Plate Tectonics And Associated Hazards

Question 0

How dense are oceanic plates?

Answer 0

2. 9 g/cm^3

- 6/10km thick

Question 1

What are the different layers of the Earth?

Answer 1

1. Inner core
2. Outer core
3. Mantle
4. Asthenosphere
5. Lithosphere

Question 2

How dense are Continental plates?

Answer 2

2. 7g/cm^3

- 30/70km thick

Question 3

How do convection currents in the mantle work?

Answer 3

1. Radioactive decay of some elements in the Gutenberg Discontinuity generates a lot of heat.
2. When lower parts of the asthenosphere heat up, they become less dense and slowly rise.
3. As they move towards the top of the asthenosphere they cool down, becoming more dense, slowly sinking.
4. These circular movements of semi-molten rock are called convection currents.
5. They create drag on the base of tectonic plates causing them to move.

Question 4

Who proposed the theory of continental drift in 1912?

Answer 4

Alfred Wegener

Question 5

What did Wegener base his theory on?

Answer 5

1. Geological evidence

2. Fossil records

Question 6

Who created the theory for motion, concerning thermal convective cells in the asthenosphere in the 1930s?

Answer 6

Arthur Holmes

Question 7

Who discovered mid-oceanic ridges in 1948?

Answer 7

Maurice Ewing discovered that oceanic crust was much thinner than the continental crust.

Question 8

Who discovered sea floor spreading, providing the mechanism for plate motion in 1962?

Answer 8

Harry Hess

Question 9

What evidence is there for plate tectonic theory?

Answer 9

1. Geological
2. Fossil records
3. Climatology
4. Palaeomagnetism

Question 10

What geological evidence for plate tectonics theory is there?

Answer 10

1. Areas of South America and Africa have rocks of the same age and composition.
2. Similar rock types and distribution of mountain ranges e.g Scotland, Norway, Sweden and Finland are similar to those on the east coast of North America.
3. These rocks and mountains must have formed under the same conditions and in the same place in order to match so well.

Question 11

What fossil evidence is there for plate tectonics theory?

Answer 11

1. Fossil Branchiopoda found in some Indian limestones are comparable with similar fossils found in Australia.
2. Fossil remains of the reptile; Mesosaurus are found in both Southern Africa and South America - unlikely to have developed in two such different locations.

Question 12

What climatological evidence for plate tectonics theory is there?

Answer 12

1. Similar glacial deposits are found in Antarctica, Africa, South America, India and Australia.
2. Large coal deposits found in tropical conditions have been found in North America and parts of Europe - suggesting these regions were once closer together.

Question 13

What is Palaeomagnetism?

Answer 13

1. It is the study of the history of the earth’s magnetic field.
2. Once every 200,000 years, the Earth’s magnetic field reverses polarity.
3. Paleomagnetism has provided evidence for sea floor spreading.
4. As magma erupts from the mid-oceanic ridges, magnetic materials in the molten rock align themselves with the direction if the Earth’s magnetic field.
5. When the new crust has solidified, the alignment is fixed.
6. The magnetic minerals in the crust, are created in periods of normal polarity (magnetic north near North Pole) or are crated in periods of reverse polarity (magnetic north near the South Pole).
7. This creates a series of alternating magnetic stripes along the sea floor.
   - this striped pattern is mirrored either side of the mid-ocean ridge, suggesting that the ocean crust is slowly spreading away from the boundary.

Question 14

What happens at a Constructive (divergent) margin?

Answer 14

1. It is where two plates are moving part from each other due to convection currents in the asthenosphere.
2. The convection currents are created by decaying radioactive particles in the core/mantle which generates a lot of heat, causing the lower parts of the asthenosphere to melt.
3. These parts then rise to the surface as they are less dense than the surrounding magma.
4. As they rise they cool and become more dense, subsequently causing them to sink.
5. As they sink they drag the plates above them causing the plates to move apart.
6. When the plates move part, they release pressure causing the mantle to melt, producing magma which fills the space between the two plates as it has risen up because it is less dense than the surrounding plates.
7. As it reaches the surface it can erupt to form a volcano.

Question 15

What landforms are created at Constructive Margins?

Answer 15

1. Mid-Oceanic Ridge e.g Iceland

2. Rift Valley e.g East African Rift Valley

Question 16

What happens at rift valleys?

Answer 16

1. Plates diverge beneath land, leading to magma rising, causing the continental crust to bulge and fracture, forming fault lines.
2. As the plates keep moving part the crust between parallel faults drops down to form a Rift Valley
   E.g East African Rift Valley - stretches 400km formed because of the Namibian and Somalian plates are diverging.

Question 17

How are Mid-Ocean ridges formed?

Answer 17

1. When diverging plates are underwater, a mid-ocean ridge forms.
2. Underwater volcanoes erupt along the mid-ocean ridges and they can build up to be above sea level e.g Surtsey, Iceland.

Question 18

What happens at a destructive margin?

Answer 18

A destructive margin occurs when two plates are moving towards each other.
- there are three types of convergence.

Question 19

What happens at an Oceanic-Continental Convergence boundary?

Answer 19

1. This occurs when continental crust and oceanic crust converge.
   - the more dense oceanic crust is forced under the less dense continental crust (subduction).
2. This forms a Deep Sea Trench e.g Peru-Chile trench in the Pacific Ocean (the Nazca plate subducts under the South American plate)
3. Fold mountains are formed where the plates meet, sediments accumulating on the continental shelf are forced upwards and so are deformed by folding and faulting. E.g Andes
   (Fold mountains occur at oceanic-continental or continental-continental boundaries)

Question 20

How are volcanoes formed at Destructive margins?

Answer 20

1. They are formed when the denser oceanic plate is subducted and then melts due to friction with the continental plate. (Area where this takes place is the Benioff zone)
   - it also melts due to extreme pressure and heat in the mantle.
2. The magma is less dense than the continental crust above and so will rise back to the surface to form a volcano, via mantle plumes.
3. Lava produced is andesitic by nature, meaning it is more viscous, thus creating complex, composite, explosive volcanoes.

Question 21

How do earthquakes occur at convergence boundaries?

Answer 21

They occur when one plate gets stuck subducting under another plate, which creates pressure build up,many after a sudden release of pressure and earthquake occurs.

Question 22

What happens at Oceanic-Oceanic convergence boundaries?

Answer 22

The denser of the two plates will be subducted, forming a deep sea trench and melting.
- earthquakes are also triggered as the plate subducts.
The resulting rising magma from the Benioff Zone forms submarine volcanoes along the plate margins which mag grow to form island arcs.
E.g the Marianas Trench (Pacific plate subducted beneath the smaller Philippine plate.

Question 23

What happens at Continental-Continental plate boundaries?

Answer 23

1. Continental plates are of lower density than the asthenosphere beneath them, therefore subduction does not occur.
2. The colliding plates, and any sediments deposited between them, simply become uplifted and buckle to form high fold mountains e.g the Himalayas.(Eurasian plate colliding with the Indian Plate)
   - volcanic activity does not occur at these margins due to no subduction or melting; however shallow-focus earthquakes can be triggered.

Question 24

What happens at a Conservative plate margin?

Answer 24

1. This occurs when two plates slide past each other.
2. There is no destruction or subduction of crust, and so there is no melting of crust. Thus, there is no volcanic activity or formation of new crust.
3. Despite, the absence if volcanic activity, these margins are tectonically extremely active and are associated with powerful earthquakes.
4. Friction between the two moving plates leads to stresses building up whenever ‘sticking occurs’.
5. These stresses may eventually be released suddenly as powerful shallow-focus earthquakes e.g Los Angeles 1994
   - The San Andreas Fault occurs due to the North American plate striking and slipping past the Pacific plate.

Question 25

How do hot spots form?

Answer 25

1. A hot spot is where a concentration of radioactive elements in the mantle causes very not temperatures, which in turn causes a plume of magma to rise towards the surface.
2. Eventually it burns through the lithosphere, causing a volcano to be formed. (They are often found where the crust is thinner (often due to a divergent boundary)
3. Hot spots are stationary, so as the plate moves, the volcanic islands drift away from the not spots, becoming extinct. -> whilst the volcano above the hot spot remains active.
   Example: the Hawaiian Islands are near the centre of the Pacific Plate and are a chain of active and extinct volcanoes.
   - they prove tectonics theory as they prove the Earth’s crust is moving.

Question 26

How much do the Hawaiian Islands move per year?

Answer 26

The islands are moving north west by 10cm/y^-1