How Metamorphic Rock Is
Formed?
comes
from
pre-existing
rock
Answer :
By J. Piccirillo
• Metamorphic rocks - are
rocks that have "morphed" or
changed into another kind of
rock
• These rocks were once
igneous or sedimentary rocks,
or they could have been preexisting metamorphic rock
that’s been re-formed
• These rocks have been put
under tons and tons of
pressure, which fosters heat
build up, and this causes
them to change
• This is happening below the
Earth’s Surface
• Examine metamorphic rock
samples closely, you'll
discover how flattened some
of the grains in the rock
appear
2 Main Types of Metamorphism
• 1. Contact (local, small
area) when molten rock
(Magma) touches cold preexisting surrounding rock
and partially alters them
• Contact metamorphism
helps us to determine
relative ages of rocks in an
area.
• 2. Regional Metamorphism
occurs when Large areas
are affected by intense heat
and pressure
• Example – Mountain
Building in a chain
Re-crystallization – that’s
when the minerals in a rock
partially melt to form new
minerals and hence a new
rock. Again pressure and
heat at play
2 Types of Structures in
Metamorphic Rock
1) Foliated - relating to a layered
structure. Or an alignment of the
minerals oriented along visible
planes (banded). Generally
comes from many minerals in
the rock
Ex. Slate, Schist, Gneiss
Foliated
• Slate
Foliated
• Schist –notice it has flat types of flakes that
glitter from the mica crystals
2) Non-Foliated - (no banding)
usually rock of one mineral.
Looks like one appearance or
like a uniform mass. Generally
comes from one mineral in the
parent rock
Ex. Marble, Quartzite
Non- Foliated
• Quartzite
Non-Foliated (notice just one look)
• MetaBasalt (Greenstone)
Non-Foliated
• Metaconglomerate
For examples of
metamorphic rocks, take a
look at your ESRT and the
following pictures
Schist - can be formed from
basalt or shale, or slate
Gniess
Gneiss
Slate
Marble
Marble
Notice no ordered layering or structure.
However, it does have streaks of partially
melted mineral running in it
Quartzite
• Made of smoky and dark quartz