Physical Geology Introduction
METAMORPHIC ROCKS
Metamorphism refers to a set of processes
that result in changes in mineralogy and
texture accompanying changes in
temperature and pressure.
As noted previously, the boundary between
diagenesis (sedimentary) and the onset of
metamorphism is truly fuzzy.
Metamorphic Types
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
Regional Metamorphism - no obvious heat
source - increasing depth of burial plus
deformation results in an increase in
temperature and pressure
Dynamic Metamorphism - variable pressure
at relatively low temperatures - often
associated with fault zones
n High Pressure - Low Temperature
Metamorphism - associated with subduction
zones
Effects of Metamorphism
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 degree of metamorphism
Classification
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
Phyllite - fine - barely visible
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 metamorphsed quartz arenite
Degree of Metamorphism
A function of the 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
Plate Tectonics and Metamorphism
Divergent Boundaries - contact
metamorphism : basaltic lavas in contact
with sediments
Convergent Boundaries : Subduction Zones :
high pressure/low temperature
metamorphism
Convergent Boundaries :
Continent/Continent collision - regional
metamorpism