Continental drift

Although we are not directly conscious of it normally, the ground under our feet is continually moving. This is due to the natural phenomenon of continental drift due to plate tectonics. The continents and islands on which we live do not float upon the seas around us, but are part of the Earth's crust which itself rests on a molten lava-like mass called the upper mantle.



The earth's crust can be best imagined as a thick jigsaw covering the whole planet. The jigsaw pieces vary greatly in size and are known as plates. These plates are moving across the upper mantle at varying speeds and, sometimes, in different directions. Over time the continents we know today were connected in different combinations and were placed elsewhere on the surface of the upper mantle.

The breakup of Laurasia and Gondwana into the respective continents we know today is of vital importance in understanding the modern distribution of species (zoogeography). For example:

In the late Cretaceous about 90 million years ago, subsequent to the splitting off from Gondwanaland of a conjoined Madagascar and India, the India Plate split from Madagascar and began moving north at about 20 cm/yr (8 in/yr). It began colliding with Asia between 50 and 55 million years ago in the Eocene. During this time, the India Plate covered a distance of 2,000 to 3,000 km (1,200 to 1,900 mi) and moved faster than any other known plate. German geologists determined in 2007 that the reason the India Plate moved so quickly was that it is only half as thick as the other plates which formerly constituted Gondwanaland. The collision with the Eurasian Plate along the boundary between India and Nepal formed the orogenic belt which created the Tibetan Plateau and the Himalaya Mountains as sediment bunched up like earth before a plough. The India Plate is currently moving northeast at 5 cm/yr (2 in/yr), while the Eurasian Plate is moving north at only 2 cm/yr (0.8 in/yr). This is causing the Eurasian Plate to deform and the India Plate to compress at a rate of 4 mm/yr (0.15 in/yr).

Gondwana was one land mass about 90 to 120 million years ago and animals were able to travel across it. Once the continent began to split into sections, animals species which could not fly or swim would have been isolated on their segment. This concept of Gondwanan fauna explains the modern distribution of the ancestors of species separated by vast ocean distances.
The extant ratite taxa, Rhea, Ostrich, Cassowary, Emu, Kiwi and until recently Moas, can be found in South America, Africa and Australasia. They are not found in India, but the Toba Volcano event mentioned below would have made them extinct there. These birds, which cannot fly, share a common ancestor which lived about 90 million years ago. During that time the ancestral ratite, probably nearer the size of a chicken rather than an Ostrich, would have been widespread across Gondwana. Fossil and DNA evidence supports this hypothesis although not many fossils have been found yet. It is to be expected that future excavation in Antarctica will add to our knowledge.

When plates collide they do so very slowly, mostly only a few centimetres a year, but with continuous pressure various types of folding happen:-