is the solid state transformation of preexisting rock into texturally or
mineralogically distinct new rock, as a result of high temperature and pressure,
new rock ===== metamorphic rock in nearly all cases has a different texture and
very common different mineral composition from the original (called) parent
sandstone+calcite cement === wollastonite+CO2
changes occur in order to restore equilibrium for the gradually changign
Pressure and temperature.
agents of metamorphizm are:
of the original (parent) rocks
1. Temperature Changes
is said to be STABLE, if given enough time it does not react to become a new
Temperature is probably the most important factor in metamorphism. As the
temperature of the rocks changes during metamorphism its minerals become
unstable and react with other minerals to form a new mineral
assemblage, that are stable under the new conditions.
is also important because it increases the reaction rate.
higher temp. burn everything)
200°C the reaction rate is low, this is the sedimentary environment, and rocks
are stable for millions of years.
extreme temperature increase, minerals start to melt, and become igneous rocks.
At the transitional boundary, some rock forms. The rocks forming here are called
a high temperature metamorphic rock cools down, the minerals became unstable
again, however at the low temperature environment reaction rate is low, these
minerals are called METASTABLE
2. Pressure changes
are two kinds of pressure:
equally on all surfaces (Hydrostatic)
in the water (his whole body has the same pressure everywhere)
high pressure tend to reduce the space occupied by the different minerals,
and can drive reactions which produce new minerals with closer atomic
applied unequally on the surface of a body is directed or (DYNAMIC) pressure.
This pressure tend to deform objects into spindle-shaped or flattened
form.directed compressive pressure
by directed pressure also, but here the parts of the body move or slide relative
to one another and paralell to the forces. (rolling pin)
pressure usually changes abruptly along tectonically active zones. (huge
pressure the minerals became ductile and can be slowly deformed as the grains
are slowly move and rotate. Deformation reorients the mineral grains and form
3. Chemical activity during
active fluids mostly water containing ions in solution, are also very important
in metamorphic processes. Some fluid occur in almost every kind of rocks. In
addition minerals stable at low P,T, such as amphibol, clay, mica are contain OH
in their crystalline structure. At high T,P, this water is forced out from the
mineral structure and is good agent in the rock for further chemical changes.
(The mineral changes caused by the hot fluids are called METASOMATISM).
pressure has important role in forming met. texture.
paralell aligment of textural and structural features of rocks are called
The rock splits along nearly flet and
paralell planes indicating that
microscopic platy minerals were pushed into aligment during metamorphism.
Visible needle-shaped minerals have grown essentially parallel to one
another during directed pressure
Rock became very plastic and the new minerals are separated into bundles
of different minerals.
large crystals in metamorphic rocks
OF METAMORPHYC ROCKS
PRESSURE IS NOT IMPORTANT, THEREFORE THESE ROCKS ARE FREQUENTLY NONFOLIATED.
country rocks Contact metamorphism
(1 - 10 M WIDE)
IS changed into hornfels, with visible micas
fault zones, Pressure dominant temperature is not important
breccia, (force crashes the rock into small fragments)
flow can form milonite along fault zone at larger depth
temp. and Pressure
minerals are good indicators for certain pressure and temperature range. They
are called Index minerals
felsic igneous rock