Continental Drift

DefinitionThe very slow movement of the continental plates

on the surface of the Earth due to tectonic activity through geological time.

What is the evidence of Continental Drift?Continental Fit

Some continents such as the Western seaboard of Africa and the Eastern seaboard of South America seem to fit together as if placed beside each other.

What is the evidence of Continental Drift?Geological evidence

Rocks of the same age and type and displaying the same formations are found in south-east Brazil and South Africa.

The trends of the mountains in the eastern USA and north-west Europe are similar when placed in their old positions.

Similar glacial deposits are found in Antarctica, South America and India, now thousands of km apart.

Striations showing the same orientation when the continents are reunited, are found in Brazil and West Africa. Striations means a series of ridges, furrows

or linear marks, and is used in several ways

What is the evidence of Continental Drift?Climatic evidence

Parts of Antarctica, North America and the UK all contain coal deposits of similar age that were formed in tropical conditions.

These areas are no longer in tropical climates which suggests that they have drifted apart since the Carboniferous period.

• Glacial Striations (scratches in the rocks) can be found that match in Brazil and West Africa

• Glacial deposits are also very similar in India, South America and in the Vaal Valley in South Africa

• These landmarks suggest that although these areas are very far apart now they once had a very similar climate

What is the evidence of Continental Drift?Biological evidence

Similar fossil formations are found on either side of the Atlantic. For example, a reptile called Mesosaurus, is found only in South America and Southern African sediments. These are dated from the Permian age (aprox. 280million

years ago).Plant remains from the humid swamps that later

formed coal deposits have been found in India and Antarctica.

Marsupials are found only in Australia because it drifted away from the main supercontinent before the predators that wiped them out elsewhere had migrated there.

What is the evidence of Continental Drift? Paleomagnetism

This is when hot magma rises to the earth's surface and cools, the minerals themselves become magnetized.

Rocks formed in different places on the planet are magnetized differently. When rock layers were discovered with different magnetism than the original rock, it

points to continental drift. The Mid Atlantic Ridge was the first piece of evidence to support Wegener’s Theory. 2 British Geologists, Vine and Matthews, discovered magnetic stripes or banding

running parallel to the ridge in the 1960s. These stripes corresponded to times when the Earth’s magnetic field reversed from

North to South and so on, and iron particles in the erupting magma either side of the ridge cooled and aligned themselves with the Earth’s polarity at that time.

These symmetric stripes supported a theory by Hess on sea floor spreading, which we now know exists at the Ridge. In addition, by studying the stripes rates of spreading could be calculated.

This has been further supported by studies of the ages of rocks either side of the ridge. Rocks closest to the ridge are youngest (up to 10 million years old) and those furthest away are oldest (over 156 million years old) on both sides of the ridge.

Theory of Continental drift In 1915, the German geologist and meteorologist Alfred Wegener first proposed

the theory of continental drift, which states that parts of the Earth's crust slowly drift atop a liquid core. The fossil record supports and gives credence to the theories of continental drift and plate tectonics.

Wegener hypothesized that there was a gigantic supercontinent 200 million years ago, which he named Pangaea, meaning "All-earth".

Pangaea started to break up into two smaller supercontinents, called Laurasia and Gondwanaland, during the Jurassic period. By the end of the Cretaceous period, the continents were separating into land masses that look like our modern-day continents.

The theory of continental drift was not accepted for many years. One problem was that a plausible driving force was missing. And it did not help that Wegener was not a geologist.

Other geologists also believed that the evidence that Wegener had provided was not sufficient.

It is now accepted that the plates carrying the continents do move across the Earth's surface; ironically one of the chief outstanding questions is the one Wegener failed to resolve: what is the nature of the forces propelling the plates?

Jigsaw Evidence Francis Bacon was one of the first people to note that the

West coast of Africa and Europe seem to have a “jigsaw fit” with the Eastern seaboard of North and South America.

Wegner provided more evidence stating that the shape of the east coast of South America fits the west coast of Africa, like pieces in a jigsaw puzzle.

Alfred described how the continents would of fit together forming a super continent which he named Pangea.

However Wegener was trained as an astronomer and a meteorologist therefore many geologists did not think that he had the right background to judge or make geological theories. For this reason they dismissed much of his evidence.

Fossil Evidence in Support of the TheoryEduard Suess was an Austrian geologist who first realized that there

had once been a land bridge connecting South America, Africa, India, Australia, and Antarctica. He named this large land mass Gondwanaland (named after a district in India where the fossil plant Glossopteris was found).

This was the southern supercontinent formed after Pangaea broke up during the Jurassic period. Suess based his deductions on the fossil plant Glossopteris, which is found throughout India, South America, southern Africa, Australia, and Antarctica.

Fossils of Mesosaurus (one of the first marine reptiles, even older than the dinosaurs) were found in both South America and South Africa.

These finds, plus the study of sedimentation and the fossil plant Glossopteris in these southern continents led Alexander duToit, a South African scientist, to bolster the idea of the past existence of a supercontinent in the southern hemisphere, Eduard Suess's Gondwanaland.

This lent further support to A. Wegener's Continental Drift Theory

Ocean floor spreading theory In the 1960s a Scientist called Harry Hess said that not only were the

continents moving but also the sea floor was moving. The conveyer belt motion of the sea floor meant that youngest rocks

were found on the mid-ocean ridges and then get older, further away from the mid-ocean ridge. This shows that the sea floor is spreading and ultimately moving the continents to cause the continental drift.

Hess said that the movement of the sea floor was to do with the convection currents in themantle because the hot magma that rises is less dense than the cooler magma; this magmacools are forms new rocks when it reaches the seas surface and consequently other magma that rises pulls the sea floor apart moving the continents around. Then at the plate boundaries the cool rocks sinks and melts from the mantles heat.

2 new supercontinentSouth African geologist Alexander du Toit maps

out a northern super-continent, "Laurasia," to explain coal deposits, which presumably indicate the remains of equatorial plants, in the Northern Hemisphere.

Laurasia formed when Pangea broke up to form 2 smaller supercontinents.

The other supercontinent was locatedin the southern hemisphere and was known as Gondwanaland.

Jack OliverJack Oliver provided a

research paper on plate tectonics that proved vital in providing hard evidence for the existence of continental drift, which was released in 1968.

The 1968 paper finally proved Wegener right and continental drift was accepted by the world of science.