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Earth structure, plate tectonics theory: convection currents and sea-floor spreading. Evidence: continental drift and palaeomagnetism.

Continental Drift and Plate Tectonics

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.  However, it wasn’t until Alfred Wegener (a 1900s German Meteorologist) put forward his theory that plates moved that great strides were made into understanding this element of our Earth.  It is thought that in the past the Earth had one supercontinent, known as Pangea, 300 million years ago, which has slowly drifted apart.

Pangea reforming backwards through time


Alfred Wegener

The Evidence provided to support this theory is;

Paleomagnetism and age of rocks along the Mid Atlantic Ridge

The first piece of evidence to support Wegener’s Theory came from the discovery of a ridge of mountains running along the middle of the Atlantic Ocean known as the Mid Atlantic Ridge.  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. Watch an amazing animation here

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 10million years old) and those furthest away are oldest (over 156 million years old) on both sides of the ridge.

Graphic of crustal ages either side of the Mid Atlantic Ridge


Fossils like Mesosaurus and Glossopteris

Fossil evidence is essential in helping explain continental drift.  The plant Glossopteris is a fern that has been found in Africa, Antarctica, Australia and South America.  It is used as evidence that these continents must have at some point around 250 million years ago been joined.  Mesosaurus is an extinct reptile that has been found in both Africa and South America.  As Mesosaurus was a coastal animal, and therefore could not have+ crossed the Atlantic Ocean, this distribution indicated that the two continents used to be joined together.

Glossopteris and Pangea


Glossopteris Fossils - Source

Mesosaurus - Source
The image below shows how Glossopteris and Mesosaurus fossil evidence help to show that the continents would have been joined together as Pangea.  Source.

Click on the above for a bigger, cleaner image

Isle of Arran footprints

Fossil footprints in the Isle of Arran of the reptile Chirotherium have been matched to those in Eastern Canada, suggesting the North American and European continents used to be joined.

Glacial evidence

Widespread distribution of Permo-Carboniferous glacial sediments in South America, Africa, Madagascar, Arabia, India, Antarctica and Australia was one of the major pieces of evidence for the theory of continental drift. The continuity of glaciers, inferred from oriented glacial striations and deposits called tillites, suggested the existence of the supercontinent of Gondwana, which became a central element of the concept of continental drift. Striations indicated glacial flow away from the equator and toward the poles, in modern coordinates, and supported the idea that the southern continents had previously been in dramatically different locations, as well as next to each other (source)

Mountain chains

The continuity of mountain chains across continents also provide evidence for Pangea and continental drift. A great example of this is the Appalachian Mountains chain which goes from the northeastern United States to the Caledonides of Ireland, Britain, Greenland, and Scandinavia, despite the Atlantic Ocean being in the middle! These are also displyed on the map above.