Continents are made of lighter (less dense) crustal rock that floats on the more dense rock of the mantle, forming plates that can be pushed around.
The continental crust makes up the continents, although small parts, like central America, are made up of material that has been uplifted by tectonic forces.
Under the deep oceans, the crust is thinner, as there is much less mass of crust to support, and the mantle comes closer to the average surface of the planet.
Areas of mountain lie above very thick crust, in just the same way that large icebergs extend further above and below sea level. This is called isostasy.
In some cases, pieces of sea floor may be lifted up above the sea surface. This sort of formation is called an ophiolite and it reveals hidden processes.
The surface of the planet is shaped by plate tectonics, because plates, as they collide, cause massive upheavals in the form of earthquakes and volcanoes.
The surface of the earth is made up of plates that are in motion, driven by convection currents. Plate tectonics studies how the world changes over time.
We can measure the actual movement of tectonic plates today by GPS stations located at fixed points. Most plates move about as fast as a fingernail grows.
Good evidence of tectonic movement can be found at a mid-ocean ridge, where the sea floor can be shown to be spreading, seen in patterns of magnetic striping.
In 1595, Abraham Ortelius suggested the Americas were "torn away from Europe and Africa by earthquakes and floods", an early suggestion of continental drift.
In 1620 Francis Bacon pointed out the jigsaw fit of the opposite shores of the Atlantic Ocean, a first step to drifting continents and plate tectonics.
In 1910, Alfred Wegener noticed the close 'fit' between the west coast of Africa and the east coast of South America and started thinking of continental drift.
In 1912, Alfred Wegener developed his theory of continental drift based on fossil and glacial evidence, and first lectured about continental drift.
In 1926, geologist Arthur Holmes saw that the Earth's internal heat had to go somewhere, and argued that there may be convection currents in the Earth's crust.
In 1960, Harry Hess proposed that new sea floor might be created at mid-ocean rifts and destroyed at deep sea trenches, a key to plate tectonics.
In 1963, Vine and Matthews explained the stripes of magnetized rocks as due to sea floor spreading and the periodic geomagnetic field reversals.
The tectonic plates are thought to be moved by convection currents operating deep down in the Earth, bringing hot molten rock closer to the surface.
Evidence for past movements of tectonic plates comes from the locations of fossils, geology and the observed distributions of plants and animals today.
The movement of different tectonic plates across the globe influences the distributions of animal and plant species and how they later evolve, when separated.
Evidence that the plates are still moving comes from measurements with GPS equipment and from magnetic striping in the sea floor caused by polar reversals.
On a large scale, geological structures such as mountain chains and island arcs relate to and are caused by the movements of the Earth's tectonic plates.
Volcanoes are commonly found where plates are in contact, as there are planes of weakness there that plunge deep into the earth, and frictional effects.
Mountains are most commonly formed when forces operate as the result of two tectonic plates coming in contact with each other and forcing material upwards.
On the evidence, there was once a supercontinent that is now referred to as Pangaea. It later divided to parts now given the names Laurasia and Gondwana.
Earthquakes are most severe when one plate moves under another, a process known as subduction. Japanese earthquakes involve subduction triggering deep quakes.
The southern distribution of some groups of plants and animals reflects their origins in Gondwana. Laurasia gave rise to most of the northern continents.
This file is http://members.ozemail.com.au/~macinnis/scifun/splatstecto.htm, first created on February 16, 2008. Last recorded revision (well I get lazy and forget sometimes!) was on February 16, 2008.