How the Earth works Lecture 7: Metamorphism

Question 1

Define a metamorphic rock

Answer 1

solid state alteration of a protolith.

Question 2

Define a protolith

Answer 2

Pre-existing rocks, have not been altered/before metamorphism.

Question 3

List the variables which affect and determine Metamorphic changes in rocks

Answer 3

Temperature.
Pressure.
Tectonic stresses.
Amount of reactive water

Question 4

Explain what a foliation is in metamorphic rocks

Answer 4

An Alignment of platy minerals (i.e., micas), or:
Creation of alternating light/dark bands of minerals (quartz, feldspars and amphiboles.
) in the metamorphic rock.

Question 5

Define recrystalisation

Answer 5

Recrystallization—minerals change size and shape however the mineral identity doesn’t change.

Example: Limestone -> Marble.

Question 6

Define neocrystallisation

Answer 6

New minerals form from old minerals

For example, quartz and clay metamorphosed to quartz, garnet and mica.

Question 7

Define a pressure solution in metamorphic rocks

Answer 7

a process by which grains dissolve at intergranular or intercrystalline contacts/ when two grains are touching and under significant pressure/stress the grains will dissolve at their contacts of eachother.

Spherical grains are compressed into elliptical grains (squished)

Question 8

What are the three agents of metamorphism as a process?

Answer 8

• Heat (T).
• Pressure (P).
• Compression

Question 9

What is the general temp at which metamorphism occurs

Answer 9

Most metamorphism occurs between 250oC and 850oC.

Heat energy breaks and reforms atomic bonds.

Sources of this heat:
The geothermal gradient.
Igneous intrusions

Question 10

Describe the relationship between Pressure and depth in metamorphism

Answer 10

Pressure increases with depth in crust.

270 to 300 bars per km in depth (1 bar is almost 1 atm = 14.7 psi).

Metamorphism occurs mostly in 2–12 kbar range

Increase in Pressure packs atoms more tightly together.

Creates denser minerals.

Involves phase changes or neocrystallization.

Question 11

What are the two kinds of differential stress

Answer 11

Normal and Shear

Question 12

Describe normal stress

Answer 12

In normal stress, think of a rock being compressed or squashed from the sides.

Question 13

Describe Shear stress

Answer 13

In shear stress, think of the rock sliding in a lateral motion.

Question 14

Define Slate

Answer 14

Slate—fine-grained, low grade metamorphic shale

(Low grade metamorphism)

Question 15

Define a Phyllite

Answer 15

Phyllite—fine-grained mica-rich rock.

(Slightly higher metamorphic grade than slate. Slate develops into Phyllite)

Question 16

Define a schist

Answer 16

Schist—fine to coarse rock with larger micas.

Has mica foliation

(Medium-grade metamorphism)

Question 17

Define a Gneiss

Answer 17

Gneiss – coarse-grained, high-grade metamorphic rock

Question 18

Describe the broad compositional classes of metamorphic rocks

Answer 18

Broad compositional classes:

Pelitic—Al rich (e.g. shales)

Mafic—Fe and Mg rich, Si poor (e.g. basalt)

Quartzo-feldspathic— (e.g. granitic)

Calcareous—from Ca-rich rocks
(e.g. limestone)

Question 19

Describe Mafic rocks

Answer 19

Mafic—basalt, dolerite or gabbro protolith.

Rich in Fe and Mg and poor in Si, Al, Na, and K.

Question 20

Describe Quartzo-feldspathic metamorphic rocks

Answer 20

Quartzo-feldspathic: a granite protolith

Question 21

Describe calcerous metamorphic rocks

Answer 21

Calcareous—carbonate protolith.
Rich in Ca (limestones) and Ca and Mg (dolostones).
Recrystallize into calcite and dolomite marbles.