Plate Tectonics 1 Flashcards

Question

Explain the sea floor spreading theory

Answer 1

Oceans were shallower in the middle and identified the presence of Mid Ocean Ridges rising as a high as 1.5km In addition found that the deepest parts of the ocean trenches were found very close to the continental margins in the pacific Hess believed that oceans grew from their centres from molten material escaping the mantle along the ridges. This created new seafloor which then spread away form the ridge in both directions The ocean ridge was thermally expanded and consequently higher than the ocean floor further away. As spreading continued, the older ocean floor cooled, became more dense and eventually subjected at a deep ocean trench

Answer 2

Hess believed that ocean trenches were the locations where ocean floor was destroyed and recycled - However didn’t have enough evidence to prove it Couldn't explain Hawii

Answer 3

British geologists - their work looked at the patterns of magnetic striopes in the sea floor. Confirmed Hess' theory

Answer 4

When the material from the mantle rises up through mid-ocean ridges and cools, it preserves a record of the Earth's magnetic field. This is because the magnetite in the basalts is strongly magnetic, and aligns with the field when it cools

Answer 5

Hess lacked the evidence to assert that oceanic plates were created at Mid Ocean Ridges When basaltic lava cools on the sea floor, individual minerals separate - especially iron - and these minerals then align themselves on the sea floor in the direction of the magnetic pole Earth's magnetic field swaps direction at points throughout time Noticed there was a symmetrical pattern of magnetic stripes on either side of the mid ocean ridges. In addition when the basalts of the sea floor were dated, they were found to be the same age at similar distances away from the ridge on each side. This suggested that the ocean floor was created at the mid ocean ridges, then was split in half by later activity and pushed sideways.

Answer 6

1963, in a paper called 'magnetic anomomlies over oceanic ridges'. Became the first scientiic test for sea floor spreading But still didn't explain Hawii

Answer 7

Most volcanic activity is found at locations of sea floor spreading (Hess) or subduction zones. Some volcanic locations such as Hawaii are far away from any known sea floor spreading location or subduction zone.

Answer 8

Continental drift theories were accumulating without any explanation for land masses such as Hawii In 1963, Tuzo-Wilson proposed that plates may move over fixed hotspots in the mantle to create a string of volcanic islands Hawaii is composed of 19 volcanic islands.

Answer 9

Radioactive decay in the Earth's core generates hot temperatures. If the decay is concentrated, hotspots will form around the core Heat the lower mantle creating localised thermal currents where magma plumes rise vertically Burn through the lithosphere to create volcanic activity on the surface Hotspot remains staionary, the movement of the overlying plate reults in formation of a chain of active volcanoes. Hawii As oceanic lithosphere oves away, volcanic activity ceases, cools and becomes an island

Answer 10

Conservative plate boundries Plates move transversely – causes earthquakes but no volcanoes, without destroying or creating any oceanic crust

Answer 11

There was lots of evidence about continental drift but now evidence to prove why it was happening Heard Fredrick Vine speak about sea florr spreading and magnetic anomolies

Answer 12

He suggested there are two layers in the mantle, each of which are in motion, controlling the movement and behaviour of the tectonic plates above. Convection currents in the mantle occur when the hot + less dense magma in mantle rises, cools and sinks becuase it is more dense. This repeast in a cyclical motion forming convection currents Destructive - convection currents force the continental and oceanic plate together. Oeceanic more dense so subducts Constructive - two plates are forced apart by convection currents forning a gap between the plates

Answer 13

Mantle convection being resposnible for the movent of tectonic olates is now largely out of favour, with modern imagaing techniques unable to identify convection cells in the mantle sufficientley large to drive plate movement

Answer 14

Gravitational sliding

Answer 15

Combination of ridge push and slab pull

Answer 16

Takes place with plate movement driven because of the higher elevation of plates at ocean ridges. As fresh magma wells up at mid-ocean ridges to form new young, oceanic lithosphere, a higher elevation is formed at spreading ridges. The new oceanic crust gradually cools, gets denser and thickens with age and is pushed ‘downhill’ by gravity as new magma emerges from the active zone of divergence behind it (and thus adds distance from the ridge).

Answer 17

More significant gravitational force acting on the plates Occurs at destructive plate margins where an oceanic plate subducts under a continental plate, working in tandem with convection currents and gravity Due to the extreme density and mass, the subducting plate pulls with it the rock behind and forces it to move along as it subducts