Metamorphic Rocks

What is metamorphic?

• These rocks were at one time either sedimentary or igneous. (The parent rocks)

• A change must occur to be classified as metamorphic.

• Changes can be in texture, minerals, or chemical composition

How do rocks metamorphose?

• From the partial or complete recrystallization of minerals in the rocks over long periods of time

• Rocks remain essentially solid during metamorphism

• Factors causing it can be temperature, pressure, chemistry

1) Temperature

• Low end 100 – 200 degrees C (before?)

• High end 700 – 800 degrees C (after?)

• Where does the heat come from?

• Geo-thermal gradient– as you go deeper into earth’s crust the temperature increases.

• Plutonism – rocks close to an igneous pluton can be raised to near melting.

Effects of temperature

• Causes minerals to react or breakdown into more stable “high temperature minerals”

• Example: Amphibole = pyroxene + quartz + water. Note with bowen’s reaction series that pyroxene is more stable at high temp.

2) Pressure• Where does increased pressure come

from?

• Confining pressure – equal in all directions and all rocks subject to this. Obviously increases with depth.

• Directed Stress – not equal in all directions. (often associated with mountain building forces)

Effects of pressure

• Increased confining pressure favors denser minerals. A common one being Garnet.

• Also evident in metamorphosed sedimentary rocks such as sandstone is the tendency for the rocks texture to become more compact. (fig. 7.3 p.106)

Directed Stress Textures

• A) Slaty Cleavage – platy minerals such as clays or micas will tend to align parallel to each other. Results is “rock cleavage”. Example is slate (metamorphosed shale)

• “low grade metamorphism”

Slate road cut in BC

Directed Stress Textures

• B) Schistosity – Temperatures increase even more, crystals grow coarser (larger) and elongated.

• Example mica schist

• “intermediate grade metamorphism”

Foliation

• Both A and B are examples of foliation. Do not confuse this with bedding!

• Both are textural

• Can sometimes be termed lineation especially for needle shaped minerals like amphiboles.

Gneiss (pronounced “nice”)

• “High grade metamorphism” – highest temperature and pressure.

• Re-crystallized minerals separate into bands of differing composition.

• Usually light (quartz) and dark (ferromagnesian)

• A type of foliation but clearly a compositional difference in the layers.

3) Chemistry

• Gases or liquids passing through rock can react with existing minerals.

• =Metasomatism

Fig. 9.3

Types of Metamorphism

• RegionalRegional

• ContactContact

• SeafloorSeafloor

• BurialBurial

• Shock (impact)Shock (impact)

Types of Regional Metamorphism

Widespread changes in temperature and pressure bring about changes in rocks due to tectonic forces.

Types of Contact Metamorphism

Intrusion of magma against colder rocks. Affected area is proportional to the size and temperature of the intrusion, but is always only a local phenomenon.

Types of Seafloor Metamorphism

Changes in rocks at the Mid-ocean ridge associated with chemical reactions promoted by the infiltration of heated seawater

Types of Impact Metamorphism

Changes due to rapid increase in pressure (localized only )

Slate (parent rock = shale)

• Individual mica crystals too small to be seen

• Slatey cleavage

• More dense and more cleavage than shale.

Phyllite

• Mica crystals become visible, but still fine grained texture overall.

• Continues with slatey cleavage

Schist

• Growth of elongated larger crystals leads to schistosity.

Gneiss

• Compositionally layered rock may form from continued metamorphism of a schist.

• Or this gneissic texture may be formed from strong metamorphism of granite.

• Naming of the foliated rocks is purely based on degree of metamorphism and not composition.

• You can use mineral names to be more specific such as

• “garnet-biotite schist”

• “granitic gneiss” if the composition is that of granite (quartz, feldspar, amphibole)

Non-Foliated

• Quartzite (Parent = Sandstone)

Marble (parent = limestone)

• Will fizz with acid

• If linear features are present then they would have been there with the parent, and not as a result of metamorphism.

Begin Lab

• Identify the rocks in your lab ID table

• Answer questions from the lab manual on the back of the ID table.