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Metamorphic Rocks
Metaconglomerate
Conglomerate
All metamorphic rocks were originally igneous or sedimentary.
Heat and pressure over long periods of time alter the original rock
The pressure may be from the top
Or sides
The metamorphic process may cause any of the
following to occur to the rock:
1. An overall increase in the rock density
2. The appearance of new mineral crystals (ex. diamond, garnet
graphite, or talc)
3. An alignment of existing crystals or new crystals to
produce a certain foliation
4. A recrystallization of existing minerals (ex. calcite
crystals in the limestone become altered to
form marble)
Low grade metamorphism results from the weight
of overlying sediments plus some heat from below
This often produces slate from shale
(low grade)
Pressure from
weight of overlying
sediment
To produce high level metamorphism, much
more pressure is needed than can be provided
by the weight of overlying sediments
This pressure comes from the side from
plate collision
After sideways squeezing
This sideways squeezing
produces metamorphic
rocks like schist
Schist rock resulting from high lateral squeezing
Another schist rock
garnet schist
If the intense squeezing is
combined with extremely
high temperatures, a gneiss
is produced.
This is the highest degree
of metamorphism a rock
can undergo without
melting
banded gneiss
Garnet gneiss
hornblende
gneiss
All of the metamorphism you have seen so
far has been regional metamorphism
In regional metamorphism the entire region is subject
to pressure and or high temperature
When hot molten rock (magma) comes up to the
surface from deep inside the earth, the high
temperature of the magma alters the rock which
comes in contact with it.
This is known as contact metamorphism
Hornfels
magma
Contact metamorphism here
How can you tell if a rock is
METAMORPHIC?
Look for Banding or Foliation of Mineral
Crystals (NOT layering of sediment grains)
Look for evidence of deformation, like
intense folding
Look for new mineral crystals forming in the
rock (garnet)