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!