The Theory of Continental Drift has had a long and turbulent history since it was first proposed by Alfred Wegener in 1910. (46){{U}}Vigorously challenged yet widely ignored, the theory had languished for half a century, primarily due to its lack of a plausible mechanism to support the proposed drift.{{/U}} With the discovery of sea-floor spreading in the late 1950's and early 60's, the idea was reinvigorated. Plate tectonics is now almost universally accepted. Many details of the mechanism are to be worked out.
The surface of the Earth is divided into approximately six large plates, plus a number of smaller ones. The plates are bounded by an interconnected network of ridges, transform faults, and trenches. Ridges, also called spreading centers, occur where two plates are moving away from each other. As the plates separate, hot molten mantle material flows up to fill the void. (47) {{U}}The increased heat resulting from this flow reduces the density of the plates, causing them to float higher, thus elevating the boundaries by many thousands of feet above the colder surrounding sea floor.{{/U}} (48) {{U}}Ridges on the ocean floor form the longest continuous ranges of mountains on the planet, but only in a very few places on the Earth do these mountains rise above the ocean surface.{{/U}}
New sea floor is constantly being created along spreading centers. Obviously somewhere else old sea floor must be going away. This occurs in trenches, also called subduction zones. Trenches occur along the boundary between two plates that are moving towards each other. (49) {{U}}Where this occurs, one plate is bent downwards at about a 40o angle and plunges under the other plate's leading edge, eventually to melt back into the liquid mantle below.{{/U}} As the suhducting plate is heated back up to mantle temperatures, certain minerals in the plate melt sooner than others. (50) {{U}}Minerals that melt at lower temperatures and are lighter than the surrounding material tend to rise, melting their way up through the overriding plate to erupt as volcanoes on the ocean floor.{{/U}} As these volcanoes grow, they rise above the ocean surface to form lines of islands along the leading edge of the overriding plate. Numerous islands of Micronesia and Melanesia in the western Pacific were created in this way.