hazardous earth 6

Question 1

the development of plate tectonic theory

Answer 1

1 - Alfred Wegener: 1915, proposed idea that continents had once been connected as a large land mass called Pangea, and had drifted away over time. Evidence: cape mountain chain in S america and SA, jigsaw fit of continents, fossil evidence: glossopteris, mesosaurus, glacial deposits.
2 - Marie Tharp: 1948:used echo sounding during coldwar to map out seafloor for submaines, discovered 60,000 mid antlantic ridge with a rift valley in the centre, could only be formed if continents had drifted.
3 - Harry Hess: seafloor spreading: He suggested molten rock was erupting from the crack creating new ocean floor and then moving away. Meaning The earth was like a conveyor belt and the continents moved with the crust. Notices ocean bed was shallower in middle.
4 - Fred Vines: Paleomagnetism is the record of the strength and direction of the Earth’s magnetic field in rocks, pattern of magnetic strpes on either side of ocean ridges, rocks preserve plarity showing earths poles flipped.

Question 2

what mechanism causes plates to move?

Answer 2

Convection Currents:
Hot mantle rock rises as it is less dense and then spreads out across the surface, cools and becomes denser so sinks back down to be reheated and made to rise again. The circular flow of the mantle rocks by convection causes “frictional drag” on the overlying lithospheric plate causing plate movement.

There are 3 mechanisms that contribute to plate movement:
1 - slap pull: The weight of subducting ocean crust into the asthenosphere at an oceanic trench pulls down the rest of the plate with it
2 - Plate Drag: Convection currents in the asthenosphere “drag” the overlying plate with it.
3 - Ridge Push: Upwelling of magma at the midcauses the plates to push apart.

Question 3

divergent plate bounaries characteristics

Answer 3

Hot mantle rock rises up through asthenosphere, pressure causes solid mantle rock to partially melt (decompressional melt), basalitc magma forms. As magma rises,overlying rock thermally expands and is forced up, magma erupts on ocean floor, this adds new oceanic crust = seafloor spreading = plates move

features;
Shallow focus earthquakes, occur on transform faults. Pillow lavas - eruptions occur udnerwater magma cools rapidly. Rift valleys. Black smokers: superheated jets of water, support specialised ecosystems.

Mid ocena ridges are found here, they are not continuous as at frequent intervals they are broken into segments by transform faults, which displace the ridge sideways.

Question 4

convergent plat boundaries

Answer 4

O/C boundaries: earthquakes in the Benioff zone due to faulting. Ocean trenches: narrow deep depression adjacent to subduction zone: as crust descends into asthenosphere it causes deepening f the ocean as the leading edge of overirding continental crust buckles forming trench, explosive volcanoes.

O/O boundaries: ocean trenches, island arc: subduction occurs, as the descending plate melts, magma rises to the surface and forms chains of volcanic islands. Explosive volcanoes, earthquakes e.g. Japan trench.

C/C boundaries: fold mountains, shallow docus earthquake due to no subduction, reverse faults.

Question 5

constructive plate boundaries

Answer 5

• Two plates are sliding past each other either in opposite directions or same direction but diff speeds. No crus is created or destroyed so no volcanic activity.
• However there can be shallow focus earthquakes: as they slide past one another, they can get lockedtogether via friction, pressure builds. When pressure released sections of plate jerk forward shallow earthquake. It is a strike slip fault. e.g. Haiti EQ
  e.g. San Andreas fault, Californian fault system extends for 1200km, there has been 1000km of lateral movement in the last 25 million years.