Plate Tectonics

Question 1

What is the structure of the earth?

Answer 1

• formed approx. 4.6 billion years ago following the explosion of a star
• the heavier denser material moved to the centre of the earth as it was formed.

The crust was formed by molten material which slowly cooled. The rocks that make it up are fractured into Tectonic Plates. These move slowly due to convection currents.
The mantle is made up of molten Silicates and other minerals. Where the crust and mantle edge together is called the asthenosphere
The core is made up of iron and nickel. It is solid due to the large pressure and is 6200 c.

Question 2

What are the energy sources of the earth?

Answer 2

It has 2 internal heat sources:

1. Radiogenic heat: produced by the decay of radioactive isotopes including Uranium-238, Thorium-232 and Potassium-40
2. Primordial heat: the heat lost by the Earth as it continues to cool from its original formation. The core’s heat flow (heat leaving the core and going into the mantle) is thought to be due to primordial heat. The formation of the dense core could’ve caused superheating and rapid heat loss. Heat flow from the core is necessary for maintaining the convecting outer core and the geodynamo and Earth’s magnetic field; therefore primordial heat from the core enabled Earth’s atmosphere and thus helped retain the Earth’s liquid water

Question 3

What is the evidence for plate tectonics theory.

Answer 3

Seismic waves - the evidence for the layers of the earth. The speed of a wave will change depending on what part of the earth they are travelling through

P waves:
- they are the fastest that arrive first to recording stations
- they travel through solids and liquids
- they are compressional

S waves:
- they travel at half the speed of P waves
- they can only travel through solids
- they vibrate at right angles to their direction of travel

L waves: only travel on the surface and are the slowest
There are two types:
Love waves vibrate at right angles to their direction of travel with no vertical movement
Raleigh waves have bother vertical and horizontal displacement of the ground

Question 4

Describe the two types of crust?

Answer 4

Continental crust
- It is older (much of it is over 1 500 million years old), thicker and lighter and made up of mainly granite.
- It is 35Km thick on average and is less dense than oceanic crust
- It is mostly formed at destructive plate boundaries

Oceanic crust
- It is younger (most under 200million years old) and denser and made up of mainly basalt and Gabbro
- It is mainly formed at constructive plate boundaries or spreading mid ocean ridges.
- It is between 6 and 10km thick and is constantly being destroyed and recreated

Question 5

What are the theories for place movement over time?

Answer 5

Continental drift theory – Alfred Wegener 1912
- 300 million years ago there was one super continent called Pangea that has now split into our modern continents
Evidence:
- Geological:
The fit of South America and Africa
Deposits from a late-carboniferous glaciation period that are found in both South America, Antarctica and India
Rock sequences that are found in Scotland which match those found in Eastern Canada
- Biological:
Similar fossil brachiopods found in both Australia and India
Fossilised remains of a specific plant found in both India and Antarctica

Sea floor spreading – Harry Hess 1962
- He discovered that oceans were shallower in the middle – suggesting ocean grew from their centres with molten basalt coming up from the mantel along mid ocean ridges which were raised above the flat sea floor (abyssal plain)
- The ocean ridge was thermally expanded and consequently higher than the abyssal plain
- He believed ocean trenches were the locations where ocean floor was destroyed and recycled

Magnetic striping – Vine, Matthews and Morley
- They noticed that there was a symmetrical pattern of magnetic stripes on either side of mid ocean ridges. Because the magnetite in the basalts is strongly magnetic - so shows how the Earth’s magnetic field switches direction over time – so goes into stripes
- Similar ages of basalts were found to be similar distances away from the ridges on each side

Question 6

What are the plate boundaries?

Answer 6

Constructive: new crust is generated where plates pull apart. Has both seismic and volcanic activity

Destructive: denser oceanic crust is subducted under continental crust. Both seismic and volcanic activity

Collision: two continental crusts move towards each other and buckle upwards. Only has seismic activity

Conservative: plates slide horizontally past each other. Only has seismic activity

Question 7

What are the processes and landforms at each of the plate boundaries ?

Answer 7

Ocean ridges
- Are the longest uplifted features on the surface of the planet.
- They are created at constructive plate margins where the space between the plates is filled with basaltic lava that solidifies to form a ridge and new lithosphere.
- Submarine volcanoes can also form along the ridges
- E.g. Mid Atlantic Ridge – 3km high and 1 000 and 1 500km wide

Rift valleys
- At constructive margins where continental plates move apart.
- Rising magma causes the region to be lifted so as the crust moves apart it becomes brittle and fragments into sections. As they continue to move outwards, areas of crust can drop down between parallel faults to form a valley. The area of upstanding rock between 2 parallel rift valleys is known as a horst
- E.g. East African Rift Valley – up to 12km wide and 1 500m deep and is thought to be developed over a hot spot

Deep sea trenches
- Created in subduction zones. The down warping of the plate forms a trench
- Can also be created when 2 oceanic plates move towards each over
- E.g. Marianas trench

Island arcs
- During subduction, the descending plate encounters increased friction and heat – causing it to melt and become magma. Magma is less dense than its surroundings so rises and when it reaches the Earth’s surface it erupts.
- If this is offshore a line of volcanic islands (island arc) forms

Young fold mountains
- When continental plates collide, their edges are forced to fold upwards. Some material is forced downwards to create deep mountain roots.
- Fold mountains are also formed at destructive margins as the edge of the continental plate buckles upwards as the oceanic plate is subducted
- E.g. Himalayas and Andes

Question 8

Why do plates move

Answer 8

DOUBLE CHECK THAT HAVE ANSWERED !!!!!!!

Question 9

What is a magma plume?