3. Definition of Metamorphism
Meta 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
4. The Limits of Metamorphism 1
Lower 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
5. 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
6. Types of Pressure that causes Metamorphism
Confining Pressure Directed Pressure
The effect of the mass of The effect of tectonic forces
the overlying rocks. It acts operating at convergent
equally in all directions and (destructive) plate margins
increases with depth
8. Metamorphism - Effects
Destruction/distortion of fossils
Destruction of sedimentary structures
Hardening of the rock
Colour changes to the rock
Growth of new metamorphic minerals
Alignment of minerals in the rock
Metamorphism
Shale Schist
9. Metamorphic Grade
The extent to which the pre-existing
rocks have been changed in form/altered
Shale-Country Rock Slate-Low Schist-Medium Gneiss-High
Low Grade – slight alteration
Medium Grade – significant alteration
High Grade – extensive/total alteration
10. Metamorphic Changes 1
Are 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
11. Metamorphic Changes 2
Recrystallisation takes place in the solid state during
metamorphism, there is no melting of rock involved
Spherical grains of calcite Angular crystals of calcite
Metamorphism
Oolitic Limestone Marble
Analogy - snow recrystallises to ice without melting
12. Types of Metamorphism
Contact 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
13. 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
Sandy Shale
Recrystallisation occurs
with a random
orientation of minerals Sandstone Granite
(non-foliated)
14. Dynamic Metamorphism
Associated with major
San Andreas Fault
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
15. Regional Metamorphism
Associated 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 Regional rocks have a foliated
Metamorphism texture with minerals being
aligned parallel to each other
16. 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
