Download metamorphism and metamorphic rocks

12 September 2002
GE3025: Lecture #8
METAMORPHISM AND METAMORPHIC ROCKS
Metamorphism is the process by which conditions within the Earth alter the mineral content and
structure of solid rock without melting it. Metamorphic rocks generally form under temperature,
pressure, and chemical conditions that exist beneath the zone of diagenesis but above the 50-250
km deep “zone of melting” where magmas are formed. Metamorphic processes never break all the
bonds in a rock’s minerals- if all the bonds were broken the rock would melt, making it an igneous
process.
The four conditions that promote metamorphism are heat, pressure, fluids, and parent rock.
Temperatures sufficient to metamorphose rocks (>200° C) are generally reached at about 10km
depth. Rocks subjected to either confining or directed pressure change in a number of ways,
including the development of foliation. Fluids greatly increase the potential for metamorphism by
facilitating the migration of unbonded atoms and ions.
Some of the common metamorphic rocks are named according to their appearance and conditions of
formation, while others are classifed by their composition and specific type of parent material.
Generally, foliated metamorphic rocks are named according to their appearance, while unfoliated
metamorphic rocks are named based on their composition.
Foliated metamorphic rocks form in response to directed pressure and heat. The directed pressure
allows the plate-like clays, micas, and chlorite to align and develop one of the most diagnostic
features of foliated metamorphic rocks: cleavage. Even when the individual mineral grains are
too small to be seen (as in slate), the cleavage of the rock is a readily visible feature. Unfoliated
metamorphic rocks form in response to high temperatures and/or confining pressure from deep
burial, and do not develop cleavage.
One of the important characteristics by which foliated metamorphic rocks are names is the size of the
clay and mica mineral grains within the rock. The finest-grained foliated metamorphic rocks are
formed from shales and mudstones and are called slates and phyllites. Coarser grains of micas
and chlorite are found in schist. Gneisses usually have a wide variety of minerals including
quartz, hornblende (an amphibole), augite (a pyroxene), and feldspars. Gneisses often have a
“banded” appearance, unlike slates, phyllites, and shists, which tend to look more “sheet-like”
because of the predominance of sheet silicate minerals.
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Slates have microscopic grains of clays, micas and chlorite; phyllites have grains of micas
and chlorite that are just visible. Phyllite can be recognized from slate by its sheen, which
comes from reflections off the surface of the flake-like grains of mica and chlorite. When
the grains of mica and chlorite become larger (> 1cm) the rock is classified as a schist.
Schist is a coarsely-foliated rock that splits readily into flat slabs- a distinctive cleavage
known as schistosity.
Gneisses from through the process of metamorphic differentiation, whereby ions from
neighboring minerals form bands of dark and light minerals.
Migmatites form when minerals with low melting temperatures (quartz, potassium
feldspar) melt and form igneous textures, while minerals with higher melting temperatures
(hornblende, pyroxenes) “flow” into intricately folded bands.
Schists and gneisses may also form from metamorphism of multimineral igneous rocks. Greenschists
form when olivines, pyroxenes, and calcic plagioclase from basalt are converted to more stable
minerals that include chlorite.
Nonfoliated metamorphic rocks can form from increased heat and pressure during deep burial, or
from contact with heat from an intruding body of magma. Sedimentary rocks that consist
primarily of a single mineral (like limestone and sandstone) typically recrystallize as coarsegrained nonfoliated rocks.
Marbles form from relatively pure limestone and dolostones; less pure limestones typically form
skarns, which contain an abundance of calcium-rich silicate minerals.
Pure sandstone metamorphoses to quartzite, which is an extremely hard rock.
Hornfels forms when hot magma intrudes a shale or basalt bedrock. Water is driven off by the
heat and minerals with more compact structures are formed, forming a dark-colored, dense,
and hard rock.
Week #4
There are 2 primary types of metamorphism: contact and regional. Contact metamorphism occurs
when magma intrudes bedrock, and results almost entirely from the effects of heat and circulating
hot fluid. Most intrusive igneous bodies are surrounded by a contact metamorphic aureole.
Pressure is not an important factor in contact metamorphism.
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