The constant Creation and Destruction of the Earth’s Surface
In 1965, Sir Edward Bullard of Cambridge University used a computer to fit the shapes of the continents together, showing only a small area that didn’t match. Most people now are aware of the widely accepted theory that the continents started as a single supercontinent, known as Pangea, which split apart to form several smaller continents. This was approximately 250 million years ago, but there is evidence of other supercontinental formations long before that. As stated before, the Earth is in constant motion.
‘Plate tectonics’ is a term used to describe the movement of the Earth’s lithosphere. In the previous section we discussed the lithosphere, made up of the Earth’s crust and uppermost part of the mantle, and its movement over weaker areas known as the asthenosphere. The lithosphere is not a single unit, but a series of rigid plates. They are usually described as seven major plates and six minor ones, each carrying an amount of continental crust (areas where the Earth’s crust rises above sea level, forming continents), and oceanic crust (the thinner area of crust on the ocean floor). Each plate is in contact with its neighbours on all sides, leaving no gaps, and all are moving relative to one another.
Earthquake zones are a manifestation of plate tectonics. Volcanoes around the Pacific Ring of Fire, another example of plate tectonics, produce almost identical rock wherever they are in the world, with basically the same compounds of quartz, feldspar and hornblend, and the richest metal and ore deposits occur where they have been brought to the surface by volcanoes.
When two of these plates collide with one another, one is thrust down below the other. As oceanic crust is denser than continental crust, which is generally thicker and more buoyant, it is usually the oceanic plate that goes under the other. This process is known as subduction, and the area where it occurs is called a destructive plate boundary. Subduction zones are visible as a trench in the ocean floor. There is an old subduction zone beneath the Lake District, Wales, and Southern Ireland, and because of plate movement, this same zone is now beneath Iceland. The global distribution of volcanoes above sea level roughly correlates with destructive plate boundaries.
If two plates, both containing continental crust, collide with one another, the edges buckle and eventually the subduction zone is jammed. Near this suture the highest mountain ranges, such as the Himalayas and the Alps, are formed.
When two of these plates move away from each other, new ocean floor is created to make up the difference. The underlying asthenosphere wells up to prevent any gaps from appearing, and as it cools, becomes part of the lithosphere. The areas where this process occurs are known, not surprisingly, as constructive plate boundaries, and form the mid-ocean ridges. Constructive plate boundaries produce a record of the orientation of the Earth’s magnetic field. Periodically, the Earth’s magnetic field reverses polarity, so north becomes south and visa versa. This can be as frequently as 100,000 years, with an average of 500,000 years between reversals*. Unlike the sun, the earth’s magnetic reversals are chaotic. The reversals usually appear to take 100 – 1000 years, but there is evidence that some reversals have taken only a matter of weeks. Studies of the magmatisation of ocean floor were carried out in the 1950s through the work of Runcorn, Irving, Rutten, and Cox. They revealed a strange striped pattern, with half the stripes magnetised in the direction of the present field, and alternating stripes magnetised in the reverse direction. When the oceanic plate is newly formed, and still molten, the magnetic particles within it line up with the magnetic pole, and remain that way as the magma solidifies. This is sometimes referred to as the mid-ocean tape recorder.
*Source: http://science.jrank.org/pages/47642/geomagnetism-polarity-reversals.html ). This is an excellent article, if you would like to understand more about the history and process