Continental Drift

Question 1

What is the theory of continental drift?

Answer 1

In 1912, Alfred Wegener proposed that, 250 million years ago, in the Carboniferous period, a large single tectonic plate (Pangea) existed. Initially it broke apart into two land masses to the north and south; this spread continued to give the present-day continental land masses

Question 2

Geological evidence for continental drift

Answer 2

• The fit of continents, such as South America and Africa on either side of the Atlantic
• Evidence from about 290 million years ago of the effects of contemporaneous glaciation in southern Africa, Australia, South America, India and Antarctica, suggesting that these land masses were joined at this time, located close to the south pole
• Mountain chains and some rock sequences on either side of oceans show great similarity (e.g. northeast Canada and northern Scotland)

Question 3

Biological evidence for continental drift

Answer 3

• Similar fossil brachiopods (marine shellfish) found in Australian and Indian limestones
• Similar fossil reptiles found in South America and South Africa
• Fossils from rocks younger than the Carboniferous period, in places such as Australia and India, showing fewer similarities, suggesting that they followed different evolutionary paths

Question 4

Evidence of sea floor spreading

Answer 4

Magnetic field data showed that fresh molten rock from the asthenosphere reached the sea bed and older rock was pushed away from the ridge. This theory of sea-floor spreading linked to the theory of continental drift. It became clear that plates were being moved by sea-floor spreading

Question 5

Palaeomagnetism

Answer 5

As lava erupts it cools and the magnetic orientation of the iron particles within the lava is “locked” into the rock. The direction of the Earth’s magnetic field changes every 400,000-500,000 years (palaeomagnetism). It was palaeomagnetic data that led to the idea of sea-floor spreading

Question 6

The age of sea-floor rocks

Answer 6

In the 1960’s an ocean drilling programme showed that the thickest and oldest sediments were near the continents and younger deposits were further out in the oceans, giving further support to the idea of sea-floor spreading