Alfred Wegener, a German meteorologist, had a larger impact on plate tectonics than any other man, woman, or child. His theory has evolved into the continental drift theory and the basic foundation of modern Plate Tectonics. His formulation of the continental drift theory states that the Earth was once a huge land mass called Pangea, and it then broke apart forming into Gondwanaland and Lauarasia. These new land masses continued to break apart in this matter until they formed what is now the 7 continents of the Earth. This theory explained why the continents fit together like a jig-saw puzzle, and why rock deposits from all over the world were the same.
In 1923, Alfred Wegener presented his theorys and ideas to the American Philosophical Society in Philadelphia. This idea was met with total incredulity. Even the Society's President went so far as to call the nonsense, "utter damned rot." A professor of the University of Chicago said that Wegener "....... is blind to every fact and every argument."
For every fact that Wegener had proving his continental drift theory, disbelievieng scientists had one disproving it. "What could the plates float on?" they asked. Harold Jefferys, a British physicist, answered that question. He stated that deep inside the Earth, there was a layer of magma. The plates could float on that. The layers of magma also created heat currents, which were called convection currents.
Wegener died in 1930. Sadly, he did not live long enough to see his work accepted by the scientific community of that time, but soon after his death, the evidence started to mount. First, starting in the late 1930's, an American geologist by the name of David Griggs showed that in Earth's mantle flowed when subjected to high temperatures and pressures, thus solving the question of how the mantle could flow. This was called plasticity. Still, more evidence mounted. The Earth has a solid core, as we all know, but at that time this was also in question. When the scientists looked at the seismograph they saw no lines, which meant that there was no shadow zone. A shadow zone, if found, would prove the existence of solid core. A shadow zone is the area were some P waves have been refracted just a little bit, creating a "shadow" zone. Then in 1936, using a more advanced seismograph, scientists discovered that there was in fact a shadow zone. Danish Geophysicist, Inge Lehmann (born 1888) proposed that the inner core of the Earth was solid, which was the most probable assumption from the now visible shadow zone. This was then confirmed in 1970 with an even more sensitive seismograph.
Scientists started developing ways to study rock magnetism in the late 1950's. This enabled scientists to figure out the continents' approximate age and where they were millions of years ago. By the end of the 1950's, the scientists of that time had mapped out 37,000 miles (60,000 km) of the underwater world.
Also in the late 1950's, scientists noted that most earthquakes occurred around the ocean ridges. An American geologist by the name of Harry H. Hess explained this by creating the Seafloor Spreading Theory. This same man noticed that underwater mountains, those which he called guyots, had flat peaks. He hypothesized that these mountains had formed as volcanic islands where water was shallow, but he had one dilemma. How could the mountains move to their areas in deep water? Luckily, he had his Seafloor Spreading theory right there and handy. He figured out that everything in the ocean was shifting, and the mountains had moved to their current positions because of that.
In 1967 Jason Morgan and D.P. McKenzie each independently proposed the idea of several different plates.
The following year, in 1968, American earth scientists Bryan L. Isacks, Jack Oliver, and Lynn Sykes combined their ideas and came up with the theory that the plates floated on a soft flowing asthenosphere. In 1969, the drill ship Glomar Challenger completed its first cruise, during which many samples were taken underwater around the world. The samples taken indicated that the age of the sediment was the same age as the Seafloor Spreading Theory and paleomagnetism testing said it should be.
In present time, the theory of Plate Tectonics has evolved from simple musings to a complicate combined theory of Sea floor Spreading, paleomagnetism, and continental drift. Plate Tectonics is simply the recognized theory that continental plates rupture, drift apart, and eventually collide. Thus creating, mountain belts, islands, volcanic activity, etc. Check out technical for the more technical details about the theory of plate tectonics.