GEOL MIDTERM 3

Question 1

What is metamorphic grade?

Answer 1

The intensity of P and T conditions that lead to the alteration of a rock.
Low-grade is weaker metamorphism, and high-grade is intense metamorphism.

Question 2

What are index minerals?

Answer 2

Minerals that make good markers of metamorphic conditions, indicating a specific T and P range (The metamorphic grade)

Question 3

What are metamorphic zones defined by?

Answer 3

The index minerals.

Question 4

What are the types of metamorphic environments?

Answer 4

1. Thermal/contact (Heating by a plutonic intrusion
2. Burial
3. Dynamic (Shearing in a fault zone)
4. Regional (P and T alteration due to orogenesis)
5. Hydrothermal (Alteration by hot water leaching)

Question 5

What is exhumation due to?

Answer 5

The uplift, which eventually leads to collapsing nad thinning. Erosion takes over and removes the upper material, which exposes deeper rock.

Question 6

What are metamorphic facies?

Answer 6

Groups of metamorphic rocks that form together under the same temperature and pressure, but have different protoliths.

Question 7

What are mineral assemblages?

Answer 7

Sets of minerals that grew in association with each other.

Question 8

What is thermal contact metamorphism?

Answer 8

It’s due to heat from magma invading the host rock. When magma intrudes a rock, it cooks the surrounding rock to create bands.

Question 9

Which type of metamorphism produces bands of alteration in the host rock, zoned from high to low grade?

Answer 9

Thermal Contact.
Low-grade metamorphic rock is produced far away from the pluton. High is near the pluton.

Question 10

What is the burial metamorphism process?

Answer 10

As sediments are buried in the sedimentary basin, pressure increases because of the weight of the overburden, and temperature increases because of the geothermal gradient.

Question 11

What is dynamic metamorphism?

Answer 11

The breakage of a rock by shearing a fault zone.

Question 12

What are faults?

Answer 12

Surfaces where one piece of the crust slides or shears past another piece of the crust.

Question 13

What does the fault location determine? What happens if the location is in the upper or deeper crust?

Answer 13

The type of alteration.
If it’s in the upper crust, the rocks will behave in a brittle fashion and the minerals will crush to form fault breccia.
If it’s in the deeper crust, the rocks will warm and be ductile, and the minerals will smear to form mylonite.

Question 14

What does regional or dynamothermal metamorphism cause?

Answer 14

Mountain building.
The rocks that’re used in mountain building are heated by the geothermal gradient and plutonic intrusions. They’re squeezed and heated by deep burial and are smashed and deformed by compression and shearing.

Question 15

What does temperature and pressure orogenesis create?

Answer 15

Huge volumes of metamorphic rock; more than any other mechanism.

Question 16

What is hydrothermal metamorphism? How does this process work?

Answer 16

An alteration that’s done using hot, chemically aggressive water.
It’s a dominant process near MOR magma: the cold ocean water seeps into the fractured crust and is heated by the magma, which causes it to react with the mafic rock. The hot water rises and is ejected through black smokers, and the mafic rock is metamorphically altered.

Question 17

Which type of metamorphism creates the unique facies on blueschist?

Answer 17

Subduction metamorphism.

Question 18

What is shock metamorphism?

Answer 18

When the earth is struck by a comet or asteroid.
The impact generates a compressional shock wave with extremely high pressure, and then causes the heat to vaporize/melt large masses of rock.

Question 19

What is exhumation?

Answer 19

Due to the uplift, which leads to the eventual collapse and thinning of the rock, erosion will take over and remove the upper material to expose the deeper rock below.

Question 20

What are shields?

Answer 20

Large regions of ancient high-grade rocks.
They’re exposed in continental interiors and are eroded remnants of orogenic belts.

Question 21

What did Alfred Wegener propose? What was his evidence?

Answer 21

He hypothesized that the continents used to fit together as a single continent, Pangea.
His evidence was the ‘fit’ of the continents when pushed together, glacial deposits that’re far from polar regions, paleoclimatic belts, distribution of fossils, and matching geologic units.

Question 22

What scientific revolution did Wegener’s proposal affect?

Answer 22

That the earth continually changes.

Question 23

What is a continental drift?

Answer 23

When land masses slowly move and separate.

Question 24

What did Herry Hess propose?

Answer 24

The sea-floor spreading theory. He was the first to agree with Wegener, and used his theory to back up Wegener’s proposal.
He said that as continents drift apart, new ocean floors will form in-between.

Question 25

How many plates has the earths’ lithosphere been broken into?

Answer 25

Around 20 plates that interact with each other.

Question 26

What are the different fossils of animals Wegener found to support his proposal?

Answer 26

Cynognathus and Lystrosaurus: Lang animals.
Mesosaurus: A freshwater reptile.
Glossopteris: A subpolar plant with heavy seeds.

Question 27

Why did Wegener fail in his proposal?

Answer 27

Because he was focusing on the continents, but the evidence he needed was in the sea floor.

Question 28

What is Paleomagnetism?

Answer 28

The study of fossil magnetism.
Instruments used in paleomagnetism are used to record changes in position, and this data is used to trace continental drift.

Question 29

What is Polar Wandering?

Answer 29

Each continent has a separate polar wandering path, which tells us that the location of the magnetic pole is fixed, and it’s the continents themselves that have moved.

Question 30

How were sea-floor maps created?

Answer 30

Using echo-sounding (sonar) equipment.
They were created by ships crossing the ocean, and sonar waves are emitted and once they hit the ground, they bounce back, and the boat has a receptor that calculates the speed of the sound, which allows them to figure out the elevation of the water.

Question 31

When they mapped out the sea floor, what did they find?

Answer 31

That a mid-ocean mountain range runs through every ocean, that deep-ocean trenches occur near volcanic island chains, that submarine volcanoes poke up from the ocean floor, and that huge fracture zones segment the MOR.
These are all explained by plate tectonics.

Question 32

In the ocean, where is the heat flow the greatest?

Answer 32

At MORs.

Question 32

The oceanic crust is covered by sediment; where is it thickest and thinnest? What is it made of?

Answer 32

It’s thickest near the continents, and thinnest/absent at the MOR.
It consists primarily of basalt, and lacks the rock variety that the continents have.

Question 32

When they mapped earthquakes in 1960, what did they find that earthquakes define?

Answer 32

They define zones of movement.

Question 33

Sediment thins away from ridges. True or false?

Answer 33

False. It thickens away from ridges.

Question 34

What does a magnetometer do? What do they look like on the sea floor?

Answer 34

It’s a tool that measures the strength of the earth’s magnetic field.
There are stripes of positive (stronger) and weaker (negative) intensities on the sea floor. They’re called positive and negative anomalies.

Question 35

What do positive and negative anomalies mean?

Answer 35

Positive Anomaly: Sea floor rock has a normal polarity.
Negative Anomaly: Sea floor rock has a reversed polarity.

Question 36

Magnetic polarity reversals are imprinted in sear-floor rock as the sea floor continues to spread. What is the width of the magnetic anomaly stripes related to?

Answer 36

The spreading rates. Faster spreading has wider stripes, and slower spreading has narrow stripes.

Question 37

What type of rock/mineral records the magnetic polarity changes?

Answer 37

Basalt.