Chapter 2: Plate Tectonics Slide Set 4

Question 1

How many plates are there and what are they made of?

Answer 1

• Earth’s lithosphere is broken into ~20 plates that interact. (12 major, many microplates)
• Each plate is a piece of lithosphere and can have both continent and ocean environments

Question 2

What is pangea?

Answer 2

One vast supercontinent, later fragmented into separate continents that drifted apart, moving slowly to their present positions. This model came to be known as continental drift.

Question 3

What is sea-floor spreading?

Answer 3

American geologist, Harry Hess, proposed that as continents drift apart, new ocean floor forms between them by a process that his contemporary, Robert Dietz, also had described and named sea-floor spreading

Question 4

What is subduction?

Answer 4

Hess and others suggested that continents move toward each other when the old ocean floor between them sinks back down into the Earth’s interior

Question 5

What was Wegener’s theory on plate tectonics?

Answer 5

Wegener argued that the continents were once merged into a supercontinent called Pangea that later broke up to produce smaller continents that “drifted”
apart. The matching shapes of coastlines, as well as the distribution of ancient climate belts,
fossils, and rock units all make better sense if Pangea existed.

Question 6

What is tectonics?

Answer 6

the study of large scale movements and deformation of the Earth’s crust

Question 7

What were the evidences that Wegener found? Explain each.

Answer 7

1. Glacial Evidence
2. Paleoclimatic Evidence
3. Fossil Evidence
4. Matching Mountain Belts

Question 8

Explain Glacial evidence of Wegener

Answer 8

• Evidence of glaciers found on five separated continents from 180 million years ago.
• These continents must have been together at the Poles 180 million years ago

Question 9

Explain Paleoclimatic evidence of Wegener

Answer 9

• Placing Pangaea over the Late Paleozoic South Pole
• Wegener predicted rocks defining Pangea climate belts.
• (Tropical coals, Tropical reefs, Subtropical deserts,
  Subtropical evaporites)

Question 10

Explain Fossil evidence of Wegener

Answer 10

• Identical fossils found on widely separated land.
  **Lystrosaurus—A nonswimming, land-dwelling reptile.
  **Cynognathus—A nonswimming, land-dwelling mammal-
  like reptile.
• These organisms could not have crossed an ocean.
• Pangaea explains the distribution

Question 11

Explain Matching Mountain Belts evidence of Wegener

Answer 11

Distinctive rock assemblages and mountain belts match across the Atlantic.

Question 12

Why were there so many criticism on Wegener’s ideas?

Answer 12

• He couldn’t explain how or why continents moved.
• Wegener died in 1930 on a Greenland expedition.
• Over the next three decades, new research, new technology, and new evidence from the oceans revived his hypothesis

Question 13

What causes the Earth’s magnetic field to occur?

Answer 13

Flow in the liquid outer core creates the magnetic field.

Question 14

How much is the Earth’s magnetic field tilted by?

Answer 14

The magnetic pole is tilted ~11.5° from the axis of rotation

Question 15

Explain the concept of magnetic poles and how are the geographic poles and magnetic poles with respect to each other?

Answer 15

• The magnetic pole moves with respect to the Earth’s surface.(i.e. Geographic and magnetic poles are not parallel.)
• Magnetic poles are located near geographic poles.
• Magnetic poles move constantly

Question 16

What does N indicate on the compass?

Answer 16

A compass points to magnetic N, not geographic N

Question 17

What is declination? What does it depend on?

Answer 17

The difference between geographic N and magnetic N is called declination. It depends on:

• Absolute position of the two poles
  
  • • Geographic north
  • • Magnetic north
• Longitude

Question 18

What is inclination and what does it depend on?

Answer 18

Curved field lines cause a magnetic needle to tilt.

Angle between magnetic field line and surface of the Earth is called inclination. It depends on:
- Latitude

Question 19

What is paleomagnetism?

Answer 19

The study of fossil magnetism is called paleomagnetism

In a rock that formed millions of years ago, the orientation of the dipole representing the magnetic field of the rock is not the same as that of present day Earth

The reason for this difference is that the magnetic fields of ancient rocks indicate the orientation of the magnetic field, relative to the rock, at the time the rock formed. This record, preserved in rock, is paleomagnetism

Question 20

What are the informations that we can get from paleomagnetism?

Answer 20

Iron (Fe) minerals in rock preserve information about the magnetic field at the time the rocks formed.

• Declination and inclination preserved in rocks often vary from present latitude / longitude.
• Instruments used in paleomagnetism record changes in position.
• These data are used to trace continental drift

Question 21

How do Iron minerals archive the magnetic signal at formation? hot magma vs cooled magma

Answer 21

Hot magma

• High Temp—no magnetization
  
  • • Thermal energy of atoms is very high.
  • • Magnetic dipoles are randomly oriented.

Cooled magma
- Low Temp—permanent magnetization
** Thermal energy of atoms slows.
** Dipoles align with Earth’s magnetic field.
** Magnetic dipoles become frozen in alignment with
field.

Question 22

The term ___________ to refer to the supposed position of the Earth’s magnetic north pole in the past.

The successive positions of those trace out a curving line that came to be known as an _______

Answer 22

paleopole

apparent polar-wander path.

Question 23

What is polar wandering?

Answer 23

• Layered basalts record magnetic changes over time.

- Inclination and declination indicate change in position

Question 24

True/False

Apparent polar-wander is the same on every continent

Answer 24

False

Study of paleomagnetism indicated that the continents have moved relative to the Earth’s magnetic poles. Each continent has a different apparent polar-wander path, which is only possible if the continents move “drift” relative to each other.

Question 25

What is the recent understanding of polar-wandering?

Answer 25

• The location of the magnetic pole is fixed.
• The continents themselves have moved.
• These curves align when continents are reassembled