In the first half of the 20th Century
a German meteorologist by the name of Alfred Wegener
noticed that the coastlines of Africa
and South America
matched fairly well. Further, it was becoming clear that certain terrestrial fossil
species, such as dinosaurs
, were being found in areas across the Atlantic from each other, which could not have been connected by any land route
. To account for this, he proposed an idea called 'Continental Drift
' and suggested that South America and Africa had once been close together, and had subsequently drifted apart over millions of years. Although convenient in explaining the fossil distributions and the seemingly coincidental matching of the coastlines across the Atlantic, Wegener could not propose a mechanism that would allow the continents to move. He guessed that they might be plowing across the sea-floor.
Needless to say, the concept of incomprehensibly massive continents just 'drifting' across the solid rock for no very good reason was seen as highly improbable. So the idea 'Continental Drift' languished without a mechanism.
Then, during WWII, a geologist-cum-sub-skipper, by the name of Harry Hess was assigned to study the deep ocean. He discovered that many of the numerous seamounts that dot the floor of the Pacific had flat tops, as if they were sunken islands. As the mapping of the ocean floor progressed, revealing the mid-ocean ridge, Hess began to formulate a new mechanism that might help account for the phenomina that Wegener was attempting to explain with his 'Continental Drift' theory.
Hess reasoned that the sea-floor itself was spreading from the central rift of the mid-ocean ridges. Hot Magma from beneath the earth's crust could rise at the rift-zones, forcing the sea-floor on either side to spread apart, and cooling to form new crust.
Subsequent exploration by a research vessel called the Glomar Challenger confirmed the presence of bands of alternating magnetism in the rocks of the sea-floor parallel to the mid-ocean ridges. This could be explained by changes in the earth's magnetic polarity through time being frozen in the rocks as they form at the mid-ocean ridge and then spread away.
This was the evidence that sealed the deal and confirmed that Wegener's 'Continental Drift' and Hess's 'Sea-Floor Spreading' ideas were in fact correct, laying the framework for the modern theory of plate tectonics. (Not to be confused with Gin and Tonics or their close relatives, Whiskey and Sodas).
Some facts checked on http://www.pbs.org/wgbh/aso/databank/entries/do62se.html