Download Metamorphism Metamorphism means to “change form

Metamorphism
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Metamorphism means to “change form”.
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Metamorphism causes a change in the rock’s mineral composition and a
change in the rock’s texture.
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What causes the changes? (i.e. Change Agents)
o Heat/Temperature
o Pressure
o Hot Chemical Fluids (e.g. Water)
These changes occur at various depths beneath Earth’s surface; from 2-3
kilometres deep all the way down to the MOHO (i.e. Lithosphere –
Asthenosphere boundary).
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All three types of rocks (i.e. igneous, sedimentary, and metamorphic) can
be metamorphosed.
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Example: Granite (Igneous) To Gneiss (Metamorphic)
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Example: Sandstone (Sedimentary) To Quartzite (Metamorphic)
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Example: Slate (Metamorphic) To Schist (Metamorphic)
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Metamorphism DOES NOT involve a melt and there IS NO recrystallization
of minerals from such a melt.
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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 non-foliated.
Note that the texture of a metamorphic rock is usually coarser than the
parent rock (i.e. previous rock).
(2) Volume:
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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:
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New minerals are often the result. These minerals are different from the
minerals that were present in the parent rock.
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Hot chemical fluids are often essential as a transport medium for the
movement of ions and the formation of new minerals.
Foliated Metamorphic Rocks
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Slate, Phyllite, Schist, and Gneiss are examples of metamorphic rocks that
are foliated.
Pairs
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Sedimentary – (Limestone) TO (Marble) - Metamorphic
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Sedimentary - (Sandstone) TO (Quartzite) - Metamorphic
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Sedimentary - (Shale) TO (Slate) TO (Phyllite) TO (Schist) TO (Gneiss) –
Metamorphic
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Igneous – (Granite) TO (Gneiss) – Metamorphic
Metamorphic Environments
(1) Contact: Otherwise called baking. Look for burn marks!
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HEAT/TEMPERATURE AND HOT CHEMICAL FLUIDS ARE THE DOMINANT
AGENTS. But all three agents are involved!
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(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
Remember:
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Refer to diagram on previous slide (left side).
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Buried Lava Flow = Contact Metamorphism Below The Lava Flow (Only).
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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!
Regional Metamorphism
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WHERE DOES IT OCCUR?
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(1) Areas of mountain building; and
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A compressional environment resulting in increasing pressure. As the
mountains are forming, increasing burial depth results in increasing
temperature.
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(2) Subduction zones. This type of metamorphism creates the largest volume of
metamorphic rock.
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Ocean-continent collision (convergent plate boundary) is an example of
where subduction and mountain building occurs
Regional Metamorphism
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
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. Low Grade
Low Grade
High Grade