MetamorphismMetamorphism means to “change form”.

Metamorphism causes a change in the rock’s mineral composition and a change in the rock’s texture.

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).

What causes the changes? (i.e. Change Agents)

(1) Heat/Temperature

(2) Pressure

(3) Hot Chemical Fluids (e.g. Water)

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 non-foliated.

• Note that the texture of a metamorphic rock is usually coarser than the parent rock (i.e. previous rock).

Foliated Non-foliated

• (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.

The Changes

Foliated Metamorphic Rocks• Slate, Phyllite, Schist, and Gneiss are examples of metamorphic rocks that are foliated.

Non-foliated Metamorphic Rocks• Quartzite and marble are examples of metamorphic rocks that are non-foliated.

Quartz Sandstone

QuartziteRecrystallizesExample:

Before Metamorphism

After Metamorphism

Granite when metamorphosed forms gneiss.

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 MetamorphismIn 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

(A) contact metamorphism

(B) faulting

(C) regional metamorphism

(D) unconformity

What feature is represented by “xxxx” in the diagram below?

Once molten rock

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!

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