Lecture Nineteen

Question 1

What is a metamorphic rock?

Answer 1

The process by which the mineralogy and texture of a rock is modified (by change sin temperature, pressure and fluid chemistry within the Earth).
Within rocks, existing minerals will re-crystalise or completely new one will grow in response to change in pressure, temperature and fluid chemistry.

Question 2

What are the primary sources of heat in the Earth?

Answer 2

Radioactive decay of U, K and Th in crust.
Crystallisation of the Earth’s inner core.
Increase in temperature with depth is the geothermal gradient, averages ~25 degrees C per km, but between 60 degrees C per km to 10 degrees C per km depending on tectonic setting.

Question 3

What is the equation linking pressure and temperature?

Answer 3

P = pgh (p=density, g=gravity and h=height (distance of rock on top).
g for Earth is 9.8m/sec.
Note that density of rock increases as you go down, so it’s not a linear relationship.

Question 4

What happens when fluid chemistry changes?

Answer 4

Hydrothermal fluids are found throughout the Earth’s crust.

The presence or absence of these fluids strongly affects the chemical stability of many minerals.

Question 5

How can texture change in rocks?

Answer 5

Minerals are affected by pressure.
Long thin minerals or flat minerals align along axis of least pressure - I.e. at right angles to direction of pressure.
Round minerals can be elongated by pressure.

Any alignment of minerals is called ‘foliation.’
At low pressure, foliation causes rocks to break along flat planes = slaty cleavage.
At higher pressures, foliation makes a visible alignment of minerals = e.g. gneissosity, schistocity, depending on rock type.

Question 6

What are the names of the mineral shapes?

Answer 6

Round grains = equant.
Long thin grains = elongate.
Flat grains = platy.

Question 7

What is a protolith?

Answer 7

A protolith is the original, unmetamorphosed rock from which a given metamorphic rock is formed.

Question 8

What can increasing the pressure and temperature do to a mineral?

Answer 8

At increasing pressure and temperature, minerals re-organise their lattices, or chemically react with one another, to form an energetically more stable configuration.
This may be a simpler crystal structure, or a smaller, denser new minerals, or simply a mineral with few bonds.

Question 9

What are metamorphic facies?

Answer 9

Grouping of different minerals which form at specified temperatures and pressures.

Question 10

What is bulk composition?

Answer 10

The overall chemical composition of the rock.
This is a major control on the minerals which are able to grow in response to metamorphism.
In order to make specific metamorphic minerals, you need the tight ingredients.

Question 11

What are the five metamorphic styles?

Answer 11

Regional metamorphism.
Contact metamorphism.
Cataclastic metamorphism = at fault zones.
Hydrothermal metamorphism (metasomatism) = hot fluids flow though and chemically change rocks = MOR.
Impact metamorphism = when meteorites hit.

Question 12

Explain regional metamorphism.

Answer 12

Occurs over large areas of the crust.
Associated with orogenic (mountain) belts.
Accompanied by deformation (often compressional).
Increase in temperature and pressure.
Dominant style of metamorphism on Earth.

Question 13

Explain contact metamorphism.

Answer 13

Associated with igneous intrusions.
High heat flow from the intrusion into surrounding cooler country rocks = increased temperature and unchanged pressure.
Often no deformation.
Typically in narrow (meters to km) zones around intrusive rocks.
Forms hornfels - has ‘sugary’ textures.

Question 14

Explain compressional tectonic settings with regards to subduction.

Answer 14

Thermally, subduction zones are simple, dense cold material is thrust under more buoyant, cool material very rapidly.
The composition of the down going slab is relatively homogenous - basalts and some sediments are pulled down.

Dense cold materials are thrust under warmer material faster than it can be heated up - > blueschists to eclogites.
This releases water = hydration melting of overlying mantle.

Question 15

How do rocks that have subducted end up at the surface?

Answer 15

Rocks subducted along the blueschist and eclogite facies paths.
Uplift occurs along the geotherm, so rocks then pass though the greenschist and zeolite facies.
Rocks at surface will contain minerals formed in all facies.

Question 16

Explain compressional tectonic setting with regards to continental collision.

Answer 16

Collision zone results in thickening of the crust, cold crust is pushed to depths of up to 70-80km.
high range of protoliths: sedimentary, volcanics from previously existing arcs, fragments of subducting slabs.
Scale is large - hundreds to thousands of km.
Over time the geotherm will attempt to return to equilibrium.

Time plays a central role:
Initially have high pressure and low temperature metamorphism due to crustal thickening.
Over time, the geotherm will attempt to return to equilibrium.

Rocks closest to the centre of the collision get buried the deepest.
Those at the edges get buried only a small amount.
You get a systematic variation in metamorphose temperature and pressure over a large area.
Highest temperature and pressure is in the centre of the collision zone, lowest at the margins.

Question 17

Explain extension and metamorphism.

Answer 17

Metamorphism associated with extension is characterised by increases in temperature and decreases in pressure de to brining hot material closer to the surface of the Earth.
Extension carried to extremes results i new oceanic basins.

Rifting:
Rifting can be caused by thermal plumes within mantle rising up underneath a continent.
Rifting causes the curst to thin, making deeper crustal levels rise significantly.
When the mantle rises it melts and the melt then migrates into the lower crust where it causes heating and metamorphism.