Lecture 14 - Metamorphism and Metamorphic Rocks
Chapter 8
Types of Metamorphism
Regional Metamorphism
- the most widespread type of metamorphism, occurs where high pressures
and temperatures are superimposed over large regions of the crust; this
style of metamorphism results mainly from convergence and thickening of
crustal plates.
Contact Metamorphism
- immediately surrounding an igneous intrusion, the result of advective
heat transfer and conductive heating adjacent to the intrusion.
Cataclastic Metamorphism
- shearing and pulverization along a fault can cause changes in rock
texture; mylonites and cataclastic rocks form in faults, and provide a
record of fault activity.
Hydrothermal Metamorphism
- when large volumes of hot waters and other fluids circulate through
the crust, they can change both the texture and bulk composition of a pre-existing
rock; this style of metamorphism is most commonly associated with igneous
activity, either along mid-oceanic ridges, or in the contact metamorphic
zone around a pluton; hydrothermal metamorphism is a type of metasomatic
transformation.
Burial Metamorphism
- when rocks are buried, they tend to experience higher temperatures,
and thus the simplest style of metamorphism results from deep burial within
large sedimentary basins - where the increased pressure and temperature
is simply the result of deposition of the overlying sediment. Although
this mechanism of metamorphism was once regarded as responsible for regional
metamorphism and the formation of mountain ranges, before development of
plate tectonic theory, it is now considered to be a minor factor in the
evolution of most high-grade metamorphic rocks.
Classification of Metamorphic Rocks
-
As with igneous and sedimentary rocks, metamorphic rocks are classified
on the basis of texture and composition. The principal division in classification
is in regard to whether or not the rock has a foliation.
Foliations are planar arrangements of minerals that are evident at all
scales of observation (from rock outcrop, to hand sample, to microscopic
views). Foliations are generally defined by alignments of micas, and these
alignments are the result of stresses the rock experiences during metamorphism.
The foliations in a metamorphic rock provide a record of the ancient stresses and deformation that the rock experienced. This information about deformation can also be combined with a knowledge of the pressures and temperatures at which metamorphic minerals (like garnet) grow, to evaluate the overall metamorphic history a rock experienced while deep within the earth.
Foliated Metamorphic Rocks (from low to high grade, and
with increasing grain-size):
slate ... finely grained, metamorphic minerals too small to see,
rock has a dull sheen;
phyllite ... coarser, but minerals too small to see without a magnifying
glass; rock has a bright sheen due to increased mica content;
schist ... a coarsely-grained, foliated metamorphic rock;
gneiss ... a coarsely grained, foliated metamorphic rock with compositional
layering.
Metamorphic rocks that lack abundant micas, or that formed
in the absence of a directed stress, tend to lack an alignment of minerals,
and are classified on the basis of composition, or a particular environment
of formation. The Non-Foliated Metamorphic Rocks are:
hornfels - fine-grained, flinty, contact-metamorphic rocks;
marble - composed of calcite and/or dolomite, formed by metamorphism
of limestones and dolostones;
quartzite - composed of quartz, formed by metamorphism of sandstones;
amphibolites - composed of amphiboles, formed by metamorphism of
mafic igneous rocks.