1.2 Plate boundaries and plate movement

Question 1

what is the continental drift theory?

Answer 1

• Alfred Wegener believed there was a single continental called Pangaea which existed 300 million years ago
• he had several pieces of evidence to support the theory but he could not explain the mechanism driving plate movement

Question 2

what are some evidence for pangaea and continental drift?

Answer 2

• trilobite fossils from SW Scotland are also found on the other side of the Atlantic Ocean
• fossils of glossopteris were found in all of the southern continents, showing they were once joined

Question 3

what is the theory of convection currents?

Answer 3

• rocks in upper mantle are cool and brittle and can break under pressure to produce earthquakes, those in lower are hot and plastic and flow when subjected to pressure
• radioactive decay in the core produces heat which rises, drives convection currents and moves the tectonic plates
• convection currents operate as cells

Question 4

what is the theory of ridge push?

Answer 4

• the molten magma that rises at a mid ocean ridge is very hot and heats the rocks around it
  > causing them to expand and become elevated above the surrounding sea floor, producing a slope down away from the ridge push
• 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
• this cooling rock exerts a force on the spreading plates called ridge push

Question 5

what is slab pull?

Answer 5

• at a subduction boundary the denser oceanic the denser oceanic plate subducts beneath the less dense continental plate
• continues to sink
• the force the edge of the plate exerts on the rest of the plate is called slab pull

Question 6

what is paleomagnetism?

Answer 6

• sea floor spreading occurs at constructive boundaries under the oceans
  -when magma is created on land a rift valley forms e.g. the east african rift valley
• when magma cools at constructive plate boundaries it ‘locks in’ to the earths magnetic polarity
• this can be used to determine tge age of new tectonic crust