Metamorphic Rocks

Metamorphic Rocks

• Chemical and physical alterations of pre-existing rocks brought about by heat and pressure

• 2 Types– Regional metamorphism– Contact Metamorphism

4 Main Factors

•Temperature•Fluids•Pressure•Deviatoric (directional) Stress

Regional Metamorphism

• Large scale deformation and alteration brought about by mountain building (orogeny) and related processes

• Usually on a larger scale

Contact Metamorphism

• Smaller scale alteration adjacent to intruding magma

Changes in Temperature and Pressure• The heat source is Earth’s

internal heat.– Burial is inevitably

accompanied by an increase in pressure due to the weight of the overlying rocks.

Both temperature and pressure increase with depth

in the Earth resulting in higher metamorphic “grades”

with depth.

Once we have melting, it is no longer a metamorphic rock, it is igneous!!

Differential Stress

• Differential stress occurs if pressure is different in different directions.

– Sometimes called Deviatoric stress

Differential Stress

• Foliation vs Lineation

Foliation

• Alters orientation of existing minerals

Foliation--Slaty Cleavage

Foliation--Schistocity

• Alters orientation of existing minerals

• Crystallization of new minerals

Foliation--Schistocity

• Parent rock is often a fine grained rock– Shale, mudstone, siltstone…

Picture of schist

Foliation--Gneissic banding

• Alters orientation of existing minerals

• Crystallization of new minerals

• Minerals become segregated into bands

Foliation--Gneissic banding

• Parent rock is often a fine grained rock (as is both slate and schist)– Shale, mudstone, siltstone…

http://www.tulane.edu/~sanelson/geol212/contactmeta.htm

Original Rock Sedimentary rock with distinct bedding.

Metamorphism

Foliation perpendicular to direction of maximum differential stress

Foliation and crystallization perpendicular to direction of maximum differential stress

Foliation, crystallization and differentiation perpendicular to

direction of maximum differential stress

Low Grade High Grade

SchistSchistSlateSlate GneissGneiss

Index Mineral – a mineral in a metamorphic rock which indicates a certain metamorphic grade

Isograd – line marking the first appearance of an index mineral on a map or cross-section

Metamorphic Grade