Plate Tectonics: Theory

Question 1

Oceanic vs Continental Crust

Answer 1

1. Composition: SIMA (Basalt, Si, Mg, O) vs SIAL (Granite, Si, Al, O).
2. Age: <200 million - 1500 million vs >1500 million years.
3. Thickness: 6 - 10 km vs 5 - 70 km.
   4: Density: Oceanic crust is denser than continental.

Question 2

The Core

Answer 2

Outer Core: Semi-molten, 5000 - 6000°C.

Inner Core: Solid because the pressure is too high for liquids to exist, 6000°C.

Question 3

The Mantle

Answer 3

2900 km thick.
5000°C.
Semi-molten, gets denser with depth.
Silicate rocks.

Question 4

Asthenosphere

Answer 4

The section of the upper mantle where convection currents occur.

Question 5

Lithosphere

Answer 5

The crust and rigid upper mantle.

Question 6

The Mohorovic Discontinuity

Answer 6

The point at which the crust ends and the mantle begins.

Question 7

What are convection currents and how do they interact with the crust?

Answer 7

Circular movements of semi-molten rock in the mantle which create drag on the crust, causing the tectonic plates to move.

Question 8

What causes convection currents?

Answer 8

Radioactive decay of elements in the lower mantle and core generates heat.
Lower parts of the asthenosphere heat up and rise.
Upon reaching the lithosphere, the molten rock cools and sinks again.

Question 9

Where does sea floor spreading take place?

Answer 9

Mid-ocean ridges such as the Mid-Atlantic Ridge.

Question 10

The 4 Characteristics of Sea Floor Spreading

Answer 10

1. The age of rocks increases as distance from the ridge increases.
2. The height of the sea floor decreases as distance from the ridge increases.
3. The magnetic history of the sea floor is striped (paleomagnetism).
4. Accumulation of sediment increases as distance from the ridge increases.

Question 11

Paleomagnetism

Answer 11

When magma cools, the iron it contains aligns to the Earth’s magnetic field, which reverses every 400,00 years. This creates symmetrical ‘stripes’ of alignment, alternating between North and South.
This supports sea floor spreading because it shows that crust is formed from cooling magma, and that that crust is emerging from mid-ocean ridges and moving outwards.

Question 12

What are tectonic plates?

Answer 12

Sections of crust floating on the asthenosphere.
Some plates contain both oceanic and continental crust.
Tectonic plates cannot overlap.

Question 13

How do tectonic plates move?

Answer 13

Convection currents create drag on the plates, which move slowly and continuously.

Question 14

Plate Boundaries

Answer 14

1. Convergent (Destructive): Plates move towards each other.
2. Divergent (Constructive): Plates move away from each other.
3. Conservative (Transform): Plates move alongside each other either in opposite directions or the same direction.

Question 15

O-O Divergence

Answer 15

Form mid-ocean ridges.
Rifts form in these ridges when sections of crust in the ridge’s centre sink.
Transform faults can occur as the sea-floor speads at different rates.
Earthquakes are shallow, with a magnitude of < 5.
Example: The Mid-Atlantic Ridge.

Question 16

C-C Divergence

Answer 16

Form rift valleys of horsts and grabens as magma rises up through faults.
Example: The East African Rift Valley (Mt. Kilamanjaro, Lake Victoria, stretches from the Red Sea to Mozambique).

Question 17

Horst

Answer 17

A remaining area of raised land in a rift valley.

Question 18

Graben

Answer 18

An collapsed section of crust in a rift valley.

Question 19

C-C Convergence

Answer 19

Young fold mountains form because plates cannot easily subduct. Instead of subduction, rock is compressed and bent upwards.
Example: The Himalayas (meeting point of the Indo-Austrailian and Eurasian plates).

Question 20

C-O Convergence

Answer 20

Form deep ocean trenches, young fold mountains and volcanoes.
The oceanic crust is subducted because it is denser than the continental crust.
Example: Chile, SAmerica - Andes Mountains.

Question 21

Benioff Zone

Answer 21

The area where crust is destroyed.

Question 22

O-O Convergence

Answer 22

Subduction still occurs because one plate may me slightly denser than the other or moving faster.
Form deep ocean trenches (e.g. Marianas Trench) and island arcs (e.g. Eastern Caribbean).

Question 23

Conservative Boundaries

Answer 23

No crust is created or destroyed.
Example: San Andreas Fault, California.
Large Earthquakes occur here. They are shallow with high potential magnitude.

Question 24

Hotspots

Answer 24

Hot spots are a concentration of radioactive elements which form a mantle plume.
They can be detected with seismometers.
Example: Hawaii.
Form shield volcanoes with pahoehoe lava, and coral reefs. The further away from the hot spot a volcano gets, the more likely it is to form a cinder or spatter cone.

Question 25

Hazard

Answer 25

A hazard is a natural/geophysical event that is a potential threat to life and property.

Question 26

Disaster

Answer 26

A disaster is the realisation of a hazard, which causes significant impact on a vulnerable population.
The number of disasters may be increasing. This is because global warming in increasing the frequency of weather related disasters.
Less people are being killed by disasters in modern times due to an increase in preparedness, education and efficiency of response.

Question 27

Risk

Answer 27

Risk is the exposure of people to a hazardous event.
The factors which affect risk are:
- Unpredictability.
- Lack of alternatives.
- Changing dangers.
- Optimism.
- Costs vs. Benefits.

Question 28

Risk Assessment

Answer 28

Defines the likelihood of harm/damage. Depends on the type of hazard and the human conditions.

Question 29

Vulnerability

Answer 29

An areas risk combined with it’s ability to cope.

Conditions which amplify vulnerability are poverty and low economic status.

Question 30

Vulnerability: Root Causes

Answer 30

1. Limited access to power and resources.

2. Political and economic instability.

Question 31

Vulnerability: Dynamic Pressures

Answer 31

1. Lack of training and skills.
2. Rapid population change.
3. Rapid urbanisation.
4. Debt repayment issues.
5. Over exploitation of resources.
6. Deforestation.

Question 32

Vulnerability: Unsafe Conditions

Answer 32

Physical environment:
1. Dangerous locations.
2. Unprotected buildings.
Socio-economic environment:
1. Weak local economy.
2. Lack of preparedness.
3. Prevelance of hunger and disease.

Question 33

Factors Affecting Response

Answer 33

1. Remoteness.
2. Topography.
3. Climate.
4. Population.
5. Technology.
6. Education/Training.
7. Policy.

Question 34

The 4 Types of Response

Answer 34

Modify the:

1. Loss.
   2: Vulnerability.
2. Event.
3. Cause.

Question 35

The 3 Rs

Answer 35

Relief (hours/days).
Rehabilitation (days/weeks).
Reconstruction (months/years).

Question 36

Who is involved in response?

Answer 36

1. Government (local/international).
2. Scientists.
3. Insurers.
4. Planners.
5. Charities.
6. Media.
7. Communities.