Plate tectonics is a scientific theory describing the large-scale motion of seven large plates and the movements of a larger number of smaller plates of the Earth's lithosphere, since tectonic processes began on Earth between 3 and 3.5 billion years ago. The lithosphere, which is the rigid outermost shell of a planet (the crust and upper mantle), is broken into tectonic plates. The Earth's lithosphere is composed of seven or eight major plates (depending on how they are defined) and many minor plates. Where the plates meet, their relative motion determines the type of boundary: convergent, divergent, or transform.
Types of boundaries are following:
Convergent boundaries (Destructive) (or active margins) occur where two plates slide toward each other to form either a subduction zone (one plate moving underneath the other) or a continental collision. At zones of ocean-to-continent subduction the dense oceanic lithosphere plunges beneath the less dense continent. For example - the Andes mountain range in South America, and the Cascade Mountains in Western United States. At zones of ocean-to-ocean subduction, older, cooler, denser crust slips beneath less dense crust. This motion causes earthquakes and a deep trench to form in an arc shape. For example - Aleutian islands, Mariana Islands, and the Japanese island arc.
Divergent boundaries (Constructive) occur where two plates slide apart from each other. At zones of ocean-to-ocean rifting, divergent boundaries form by seafloor spreading, allowing for the formation of new ocean basin. As the ocean plate splits, the ridge forms at the spreading center, the ocean basin expands, and finally, the plate area increases causing many small volcanoes and/or shallow earthquakes. At zones of continent-to-continent rifting, divergent boundaries may cause new ocean basin to form as the continent splits, spreads, the central rift collapses, and ocean fills the basin.
Transform boundaries (Conservative) occur where two lithospheric plates slide, or perhaps more accurately, grind past each other along transform faults, where plates are neither created nor destroyed. The San Andreas Fault in California is an example of a transform boundary exhibiting dextral motion.
Continental Drift Theory (Alfred Wegner, 1922)
Wegner was a climatologist and wanted to investigate the relative distribution of land and sea and the climatic aberration of the past. He postulated that originally there existed one big landmass which he called Pangaea which was covered by one big ocean called Panthalassa. A sea called Tythys divided the Pangaea into two huge landmasses: Laurentia to the north and Gondwanaland to the south of the Tethys. The landmasses consisted of Sial (lighter) crust mostly, while the ocean had a Simatic (heavier) base.
According to Wagener, the drift started around 200 million years ago, and the continents began to break up and drift away from one another. The drift was in two directions- equatorwards due to the interaction of forces of gravity and buoyancy, and westwards due to tidal currents because of the earth’s motion.
Critical Analysis of Evidence for Continental Drift
Apparent Affinity of Physical Features
South America and Africa seem to fit in with each other, especially, the bulge of Brazil fits into the Gulf of Guinea.
Greenland seems to fit in well with Ellesmere and Baffin islands. The east coast of India, Madagascar and Africa seem to have been joined. North America and South America on one side and Africa and Europe on the other fit along the mid-Atlantic ridge. The Caledonian and Hercynian mountains of Europe and the Appalachians of USA seem to be one continuous series. Sierra de Tendill of South Africa seem to exhibit a similar tendency.
Presence of Ice Sheets
Evidence of such ice cover dating back to the carboniferous period in the Falklands and other places in the southern hemisphere suggests that they were closer once.
The normal temperature gradient on the sea floor is 9.4°c/300 m but near the ridges it becomes higher, indicating an upwelling of magnetic material from the mentle.
The continents have moved a great deal in the history of the planet, but they carry records of where they’ve been. Some of this evidence is the fossils of animals and plants. Tropical species found in the Antarctic and similar fossils found in western Africa and eastern South America tell a story of where those land masses used to be. Paleomagnetic evidence is an even stronger piece of evidence. Magnetic strata within the fossil record show how the land masses were oriented at different times during Earth’s history. By constructing detailed records of changes in land mass orientation, scientists can reconstruct paths of tectonic movement much further back in history than they can from the magnetic striping on the sea floor.
Modern technology gives us a range of ways to directly measure the movement of tectonic plates. These methods are based around the idea of measuring distance between two points on Earth by using some intermediary transmitter in space. For example, SLR (Satellite Laser Ranging) uses two lasers on Earth, each of which fires a laser to a satellite orbiting the planet. The beam is reflected back to each laser and the difference in laser beam travel time is used to estimate the distance between the two lasers on Earth. Another approach using radio-telescopes measuring natural radio signals from deep space called VLBI (Very Long Baseline Interferometry) achieves a similar effect. Both SLR and VLBI allow for repeated measurements of distance between two points. As the continents move, the distances change. Using these measurements, scientists can accurately estimate movement of the tectonic plates today.
Older rocks from the continents while younger rocks are present on the ocean floor. On continents, rocks of upto 3.5 billion years old can be found while the older rock found on the ocean floor is not more than 75 million years old (western part of pacific floor). As we move towards ridges, still younger rocks appear. This points to an effective spread of sea floor along oceanic ridges which are also the plate margins.
As upwelling of magma continues, the plates continue to diverge, a process known as seafloor spreading. Samples collected from the ocean floor show that the age of oceanic crust increases with distance from the spreading center—important evidence in favour of this process. These age data also allow the rate of seafloor spreading to be determined, and they show that rates vary from about 0.1 cm (0.04 inch) per year to 17 cm (6.7 inches) per year. Seafloor-spreading rates are much more rapid in the Pacific Ocean than in the Atlantic and Indian oceans. At spreading rates of about 15 cm (6 inches) per year, the entire crust beneath the Pacific Ocean (about 15,000 km wide) could be produced in 100 million years.