Plate tectonic theory

Question 1

Who made the plate tectonic theory and in what year?

Answer 1

Alfred Wegener - 1912

Question 2

What did Wegener propose?

Answer 2

A single supercontinent existed about 300 million years ago called Pangaea

Question 3

What happened to Pangaea?

Answer 3

It split into two continents: Laurasia in the North and Gondwanaland in the south

Question 4

How did today’s continents form?

Answer 4

By Laurasia and Gondwanaland splitting off as a result of continental drift

Question 5

What is the main geological evidence for continental drift?

Answer 5

The bulge of south America fits into the indent below West Africa

Question 6

What is geological evidence for continental drift - glaciation

Answer 6

Deposits from a late-Carboniferous glaciation, 290 million years ago, are found in South America, Antarctica and India. The formation of these deposits cannot be explained by their present position; they must have been formed together and then moved. There are all striations of rocks in Brazil and West Africa which point to a similar situation.

Question 7

What is geological evidence for continental drift - rock sequences

Answer 7

Rock sequences in northern Scotland closely agree with those found in eastern Canada, indicating that they were laid down under the same conditions in one location

Question 8

What is biological evidence for continental drift - fossil brachiopods

Answer 8

Fossil brachiopods found in Indian limestone are comparable with similar fossils in Australia

Question 9

What is biological evidence of continental drift - reptile

Answer 9

fossil remains fo the reptile mesosaurus are found in both South America and southern Africa. It is unlikely that the same reptile could have developed in both areas or that it could have migrated across the Atlantic

Question 10

What is biological evidence for continental drift - plant

Answer 10

the fossilised remains of a plant which existed when coal was being formed have been located only in India and Antarctica

Question 11

Why did people not take Wegener seriously?

Answer 11

His theories were unable to explain how continental movement could have taken place and his ideas gained little ground

Question 12

When did evidence start to accumulate for Wegener’s theory?

Answer 12

1940s onwards

Question 13

What is sea floor spreading?

Answer 13

The mid-Atlantic ridge was studied and they found that the ocean crust on either side of it suggested sea-floor spreading was occurring. This is because the alternating polarity of the rocks that form the oceanic crust. Iron particles in lava erupted on the ocean floor are aligned with the Earth’s magnetic field. As the lavas solidify, these particles provide a permanent record of the Earth’s polarity at the time of eruption, known as palaeomagnetism. However, the Earth’s polarity reverses every 400,000 years resulting in a series of magnetic ‘stripes’ with rocks aligned alternatively towards the north and south poles. This stripped pattern, which is mirrored exactly on either side of a mid-oceanic ridge suggests that the oceanic crust is slowly spreading away from this boundary. Moreover, the oceanic crust gets older with distance from the mid-oceanic ridge.

Question 14

What are the implications of sea floor spreading?

Answer 14

The earth must be getting bigger, as this is not the case then plates must be being destroyed somewhere to accommodate the increase in their size at mid-oceanic ridges. Evidence of this was found in the discovery of huge oceanic trenches where large areas of ocean floor were being pulled downward in a process known as subduction.

Question 15

Describe the role of hot spots

Answer 15

Hot spots around the core of the earth generate thermal convection currents within the asthenosphere, which cause magma to rise towards the crust and then spread before cooling and sinking. This circulation of magma is the vehicle upon which the crustal plates move. The crust can be thought of as ‘floating’ on the denser material of the asthenosphere. This is a continuous process with new crust being formed along the line of constructive boundaries between plates and older crust being destroyed at destructive boundaries. Where two crustal plates slide past each other and the movement of the plates is parallel to the plate margin, there is no creation or destruction of crust. At these conservative margins, there is no subduction and therefore no volcanic activity.

Question 16

How are ocean ridges formed?

Answer 16

They are the longest continuous uplifted features on the surface of the planet formed when plates move apart in oceanic areas. The space between the plates is filled with basaltic lava upwelling from below to form a ridge. Volcanic activity also occurs along these ridges, forming submarine volcanoes which sometimes rise above sea level, such as Surtsey to the south of Iceland.

Question 17

How are rift valleys formed?

Answer 17

They form when plates move apart on continental areas. In East Africa, for example, the brittle crust fractures as sections of it move apart. Areas of crust drop down between parallel faults to form the valley. An area between two parallel rift valleys forms an upstanding block, known as horst. The line of the African rift is thought to be an emergent plate boundary, the beginning of the formation of a new ocean as eastern Africa splits away from the remainder of the continent.

Question 18

How are deep sea trenches formed?

Answer 18

Where oceanic and continental plates meet, the denser oceanic plate is forced underneath the lighter continental one due to subduction. The downwarping of the oceanic plate forms a very deep part of the ocean known as a trench. Off the western coast of South America, the Nazca plate is subducted under the South American Plate forming the Peru-Chile trench. A similar process happens when two oceanic plates move towards each other. On the western side of the Pacific Ocean, the Pacific plate is subducted beneath the smaller Philippine plate forming the very deep Marianas Trench.

Question 19

How are island arcs formed?

Answer 19

During subduction, the descending plate encounters hotter surroundings and this coupled with the heat 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 the eruptions take place offshore, a line of volcanic islands form. These are known as island arcs. The Marianas islands and Guam form a very good example, running parallel to the Marianas Trench.

Question 20

How do young fold mountains form?

Answer 20

The plates forming continental crust have a much lower density than the underlying layers, so there is not much subduction when such plates meet. As such plates move towards each other their edges and the sediments between them are forced up into fold mountains. As there is little subduction, there is no volcanic activity. Material is also forced downwards to form deep mountain roots. At present, the Indo-Australian plate is moving northwards into the Eurasian plate. The previous intervening ocean, known as the Sea of Tethys, has had its sediments forced upward in large overfolds to form the Himalayas, a process which is continuing today. Sediments that have accumulated on the continental shelf, along the edge of a plate, can also be uplifted as the plate edges buckle during the subduction of a denser oceanic plate. The Andes, running down the area where the Nazca plate is being subducted beneath the lighter South American plate, are a good example. Molten magma rising from depth also gives rise to a number of explosive volcanoes in the Andes.

Question 21

What are convection currents?

Answer 21

The convection currents in the asthenosphere act as a conveyer belt to move the lithosphere above it. The heat comes from radioactive materials such as uranium in the earth’s core and works to encourage the processes of ridge push and slab pull. Different sections of convection cells give rise to difference forces, the rise results in ridge push and the fall results in slab pull.

Question 22

What is ridge push?

Answer 22

The molten magma that rises at a mid-ocean ridge is very hot and heats the rocks around it. As the asthenosphere and lithosphere at the ridge are heated, they expand and become elevated above the surrounding sea floor. This elevation produces a slope down and away from the ridge. Because the rock that forms from the magma is very hot at first, it is less dense and more buoyant than the rocks farther away from the mid-ocean ridge. However, as the newly formed rock ages and cools, it becomes more dense. Gravity then causes this older, denser lithosphere to slide away from the ridge, down the sloping asthenosphere. As the older, denser lithosphere slides away, new molten magma wells up at the mid-ocean ridge, eventually becoming new lithosphere.

Question 23

What is ridge push also known as?

Answer 23

Gravitational sliding

Question 24

What is slab pull?

Answer 24

At a subduction boundary, one plate is denser and heavier than the other plate. The denser, heavier plate begins to subduct beneath the plate that is less dense. The edge of the subducting plate is much colder and heavier than the mantle, so it continues to sink, pulling the rest of the plate along with it. The force that the sinking edge of the plate exerts on the rest of the plate is called slab pull.

Question 25

How do hot spots form?

Answer 25

Radioactive decay in the earth’s core generates extremely hot temperatures. Where this decay is concentrated in a particular area, hot spots will form around the core. These hot spots heat the lower mantle creating localised thermal currents where magma plumes rise vertically.

Question 26

What are magma plumes?

Answer 26

Experimental data suggests that they rise as a plastically deforming mass that has a bulbous plume head fed by a long, narrow plume tail. As the head impinges on the base of the lithosphere it spreads outward into a mushroom shape. Such plume heads are thought to have diameters between 500 to 1000 km.

Question 27

How do magma plumes create volcanic activity?

Answer 27

Where these magma plumes occasionally rise within the centre of plates they ‘burn’ through the lithosphere to create volcanic activity on the surface. As lithospheric plates move across these stationary hotspots, volcanism will generate volcanic islands that are active above the mantle plume, but become inactive and progressively older as they move away from the magma plume in the direction of plate movement. Thus, a linear belt of inactive volcanic islands and seamounts will be produced. A classic example of this mechanism is demonstrated by the Hawaiian islands.

Question 28

Give an example of an island formed from a magma plume

Answer 28

The “Big Island” of Hawaii lies above the magma plume. It is the only island that is currently volcanically active. The seven Hawaiian Islands become progressively older to the northwest. The main phase of volcanism on Oahu ceased about 3 million years ago, and on Kauai about 5 million years ago. The evidence here suggests that the magma plume currently generating basaltic lavas on the Big Island has been in existence for at least 70 million years!