Chapter 2: Plate Tectonics Slide Set 5

Question 1

Associate the Earth’s layers with solid fluid, liquid fluid, gas fluid

Answer 1

Solid Fluid:

• The asthenosphere
• Mesosphere (mantle below lithosphere)

Liquid Fluid:
- The ocean, or the outer core

Gas Fluid:
- The atmosphere!

Question 2

Describe Earth’s outer core in terms of magnetic field

Answer 2

‣ Liquid iron (a conductor)
‣ Convecting to dissipate heat (electric current)
‣ Generates magnetic field
‣ Shaped in part by Earth’s rotation

Question 3

How does Earth’s Dynamo work?

Answer 3

A convecting conducting fluid supported by internal heat and planetary rotation could self-sustain a magnetic field over astronomical timescales

Question 4

What is the Geomagnetic Axial Dipole hypothesis?

Answer 4

Geomagnetic Axial Dipole hypothesis (Hospers, 1954)
‣ Earth’s field is dipole- dominated
‣ If you average the field over long enough, only axial
dipole with remain
‣ Field strength remained constant for the past ~ 3 Gyr

Question 5

What is Palaeo-magnetic Pole?

Answer 5

‣ Field recorded when rocks formed
‣ Different inclination at different latitudes
‣ Rocks of the same age record the same field,
i.e., they point to the same palaeo-pole

Question 6

What is the difference between the Apparent-Polar Wander and the True Polar Wander?

Answer 6

Comparing APWs across each continent allows geologists separate tectonics motions from TPW

The (big) assumption for measuring TPW:
Over geological time scales, the magnetic pole follows the rotation pole

Question 7

What is bathymetry?

Answer 7

• Depth variation on sea-floor.
• Measured by sonar
  Sea-floor maps created by ships crossing the oceans. Bathymetric maps are now produced using satellite data

Question 8

Many bathymetric features were discovered by sonar. Name the features

Answer 8

These include deep-sea trenches, oceanic islands, seamounts, and guyots, mid-ocean ridges, abyssal plains, fracture zones

Question 9

A bathymetric profile across the Atlantic Ocean (from X to X’ on the map) illustrates that mid-ocean ridges are elevated above deeper ________.

Answer 9

abyssal plains

Question 10

True/False

All mid-ocean ridges are roughly symmetrical—bathymetry on one side of the axis is nearly a mirror image of
bathymetry on the other side.

Answer 10

True

Question 11

What are marine sediments composed of?

Answer 11

minerals eroded from the continents and shells of dead micro-organisms, thicken away from the mid-ocean ridge.

Question 12

What are deep-ocean trenches?

Answer 12

The deep areas occur in elongate troughs that are now referred to as trenches. Trenches border volcanic arcs,
curving chains of active volcanoes.

Question 13

What are seamount chains?

Answer 13

Numerous volcanic islands poke up from the ocean floor: for example, the Hawaiian Islands lie in the middle of the Pacific.
In addition to islands that rise above sea level, sonar has detected many seamounts (isolated sub-marine mountains), which were once volcanoes but no longer erupt.
Volcanic islands and seamounts typically occur in chains, but in contrast to the volcanic arcs that border deep-ocean trenches, only one island at the end of a seamount and island chain remains capable of erupting volcanically today.

Question 14

What are fracture zones?

Answer 14

Surveys reveal that the ocean floor is diced up by narrow bands of vertical cracks and broken-up rock. These fracture zones lie roughly at right angles to mid-ocean ridges. The ridge axis typically steps sideways when it intersects with a fracture zone.

Question 15

What is the rock composition of the oceanic crust?

Answer 15

Beneath its sediment cover, oceanic crust bedrock consists primarily of basalt—it does not display the great variety of rock types found on continents

Question 16

Explain the concept of heat flow and what theory it led to?

Answer 16

Heat flow, the rate at which heat rises from the Earth’s interior up through the crust, is not the same everywhere in the oceans. Rather, more heat rises beneath mid-ocean ridges than elsewhere. This observation led researchers to speculate that hot magma might be rising into the crust just below the mid-ocean ridge axis

Question 17

Today’s view of the ocean floor reveals the location of:

Answer 17

• Mid-ocean ridges
• Deep-ocean trenches
• Oceanic fracture zones

Question 18

Explain Harry Hess’ concept of Sea-Floor Spreading (a mechanism to explain continental drift)

Answer 18

The ocean floor is formed at the ridges, drifts away from them on both sides and then plunges into the mantle at the trenches

The seafloor viewed as a conveyor belt for marine material (marine sediments and oceanic crust), explaining the absence of old oceanic rocks and the thinness of marine sediment.

Question 19

Basalt, an extrusive igneous rock, is formed at ________

Answer 19

mid-ocean ridges

Question 20

Explain the magnetism on the sea-floor

Answer 20

Magnetism in sea-floor rocks varies farther from Mid-Ocean Ridges.
- Stripes of positive (stronger) and negative (weaker)
magnetic intensity
- Recorded in sea-floor basalts

Question 21

What is magnetic anomaly

Answer 21

difference between the expected strength of the Earth’s magnetic field at a certain location and the actual measured strength at that location

Question 22

How does the new sea-floor form and what happens to the old ocean floor?

Answer 22

New studies of the sea floor led to the proposal of sea-floor spreading.
New sea floor forms at mid-ocean ridges and then moves away from the axis, so ocean basins can get wider with time.
Old ocean floor sinks back into the mantle by subduction. As ocean basins grow or shrink, continents drift.

Question 23

What are the two main evidences of sea-floor spreading?

Answer 23

1. the existence of orderly variations in the strength of the measured magnetic field over the sea floor, producing a pattern of stripes called marine magnetic anomalies
2. the variation in sediment thickness on the ocean crust, as measured by drilling.

Question 24

At any given location on the surface of the Earth, the magnetic field that you measure includes two parts:

Answer 24

• produced by the main dipole of the Earth generated by circulation of molten iron in the outer core
• produced by the magnetism of near-surface rock

Question 25

A time when the Earth’s field flips from normal to reversed polarity, or vice versa, is called a _______

Answer 25

magnetic reversal

Question 26

When the Earth has reversed polarity, the south magnetic pole lies near the north geographic pole, and the north magnetic pole lies near the south geographic pole. The time interval between successive reversals is called a ________

Answer 26

chron or epochs

Question 27

Why do marine anomalies form and what did the discovery led to?

Answer 27

Marine magnetic anomalies form because reversals of the Earth’s magnetic polarity take place while sea-floor spreading occurs. The discovery of these anomalies, as well as documentation that the sea-floor gets older away from the ridge axis, proved that the sea-floor spreading hypothesis is correct.

Question 28

Sea-floor spreading predicts that magnetic anomalies should be ________

Answer 28

mirror images across the mid-ocean ridge…

Question 29

What are Subchrons or events?

Answer 29

short-term magnetic reversals (1000’s to 200,000 years).

Question 30

What is the record of magnetic reversals can be used for?

Answer 30

Whereas the cause of reversals remains to be elucidated, the record of magnetic reversals can be used to date thermoremanent magnetization in geologic cross-sections.

Question 31

What does the width of the magnetic anomaly stripes related to and how does it defer by the rate?

Answer 31

The width of the magnetic anomaly stripes is related to the spreading rate.

• Faster spreading = wide stripes
• Slower spreading = narrow stripes

Question 32

Continental lithosphere vs. oceanic lithosphere

Explain the differences

Answer 32

Differ markedly in their thicknesses.
On average, continental lithosphere has a thickness of 150 km, whereas old oceanic lithosphere has a thickness of about 100 km. (new oceanic lithosphere at a mid-ocean
ridge is much thinner)

Question 33

How do the geophysicists able to determine how fast the ocean opened up?

Answer 33

Using the magnetic stratigraphy established on land (dated lava flows), geophysicists were able to determine how fast the ocean opened up.

Question 34

What are isochrons?

Answer 34

are contours of equivalent age of the seafloor.
They show the time that has elapsed and the amount of spreading that has occurred since the magnetized rocks were injected as lava into a mid-ocean rift.

Question 35

What are plate tectonics and what do they explain?

Answer 35

• Earth’s outer shell is broken into rigid plates that move and interact in three types of plate boundaries.
• Plate tectonics explains the origin and distribution of earthquakes and volcanoes, mountain belts and seamount chains, mid-ocean ridges and deep- ocean trenches, and changes in past positions of continents and ocean basins.

Question 36

What are continental margins?

Answer 36

Where land meets the ocean

Question 37

What are the types of continental margins?

Answer 37

• Margins near plate boundaries are “active.”
• Margins far from plate boundaries are “passive.” “

Earthquakes common along active margins.

Question 38

What is the significance of the passive plate boundaries?

Answer 38

• Passive-margin continental crust thins seaward.
• Traps eroded sediment.
• Develops into the continental shelf.

Question 39

Lithosphere is fragmented into ~____ major tectonic plates.

Answer 39

12

Question 40

What are plate boundaries?

Answer 40

• Locations on Earth where tectonic plates meet.
• Identified by concentrations of earthquakes.
• Associated with many other dynamic phenomena.

Question 41

True/False

Plate interiors are almost earthquake-free

Answer 41

True

Question 42

What are the types of plate boundaries?

Answer 42

Divergent boundary
Convergent boundary
Transform boundary

Question 43

Explain the divergent plate boundaries and give an example on Earth

Answer 43

• tectonic plates move apart
• Lithosphere thickens away from the ridge axis.
• New lithosphere created at divergent boundary
• Also called: mid-ocean ridge, ridge.
  EX: Iceland - Mid Atlantic Ridge

Question 44

Explain the convergent plate boundaries

Answer 44

• tectonic plates move together

- The process of plate consumption is called subduction.

Question 45

Convergent plate boundaries are also called

Answer 45

convergent margin, subduction zone, trench.

Question 46

Convergent margins are the sites of lithospheric destruction. There are 3 distinct types:

Answer 46

• Oceanic-Oceanic
• Oceanic –Continental
• Continental-Continental

Question 47

Explain the transform plate boundaries and give an example on Earth

Answer 47

• tectonic plates slide sideways.
• Plate material is neither created nor destroyed.
  The San Andreas Fault

Question 48

Transform plate boundaries are also called

Answer 48

transform fault, transform