Hazards: Plate Tectonics

Question 1

What is the inner core?

Answer 1

It is a solid ball containing lots of iron and nickel
- 1270km

Question 2

What is the outer core?

Answer 2

It is semi-molten and also contains iron and nickel
- 2200km

Question 3

What is the mantle?

Answer 3

It is mostly made of silicate rocks
- the lower mantle (closets to the core) is solid
- the middle of the mantle is semi-molten (flows) and is called the asthenosphere
- the upper mantle is solid
—> overall the mantle is 2850km

Question 4

What is the crust?

Answer 4

The outer layer of the earth
- there are two types: continental crusts (thicker and less dense) and oceanic crusts (thinner and denser)
- 0-100km

Question 5

What is the lithosphere?

Answer 5

The upper mantle and the crust

Question 6

What is radiogenic heat?

Answer 6

• Radiogenic heat is generated from radioactive decay
• When a radioactive isotopes decays to become more stable, subatomic particles are expelled from the atom.
• The kinetic energy from particles is converted to heat by the collision with surrounding matter.
• Although not many isotopes on Earth, sufficient amount so large amounts of heat generated

Question 7

What is primordial heat?

Answer 7

• heat generated from the formation of Earth (4.5 billion years ago)
• believed to be 50% of the Earth’s heat
• formed as a result of collisions in the giant cloud of material
  • Gravity slowly gathered this gas and dust into clumps creating asteroids and small planets. collided repeatedly and got bigger, building up the Solar System.

Question 8

What is the plate tectonic theory?

Answer 8

original hypothesis, (Alfred Wegener) - CONTINENTAL DRIFT - to show how present continents were in different positions and joined in different configurations
- continental fit
- biological fit (where dinosaur species lived gives evidence that the continents were together at some point)
- Palaeoclimatic evidence - glacial striations (scratches caused by movement of ice sheets across rock surfaces, e.g. in Central India) and distinctive deposits of glacial till (unescorted debris dumped by ice sheets, e.g. in N Brazil)
—> basically glaciers found where they shouldn’t be

Question 9

Why was continental drift theory not widely accepted?

Answer 9

• No explanation of any mechanism (i.e. how the continents could move over solid earth).
• The idea of continental drift has now been subsumed by the plate tectonic theory, which explains how the continents move.

Question 10

How do convection currents cause plates to move?

Answer 10

• Heat from radioactive decay is transferred from outer core to the mantle.
• This plume of hot magma in the mantle moves toward the upper mantle, cools, then sinks again.
• Creates convection currents beneath the plates that cause the plates to move.
• Movement of plates caused by thermal convection of rocks in the asthenosphere —> drag along the overlying lithospheric plates.

Question 11

What is slab pull?

Answer 11

At destructive plate margins, denser crust is forced under less dense crust.
- the sinking of the plate edge pulls the rest of the plate towards the boundary
- scientists consider slab pull a stronger factor than ridge push/mantle convection in driving plate movements. As plates being subducted are the faster-moving ones.

Question 12

What is ridge push?

Answer 12

At constructive plate margins, magma rises to the surface and forms new crust, forming a slope.
- new crust cools and becomes denser. Gravity causes this rock to move downslope, from margin.
- this puts pressure on the tectonic plates, causing them to move apart
- ridge push is also know as gravitational sliding.

Question 13

What is sea floor spreading?

Answer 13

Atlantic Ocean revealed a linear chain of undersea mountains forming the mid-Atlantic ridge.
- triggered the theory of sea floor spreading, showing there was a mechanism allowing the opening of ocean basins and thus the movement of continents.

Question 14

What is the evidence for sea floor spreading?

Answer 14

• the alternating polarity of the rocks that form the oceanic crust.
• iron particles in lava erupted on ocean floor are aligned with earths magnetic field. As lava solidifys, these particles are a permanent record of earths polarity at time of eruption.
• polarity reverses (approx every 400,000 years). result is a series of magnetic stripes, with rocks aligned alternatively towards north and south poles. The striped pattern, which is mirrored exactly either side of mid-oceanic ridge, suggesting oceanic crust is slowly spreading away from boundary.
• further it gets from ridge, the older it gets. If new crust is being formed, old crust is destroyed somewhere else.

Question 15

What is the actual process of sea-floor spreading?

Answer 15

As plates move apart, magma rises up to fill the gap created, then cools to form a new crust. Over time the new crust is dragged apart and even more new crust forms between it.

Question 16

What are oceanic constructive plate boundaries?

Answer 16

Magma rises in the gap between the two plates separating, forming new land when it cools.
- less explosive underwater volcanos formed as magma rises.
- mid-ocean ridges - forms chains of submarine mountain ridges that extend for 1000s of kms. Breaks are transform faults cut across the ridges (at right angles to boundary, separating sections of the ridge).
- may widen at different rates, leading to frictional stresses building up, with shallow earthquakes to release tension.
- They can rise upto 4000m above ocean floor. Volcanic eruptions along the ridges can build submarine volcanos. these may grow and rise above sea level, creating volcanic islands such as Ascension Island and Surtsey in the Mid-Atlantic ocean.

Question 17

What is the formation of an ocean ridge (4 mark question)?

Answer 17

• formed where oceanic plates pull apart (1) - this often occurs on the sea floor/under the ocean
• as the plates diverge, pressure of the crust on the mantle is released (1) allowing molten rock to rise through the crust of the sea floor (1)
• as magma (now lava) erupts (as submarine volcanos) it cools, solidifies and forms shallow, linear ridges (1)
• the speed of the spreading plate affects the shape - e.g. slow rates result in steep ridges, while fats spreading produce wide, gentle slopes.

Question 18

What is a continental and continental constructive plate boundary?

Answer 18

• any land in the middle of the separation is forced apart, causing a rift valley
• volcanoes form where the magma rises
• eventually the gap will most likely fill with water and separate completely from the main island.
• the lifted areas of rocks are known as horses, whereas the valley itself is known as a graben.

Question 19

What is the formation of rift valleys (4 marks)?

Answer 19

• formed where continental plates pull apart (1) becoming an emergent constructive plate boundary (1)
• hot magma rises in the upper mantle (1) causing the crust to dome (1)
• linear faults/cracks form in the crust (1)
• as the plate is pulled apart, the crust between the faults sink (1), forming linear flat-bottomed and steep sided valley (1)
• where the rift is well developed the valley floor can often sink below sea level (1) forming lakes or inland seas.

Question 20

What’s a good example of a rift valley?

Answer 20

The great African rift valley
—> may mark formation of a new ocean, as Eastern Africa splits away from rest of the continents

Question 21

What are the 3 types of convergence for destructive plate boundaries?

Answer 21

1. Oceanic plate meeting continental plate, such as along the pacific coast of South Africa
2. Oceanic and oceanic, such as along the Marina Trench in the Western Pacific
3. Continental and continental plates, such as the Himalayas

Question 22

What is the oceanic plate and continental plate convergence for destructive plate boundaries?

Answer 22

• the denser oceanic plate subducts below the continental
• the plate subducting leaves a deep ocean trench as it bends (e.g. the Peru-Chile Trench along the pacific coasts of South America)
• as the 2 plates converge, the continental land mass is uplifted, compressed, buckled and folded into chains of fold mountains (such as the Andes) - as folding continues, it can become asymmetrical and then over folded (making a recumbent fold), even further would make the middle section so thin it might break, creating a nappe.
• the descending oceanic plate starts to melt at depths of 100km - completely destroyed by 700km. (This zone of melting is called the Benioff zone)
• the melting is caused by both increasing heat and friction - which also leads to tension, which may need to be released as intermediate or deep-focus earthquakes
• the melted plate also creates extra magma, which is less dense then the surrounding asthenosphere, so rises in great plumes, passing through cracks in the buckled continental plate and magma may eventually reach the surface to form explosive volcanos.

Question 23

What happens when a oceanic meets an oceanic plate for destructive plate boundaries?

Answer 23

• the denser and faster plate subducts beneath the other.
• this leads to deep ocean trenches.
• the resulting rising magma from the friction and tension from melting in the Benioff zone forms crescents of submarine volcanoes along the plate margins which may grow to form island arcs. Island chains may develop and form land masses over time (e.g. japan and Indonesia)
• the Marina Trench and the Marina islands in the Western Pacific illustrate this

Question 24

What happens at a continental and continental (collision) destructive plate boundary?

Answer 24

• both plates are not as dense as oceanic, so lots of pressure builds.
• no subduction of any crusts here
• pile up of continental crust on top of the lithosphere due to pressure between the plates
• fold mountains are formed. - e.g. the Himalayas
• volcanic activity does not occur as no subduction, but shallow focus earthquakes can be triggered
• young fold mountains are continuously compressing and growing higher - Everest is growing 5mm each year.

Question 25

What is a conservative plate boundary?

Answer 25

When two plates slide past each other
- along these margins the crust is not being destroyed by subduction and there is no melting of rock, therefore no volcanic activity or formation of new crust.
- however these margins can cause powerful earthquakes.
- friction between the two moving plates leads to stresses building up whenever any ‘sticking’ occurs. These stresses are eventually released as powerful shallow-focus earthquakes, such as Los Angeles (1994) and San Francisco (1906 and 1989)
- can cause fault lines where the ground is cracked by the movement (e.g. San Andreas faults)

Question 26

What are hotspots?

Answer 26

A small localised area of the earths crust where an unusually high heat flow is associated with volcanic activity.

Question 27

What is the formation of hotspots?

Answer 27

• radioactive decay in the Earths core
• heating of the lower mantle generating localised thermal currents
• plumes of magma rise through the mantle and burn through the earths lithosphere to create volcanic activity on the Earths surface
• magma plumes remain stationary whilst the overlying plates move.
• e.g. Hawaii
• this leaves a chain of extinct volcanoes which are no longer fed by the source of magma
• eventually, these erode, subside and become seamounts
• bends in the island chain show changes in the direction of plate movements.
  —> this is more evidence for plate tectonic theory

Question 28

How is Hawaii a good example of hotspots?

Answer 28

• the newest island is furtherest southeast, suggesting the pacific plate is moving north west.
• the older islands eventually subside due to their weight and are reduced to seamounts by marine erosion.