4. Plate Tectonic Theory

Question 1

The core

Answer 1

• at the centre
• Inner core: at the very centre of the Earth and hottest part. It’s solid, mostly consists of iron
• Outer core: semi-molten and mostly consists of liquid iron and nickel

Question 2

The mantle

Answer 2

• Surrounds the core and is the widest layer
• The upper part is solid but below it, the rock is semi-molten- forming the asthenosphere

Question 3

The crust

Answer 3

• Forms the outer shell of the Earth
• Oceanic: thin, dense layer which lines the ocean floors
• Continental: older, thicker layer, less dense layer which makes up the Earth’s landmasses

Question 4

What is the lithosphere?

Answer 4

The crust and upper mantle, the solid layer from which tectonic plates are formed

Question 5

What is continental drift?

Answer 5

The gradual movement of the continents across the earth’s surface through geological time.

Question 6

Who was Alfred Wegener?

Answer 6

• Creator of Plate Tectonic Theory
• Believed continents gradually began to drift apart around 300 million years ago
• Holmes supported the idea by creating convection current theory

Question 7

What happened in 1912?

Answer 7

Wegener theory of continental drift

Question 8

What happened in 1940s?

Answer 8

Harry Hess reveals shape of ocean floors , using sonar and radars

Question 9

What happened in 1960s?

Answer 9

A drill ship collects crystal samples and dates them

Question 10

What happened in 1963?

Answer 10

J Tuzo Wilson proposes theory of sea floor spreading

Question 11

What happened in 1965?

Answer 11

J Tuzo Wilson and Harry Hess propose theory of plate tectonics

Question 12

What is the geological evidence for the theory?

Answer 12

• the fit of South America and West Africa
• evidence of a late carboniferous glaciation deposits from which are found in South America, Antarctica and India. Formation of these deposits cannot be explained by their present position- must have formed together and moved
• rock sequences in northern Scotland closely agree with those found in eastern Canada

Question 13

What is the biological evidence for the theory?

Answer 13

• fossil brachiopods found Indian limestone are comparable with similar fossils in Australia.
• fossil remains of Mesosaurus found in South and North Africa
• fossilised remains of a plant which existed when coal was being formed have been located only in India and Antarctica

Question 14

What is paleomagnetism?

Answer 14

• In the 1950s, studies of paleomagnetism confirmed the sea floor was spreading.
• Every 400,000 years the Earth’s magnetic fields change direction- causing magnetic N+S poles to switch
• When lava cools to become rock, minerals line up with the Earth’s polarity at the same time. Scientists studying mid ocean ridges found the same pattern of magnetic direction at either side of the ridges (something that could only happen if new rock was being formed at the same time on both sides)

Question 15

What is slab pull?

Answer 15

Denser oceanic plate sinks below less dense continental plate and the gravitational pull of the oceanic plate pulls the rest of the plate along with it.

Question 16

What is ridge pull?

Answer 16

• Magma rises as the plate moves apart.
  The magma cools to form new plate material; as it cools, it becomes denser and slides down away from the ridge which causes other plates to move away from each other.

Question 17

How does an oceanic ridge form?

Answer 17

• Continuous features created when plates move apart in oceanic areas.
• The space between the separating plates is filled with basaltic lava upwelling form a ridge
• Volcanic activity occurs along the ridges forming submarine volcanoes which may eventually rise above sea level to create islands

Question 18

Example of an oceanic ridge

Answer 18

Surtsey south of Iceland

Question 19

How are deep sea trenches formed?

Answer 19

• Destructive boundary
• Oceanic and continental plates meet and the denser oceanic plate is forced underneath the lighter continental one. The downwarping of the oceanic plate forms a very deep part of the ocean known as a trench.

Question 20

Example of a deep sea trench

Answer 20

Off the Western Coast of South America, the Nazca plate is subducted under the South American plate forming the Peni-Chile trench

Question 21

How are Island Arcs formed?

Answer 21

• During subduction, the descending plate encounters hotter surroundings and this and heat is generated from friction begins to melt the plate.
• As this material is less dense than the surrounding asthenosphere, it begins to rise towards the surface as plutons of magma.
• Eventually, these reach the surface and form complex, composite and explosive volcanoes.
• If eruptions take place offshore, a line of volcanic islands forms.

Question 22

Example of an Island Arc

Answer 22

Marianas Islands and Guam

Question 23

How are rift valleys created?

Answer 23

• Form at constructive plate margins in continental areas
• Magma rises from below and heats the crust which expands and starts to rift
• As the brittle crust rifts and fractures, sections of it move apart.
• Areas of crust drop down between parallel faults to form rift valleys.
• Through the thinning and stretched crust, magma forces its way to the surface to form volcanoes

Question 24

Example of a rift valley

Answer 24

Mount Nyragongo

Question 25

How are young fold mountains created?

Answer 25

• Destructive plate boundary
• Continental plates have a lower density so there’s not much subduction when plates meet.
• When these plates moves towards each other, their edges and sediments are forced up into fold mountains
• As there’s little subduction, there’s no volcanic activity.
• Material is also forced down to form deep mountain roots.

Question 26

Example of young fold mountains

Answer 26

The Himalayas