The Continental drift

All of you must be familiar with the map of the world by now. Some of you may be able to even say in which part of the globe a particular country is situated because you see the map of the world so often when you study geography at school and also watch television, read books and newspapers.

However, the world map was not always like the one we know today with North America on top of South America, Europe north of Africa, Australia off at the bottom right and so on.

Why was it different to what it is today? The answer to this important question is the continental drift (the movement of the Earth's continents relative to each other by appearing to drift across the ocean bed) or the plate tectonics as the modern theory goes.

The theory of continental drift put forward by early scientists was superseded by the theory of plate tectonics, which builds upon and better explains why the continents move.The speculation that continents might have 'drifted' was first put forward by Abraham Ortelius in 1596. The concept was independently (and more fully) developed by Alfred Wegener in 1912.

Some of you may be surprised to learn that the land masses we now call continents were once clumped together before they eventually drifted apart. Land masses move around in the water the way bath toys move around in the bathtub.

However, the movement of the land masses are not as visible as the bath toys because they move very slowly - usually no more than twenty centimetres in a year. So, sometimes the land masses are all clumped together in one part of the Earth, and other times they drift apart.

Do you know that since the dry land masses first appeared on Earth about four billion years ago this has happened at least three times? Just over a billion years ago, all the land clumped together in one big land mass we call Rodinia.

At this time there were only simple sea plants on Earth, and nothing much lived on land. Rodinia broke apart eventually, and then in the time of the first spiders and insects the land came back together again in another big clump we call Pannotia, about 500 million years ago, and then moved apart again.

The most recent time that the land masses all clumped together was about 250 million years ago, during the time of the dinosaurs. We call this super-continent Pangaea (pan-JAY-ah). Then, about 180 million years ago, the land masses began to drift apart again. When the first Pangaea broke into two pieces they were called the Laurasia and Gondwana by the scientists.

Then Laurasia broke up into North America, Europe and Asia. Gondwana broke up into South America, Africa, Antartica, and Australia.The land masses are still drifting away from each other today. North America and South America are gradually moving away from Europe and Africa, so that the Atlantic Ocean is getting bigger and the Pacific Ocean is getting smaller.

If you look at the continents today in the map of the world you will notice how South America would have fitted against Africa like in a jig saw puzzle.

What happens when land masses run into each other? Natural disasters! When the land masses run into each other, or slide under each other, it causes earthquakes and volcanoes, Sometimes huge mountain chains, like the Alps or the Himalayas, get pushed up high above sea level.

The California edge of the Pacific Ocean is ramming into North America and sliding past it, causing earthquakes in California and forming the Rocky Mountains.

In several hundred million years, the Pacific Ocean will disappear, and the West Coast of North America will smash into Japan and China, forming another large land mass, and probably another large mountain range.

[Fact file]

* The theory of continental drift was not accepted for many years. One problem was that a plausible driving force was missing. Today evidence for the movement of continents on tectonic plates is now extensive. Similar plant and animal fossils are found around different continent shores, suggesting that they were once joined.

* Plate tectonics is a scientific theory that describes the large-scale motions of Earth’s lithosphere. The model builds on the concepts of continental drift, developed during the first decades of the 20th century. It was accepted by the geoscientific community after the concepts of seafloor spreading were developed in the late 1950s and early 1960s.

* The lithosphere is broken up into tectonic plates. On Earth, there are seven or eight major plates (depending on how they are defined) and many minor plates.

Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust.