Metamorphism
 Metamorphism means to “change form”.
 Metamorphism causes a change in the rock’s mineral
composition and a change in the rock’s texture.
What causes the changes? (i.e. Change Agents)
(1) Heat/Temperature
(2) Pressure
(3) Hot Chemical Fluids (e.g. Water)
 These changes occur at various depths beneath Earth’s
surface; from 2-3 kilometers deep all the way down to the
MOHO (i.e. Lithosphere - Asthenosphere boundary).
Do Not Forget These Concepts!
• All three types of rocks (i.e. igneous, sedimentary, and metamorphic)
can be metamorphosed.
• Example: Granite (Igneous) To Gneiss (Metamorphic)
• Example: Sandstone (Sedimentary) To Quartzite (Metamorphic)
• Example: Slate (Metamorphic) To Schist (Metamorphic)
• Metamorphism DOES NOT involve a melt and there IS NO
recrystallization of minerals from such a melt.
•It is the hot chemical fluids that serve to dissolve away previous
minerals and serve as the medium for the crystallization of NEW
MINERALS.
The Changes
• (1) Texture:
• Two types of textures: foliated (i.e. layered or banded) and nonfoliated.
• Note that the texture of a metamorphic rock is usually coarser than
the parent rock (i.e. previous rock).
Foliated
Non-foliated
The Changes
• (2) Volume:
• The volume of a metamorphic rock is less than the volume of the
parent rock. This is due to the metamorphic agent called pressure.
•(3) Chemical Change:
• New minerals are often the result. These minerals are different
from the minerals that were present in the parent rock.
• Hot chemical fluids are often essential as a transport medium for
the movement of ions and the formation of new minerals.
Foliated Metamorphic Rocks
• Slate, Phyllite, Schist, and Gneiss are examples of metamorphic
rocks that are foliated.
Before
Metamorphism
After
Metamorphism
 Granite when metamorphosed forms gneiss.
Non-foliated Metamorphic Rocks
• Quartzite and marble are examples of metamorphic rocks that are
non-foliated.
Example:
Quartz
Sandstone
Recrystallizes
Quartzite
Pairs
• Sedimentary – (Limestone) TO (Marble) - Metamorphic
• Sedimentary - (Sandstone) TO (Quartzite) - Metamorphic
• Sedimentary - (Shale) TO (Slate) TO (Phyllite) TO (Schist) TO (Gneiss) – Metamorphic
• Igneous – (Granite) TO (Gneiss) - Metamorphic
Metamorphic Environments
• (1) Contact: Otherwise called baking. Look for burn marks!
• HEAT/TEMPERATURE AND HOT CHEMICAL FLUIDS ARE THE DOMINANT
AGENTS. But all three agents are involved!
•(2) Regional: Related to mountain building.
• PRESSURE AND HOT CHEMICAL FLUIDS ARE THE DOMINANT AGENTS. But
all three agents are involved!
Contact Metamorphism
•
•
•
•
WHERE DOES IT OCCUR?
(1) Beneath lava flows (Hint: buried lava flows)
(2) Adjacent to magma intrusions
(3) Dykes and sills
B is a magma intrusion and R is
a buried lava flow.
Dyke = Vertical
Sill = Horizontal
Contact Metamorphism
• Remember:
• Refer to diagram on previous slide (left side).
• Buried Lava Flow = Contact Metamorphism Below The Lava Flow
(Only).
• Magma Intrusion = Contact Metamorphism Above And Below The
Magma Intrusion.
Chill Out – “Chilled Margins” – Contact Metamorphism
• When lava “pours” over rock or magma “intrudes” rock, it starts to
crystallize (igneous rock activity). The contact metamorphism occurs
where the hot lava or hot magma “touches” the old, previously
existing rock.
Look at the magma intruding through a
rock unit! This magma starts to cool
slowly so crystals grow to be large.
HOWEVER, the magma cools faster on
the outsides since the cold, previously
existing rock unit causes it to cool
faster. THEREFORE, YOU GET
LARGER CRYSTALS TOWARDS
THE MIDDLE AND SMALLER
CRYSTALS BY THE OUTSIDES. The
chilled effect!
Rafts – Contact Metamorphism
• Rafts could also be called inclusions OR xenoliths;
however, rafts is the better term in relation to lava flows
and magma intrusions.
• Rafts will be included in the rock that forms from either
lava or magma. IT BELONGS TO THE ADJACENT
ROCK UNIT THAT THE LAVA IS POURING OVER
OR INTRUSING THROUGH.
A raft (a metamorphic
rock) located inside rock
formed from lava.
Regional Metamorphism
• WHERE DOES IT OCCUR?
• (1) Areas of mountain building; and
• (2) Subduction zones.
(1)
A compressional environment resulting in
increasing pressure. As the mountains are
forming, increasing burial depth results
in increasing temperature.
This type of
metamorphism creates
the largest volume of
metamorphic rock.
Ocean-continent
collision (convergent
plate boundary) is an
example of where
subduction and
mountain building
occurs.
Regional Metamorphism
(2) This is a subduction zone. The metamorphism is not occurring
within the subduction zone, but within the mountain system. As
you can see, as burial depth increases so does the GRADE OF
METAMORPHISM.
Shale
To
Slate
To
Phyllite
To
Schist
To
Gneiss
Regional Metamorphism
In the case of this oceanic-continental collision (i.e. convergent
plate boundary), pressure is created from compressional forces and
burial depth in the mountain system. Heat is created from burial
depth (i.e. geothermal gradient) and also from the magma
chambers.
Metamorphism - Grades
 Metamorphism of rocks can vary in degree from a rock
that is slightly changed (i.e. low grade metamorphism)
(Shale to Slate) to a rock that undergoes such change that
the identity of the original rock cannot be determined (i.e.
high grade metamorphism) (Shale to Schist).
 Examples:
Slate – Low Grade
Phyllite – Medium Grade
Schist – High Grade
Gneiss – Very High Grade
 Example: Shale changes to form slate.
 Example: Granite changes to form gneiss.
Sample Problem
What feature is represented by “xxxx” in the diagram
below?
Once molten rock
(A) contact metamorphism
(B) faulting
(C) regional metamorphism
(D) unconformity
Sample Problem
Which rock was subjected to the highest temperature and
pressure conditions?
(A)
gneiss
(B)
marble
(C)
shale
(D)
slate
Core Lab #3 – Metamorphic Rocks – Lab!
•Careers!
Petrology
Volcanology
Geochemistry
Sedimentologist
Hydrology