Metamorphic RocksSWHS Geology

What are they?

From the greek roots meta (change) and morphos (form):

Rocks that have been changed in form from the temperature, pressure, and fluids inside the earth.

A classic example

(Don’t take me for granite...)Gneiss!

1. Temp

2. Pressure

3. Fluids

4. Time!

Agents of metamorphism

1. Temperature

High temps (but not enough to melt) can change the structure of minerals - heat can come from the...

a) Geothermal gradient - the deeper you go, the hotter it gets!

b) Igneous intrusions - can happen at much lower pressures

2. Pressure (a.k.a. stress)

Pressure can change minerals also - the two main kinds are:

a) Confining stress - pressure applied equally on all sides and

d) Differential stress - pressure stronger in one direction than the other (could be compressive or shearing)

Confining stress

Differential stress

Question: Where would you find differential stress?

Compressive

Shearing

3. Fluids

Fluids (NOT pore fluids from when a sedimentary rock was deposited...) come from intruding magma or the breakdown (dehydration) of minerals.

They can change the minerals in a rock - this special case of metamorphism is called metasomatism.

4. Time!

Some garnets in Vermont grew at a rate of 1.4mm per 1,000,000 years!

Metamorphic rock textures:

Can be:

1) Foliated: Characterized by parallel planes formed through directed pressure and preferred growth orientations of certain platy minerals.

2) Non-foliated: Don't have those planes, usually because they are made of mineral grains which are cubic or spherical, and therefore have no preferred orientation. Two common examples are marble and quartzite.

Foliation:

1) Slaty (low grade). Splits easily along nearly flat, parallel planes - metamorphic minerals still microscopic.

Example: Slate!

Foliated texture: Can be further divided into:

2. Intermediate (between low-grade and medium-grade). Fine-grained rock with a “silky” luster. Often takes on a “wavy” texture. Minerals still not visible (too small).

Example: Phyllite

3. Schistose (medium grade). Metamorphic minerals now visible as platy and needle shaped grown from differential stress.

Example: Schist

4. Gneissic (high grade). Rock has become ductile due to high temps and pressures. New metamorphic minerals segregate themselves into light and dark bands.

Example: Gneiss!

Examples of a non-foliated texture:

MarbleCrystalline texture, effervesces in acid

(originally limestone).

Quartzite“Sugary” texture, mostly

quartz (originally sandstone).

Types of Metamorphism

1. Burial

2. Contact

3. Regional

1. Burial Metamorphism

Low temperature and low pressure

Bury rocks deeply enough and they will warm up and change. This form of metamorphism is found anywhere where sediments and rocks are buried deeply, and should strike you as being pretty similar to lithification, which we discussed last time. The line between lithification and burial metamorphism is fuzzy.

2. Contact Metamorphism

High temp and Low Pressure

Caused by high temperatures near magmatic intrusions.

Found in volcanic zones!

Rarely makes foliations

3. Regional metamorphism

High temp, high pressure

Occurs at considerable depth near mountain building regions and/or subduction zones.

Usually always makes foliations (unless starting rock does not have proper composition!)

Progressive metamorphism

Shale

Slate

Phyllite

Schist

Gneiss

Protoliths

All metamorphic rocks can be traced back to their protolith (or “parent rock”) - that is, what was the pre-existing rock before metamorphism?

For example,

A chemical limestone... becomes marble!

Or,

A conglomerate... becomes a meta-conglomerate!

Or,

A shale... becomes a slate!

P-T diagram

Metamorphic facies

A facies is like a climatic zone - different combinations of minerals are stable in different facies!

Metamorphic index minerals

Minerals characteristic of certain “grades” of metamorphic rock:

Low-grade: chlorite, muscovite, biotite

Medium-grade: garnet, staurolite

High-grade: sillimanite

And finally, know this:

If you start with two different rocks (say a basalt and a sandstone) and heat them up and squeeze them identically, so that they have gone through the same pressure/temperature conditions, you will get new minerals. Those minerals will be different for the different rocks, even under identical pressure/temperature conditions.

and...

If you start with two identical rocks (say two chunks of the same sandstone) and heat them and squeeze them differently, so that they have gone through different sets of pressure/temperature conditions, you will get new minerals. Those minerals will be different for the same rocks under different pressure/temperature conditions.