasing Grain Size:
Increasing Temperature and Pressure may aid in the recrystallization of minerals in the rock
Small grains become larger - oriented with respect to direction of applied pressure(s) - stress
Clay minerals are often enlarged with increasing metamorphism
Growth of New Minerals:
New minerals may grow during metamorphism
CaCO3 + SiO2 = CaSiO3 + CO2
The presence of wollastonite can be used as an indicator of the degree of metamorphism
ISOGRAD - a line on a map connecting points of equal degrees of metamorphism
Is the rock banded? - each band is often a single mineral - GNEISS
Does the rock exhibit foliation - parallelism of the cleavage of micas?
Schist - coarse flakes
Phyllite - fine - barely visible flakes, shiny surface
Slate - very fine
If the rock is neither foliated nor banded it is called a granofels if it is coarse grained or a hornfels if it is fine grained
Marble - a metamorphosed limestone
Quartzite - a metamorphosed quartz sandstone
A function of the pressure, temperature and composition of the parent rock - the Protolith
Marble - had a parent rich in carbonate
Quartzite - quartz sandstone parent
Slate, Schist, Gneiss - clay mineral rich parent
Metamorphic Facies - attempts to deduce degree of metamorphism by looking at index minerals, rock fabric
Divergent Boundaries: contact metamorphism, basaltic lavas in contact with sediments; dikes in contact with host rock
Convergent Boundaries: Subduction Zones: high pressure/low temperature metamorphism
Convergent Boundaries: Continent/Continent collision - regional metamorphism
Transform boundaries: two plates sliding past one another can generate higher pressures without much heat - dynamic metamorphism