Transcript metamorphic_lab

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
Foliation--Slaty Cleavage
• Alters orientation of
existing minerals
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…
Sedimentary rock with
distinct bedding.
Original Rock
Metamorphism
Slate
Schist
Low Grade
Foliation perpendicular to
direction of maximum
differential stress
Gneiss
High Grade
Foliation and crystallization
perpendicular to direction of
maximum differential stress
Foliation, crystallization and
differentiation perpendicular to
direction of maximum differential
stress
http://www.tulane.edu/~sanelson/geol212/contactmeta.htm
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 crosssection
Metamorphic Grade