Continental drift and theory of plate tectonics

Question 1

What is continental drift?

Answer 1

theory that continents are mobile and have moved across the earths surface through geographical time

Question 2

Geological and biological evidence of continental drift

Answer 2

• if all the worlds continents are pieced together, they near perfectly to form a super continent called Pangaea which existed 250ma during the carboniferous period
• The fit of eastern coastline of S America with that of s Africa
• evidence upper carboniferous glaciation is found in the southern hemisphere continents when they were once part of a supercontinent called Gondwanaland near the south pole around 290ma
• Mountain chains and some rock sequences show great similarity e.g NE Canada and N Scotland
• Coal found in Antarctica, coal unlikely to have formed in it current latitude ∴ must have moved from Pangaea location
• Fossil remains e.g Mesosaurus found in both S America and S Africa, unlikely that either could have evolved identically on separate continents
• similar marine shellfish found in Indian and Australian limestones

Question 3

What is palaeomagnetism?

Answer 3

traces of changes in the Earths magnetic field in the alignment of magnetic minerals in sedimentary and igneous rock

Question 4

What does palaeomagnetism show?

Answer 4

• technology used to detect small variations in earths magnetic field
• shows up as striped pattern across ocean floor
• this is due to igneous rocks in the ocean crust which form as a result of lava flows containing iron particles
• the iron particles cool, and orientate themselves according to the earths polarity at that time
• It was found that the earths polarity changes as does the orientation of iron particles in rocks every 400,00-500,000 yrs

Question 5

What is sea floor spreading?

Answer 5

Is the lateral movement of oceanic crust away from a mid- ocean ridge

Question 6

How does sea-floor spreading provide evidence of continental drift?

Answer 6

• Due to palaeomagnestism we know at regular intervals the polarity of the Earth reverses
• this results in a series of magnetic stripes with the sea-floor rocks aligning alternately towards north and south poles.
• This striped pattern, is mirrored exactly on either side of a mid-oceanic ridge, which suggests that the ocean crust is slowly spreading away from the MOR
• causing fresh molten rock from the asthenosphere to reach the ocean bed and then push older rock further away from the ridge
• eventually the sea floor reaches an ocean trench and is subducted
• In this way sea floor spreading moves material across ocean floors

Question 7

How does the age of sea floor rocks provide evidence for continental drift?

Answer 7

• thickest and oldest sediments found nearest to continents
• found that no rock in the oceans was older than 200ma ∴ supports fact that ocean crust gets sub-ducted and constantly recycled