Post on 22-May-2015
Introduction to Metamorphism
I. G. Kenyon
Metamorphism is just one part of the Rock Cycle
Definition of MetamorphismMeta means ‘change’, morph means ‘form’A change in the form of pre-existing rocks
Igneous, Sedimentary or Metamorphic
Change is caused by an increase in temperature, an increase in pressure or a combination of an
increase in temperature and an increase in pressure
The Limits of Metamorphism 1Lower limit for temperature is 200° C, below
this diagenesis and lithification are taking place and are classified as sedimentary processes
Upper limit for temperature is 800°C, above this rocks begin to melt and igneous processes occur
The Limits of Metamorphism 2
There is no lower limit for pressure but minimal metamorphism takes place at depths of <2km
No upper limit for pressure, but metamorphism is unlikely to occur beyond depths of 45 - 50 km
as the rocks will have melted at this depth
Types of Pressure that causes Metamorphism
Confining Pressure The effect of the mass of the overlying rocks. It acts
equally in all directions and increases with depth
Directed PressureThe effect of tectonic forces
operating at convergent (destructive) plate margins
Relationship between Metamorphism, Temperature and Pressure
Temperature / °C
Dep
th /
km
Pres
sure
/M
Pa
Metamorphism - Effects
Destruction/distortion of fossilsDestruction of sedimentary structures
Hardening of the rockColour changes to the rock
Growth of new metamorphic mineralsAlignment of minerals in the rock
Shale Schist
Metamorphism
The extent to which the pre-existing rocks have been changed in form/altered
Metamorphic Grade
Low Grade – slight alterationMedium Grade – significant alteration
High Grade – extensive/total alteration
Shale-Country Rock Slate-Low Schist-Medium Gneiss-High
Metamorphic Changes 1Are assumed to be isochemical
Bulk chemical composition of parent rock and new metamorphic rock are the same
Both contain the same % Si, Al, O, Na etc.
Only loss from the system is water as hydrous clay minerals are dehydrated by a rise in temperature
Metamorphic Changes 2Recrystallisation takes place in the solid state during metamorphism, there is no melting of rock involved
Analogy - snow recrystallises to ice without melting
Oolitic Limestone Marble
Metamorphism
Spherical grains of calcite Angular crystals of calcite
Types of MetamorphismContact or Thermal Metamorphism – changes
due to an increase in temperature alone
Dynamic Metamorphism – changes due to an increase in pressure alone
Regional Metamorphism – changes due to an increase in both temperature and pressure
Contact Metamorphism
Associated with major igneous intrusions such as plutons and batholiths
The volume of country rock altered by heat from the intrusion is known as the metamorphic aureole
Recrystallisation occurs with a random
orientation of minerals (non-foliated)
GraniteSandstone
Sandy Shale
Dynamic MetamorphismAssociated with major fault planes such as the San Andreas Fault and
the Moine Thrust
Relatively localised or small scale limited to a few metres either
side of the fault plane
Rocks are crushed and ground into angular
fragments under intense shear pressure
San Andreas Fault
Regional MetamorphismAssociated with destructive plate margins and mountain
building processes
Occurs on a large (regional) scale and involves thousands of km³ of rock
Subducted ocean floor sediments undergo regional
metamorphism from 5 km to 50 km depths
Regional rocks have a foliated texture with minerals being
aligned parallel to each other
Regional Metamorphism
Susceptibility of Rocks to Metamorphism Argillaceous sediments such as clay, mudstone and
shale undergo the greatest mineralogical change
Rich in clay minerals such as chlorite, illite and kaolinite (china clay) which are only stable at low temperatures
and pressures at or very close to the Earth’s surface
Clay minerals are chemically complex and a range of mineral types can form by recrystallization under different
temperature and pressure conditions
Metamorphic Rocks Identification Guide
Schist Happens!