Topic 8:  METAMORPHISM: Making New Crystals without Melt

Metamorphism - all changes in mineral assemblage and rock texture 
that take place in rocks within the Earth's crust as a result of chemical 
reactions in the presence of high temperature and differential stress.
Geologists are attempting to find the boundaries of ancient continents 
by mapping zones of metamorphic rocks.  They are also using the ages 
of metamorphic rocks to determine how long plate tectonics has been 
active.





Basic concepts - the following terms deal with the deformation of rocks.

Force - mass times acceleration.  Forces are best described as pushes 
or a pulls.  When you weigh yourself on a bathroom scales your body 
is forcing the scales down toward the center of the earth.  The amount 
of force exerted by your body is indicated in pounds.

Pressure - force per unit area.  Atmospheric pressure is 14.5 pounds 
per square inch.  Pressure is exerted by liquids and gases and is equal 
in all directions.

Stress - the term we use to denote force per unit area within a solid 
such as rock.  A tensional stress stretches rocks, compressional stress 
squeezes them, while a shear stress causes slippage or translation.

Differential Stress - solids such as rocks can support different 
magnitudes of stress in different directions.   The difference between 
the largest and smallest stress is called differential stress.  The largest 
and smallest stress always act at right angles. An intermediate stress 
acts normal to the plane containing the largest and smallest stress.

Strength - the maximum differential stress which a rock can absorb 
before fracturing (brittle deformation) or flowing (ductile deformation).

Strain - a measure of deformation.  Defined as a change in length per 
unit length.

Elastic deformation - a rock subject to a small differential stress will 
deform (i.e., strain) slightly but bound back to its initial shape when the 
stress is removed.  

Brittle deformation - a rock subject to a large differential stress at low 
temperature or high rate of deformation will fracture, crack, or shatter 
like glass.  This type of deformation often leads to faults and earthquakes 
in the Earth.  The differential stress at which a rock fractures is called
its brittle strength.  As differential stress often increases from zero, 
a rock will deform elastically before reaching its brittle strength.

Ductile deformation - a rock subject to differential stress at high 
temperature or slow rate of deformation will flow like putty.  This 
type of deformation often leads to folds in the Earth.





Factors controlling metamorphism - high pressure and high temperature 
causes a series of chemical reactions in metamorphic rocks.

Confining Pressure - pressure applied equally on all surfaces of a body.  
For example, a diver below the surface of the ocean is subject to water 
pressure in all directions.  In the earth we say use the term confining 
pressure when we mean that rock stress is equal in all directions.

Temperature - The most important parameter in driving reactions to 
form new minerals from old.  The geothermal gradient is a measure of 
the rate in increase in temperature with depth in the Earth's crust.  A 
typical value might be 25¡C per kilometer.

Chemically Reactive Fluids - some of the dissolved constituents of rock 
move from the fluid to form new minerals.

Differential stress - Plate tectonic forces cause larger stress in one 
direction than in other directions.  This difference in stress is responsible 
for the deformed nature of metamorphic rocks.

Time - a certain amount of time is required for chemical reactions to 
equilibrium.




Metamorphic Textures -  most metamorphic rocks develop a fabric which 
is characterized by planar surfaces composed of clay or mica flakes.  
The orientation of the planar surfaces is controlled by the stress to 
which the metamorphic rock is subjected.  

Foliation - parallel flakes of mica grown during metamorphic reactions 
produce a planar fabric called foliation.

Cleavage - another word for foliation

Slaty Cleavage - a pervasive foliation formed within a fine-grained rock 
such as shale. In these low-grade metamorphic rocks the mica flakes are 
microscopic.  Slate which is metamorphosed shale is often used as roofing 
tiles.

Schistosity - the foliation that develops when mica grains grow to be 
visible as the temperature increases.  The mica is still pervasive and 
mixed evenly between quartz and feldspar grains in a rock called a 
chist.

Gneiss -  a rock with foliation but in which quartz and feldspar become 
segregated between layers of mica.

Kinds of Metamorphism -  different processes cause changing temperature 
and stress.

Fault zone metamorphism - when rock is subject to great differential 
stress grains are crushed and shattered, particular along deep fault zones.  
Sometimes deformation along fault zones is plastic like the deformation 
of modeling clay.  If this is the case the fault rock is called a mylonite.

Contact metamorphism - occurs adjacent to bodies of hot magma that 
are intruded into cool rocks of the crust.  The zone of contact 
metamorphism is often controlled by the amount of water about the 
magma.

Burial metamorphism - when sediments are buried deep within a basin 
they can heat to a temperature of 300¡C.  Abundant pore water is present 
which helps chemical recrystallization.

Regional metamorphism - large regions of the Earth become deeply buried.





Metamorphic facies - the combination of minerals which are stable at 
certain pressures and temperatures.  In many metamorphic rocks 
microscopic relicts of earlier mineral assemblages remain; by analyzing 
these relicts, scientists can decipher the way pressure and temperature 
changed in time.





Water in metamorphism -  hot water is involved to some extent in most 
metamorphic processes.

Wet metamorphism - water merely transporting ions between grains in 
rock.

Metasomatism - water brings ions from outside the rock, and they are 
added to the rock during metamorphism.  These ions are added by incorporation 
into newly crystallizing minerals.  Other ions may be dissolved and removed.

Hydrothermal rocks - water passes through cracks or pore spaces in rock 
and precipitates minerals on the walls of cracks and within pore spaces.  
Here the new minerals are grown entirely from those ions carried in 
solution. Hydrothermal rocks are most commonly found in veins.