Studies dones in the 1950s produced new information about the earth and its' history. These new findings were combined with Wegener's continental drift theory discovering continental drift to be true on other terms. Instead of the sun-moon tidal relationship originally suggested by Wegener as the energy source for this behavior, the energy came from the asthenosphere, the plastic solid part of the mantle.
Studies also discovered that it wasn't just the continents that were spreading apart, but also the ocean floor. Some rocks and fossils found in ocean basins were considered to be less than 4.6 billion years-old, making them younger than the earth itself. There was still much curiousity on how this could be.
In 1960, a professor of geology at Princeton University, Harry Hess, presented an essay titled, "An Essay in Geopoetry." (Aylesworth.pg.23) He proposed that the earth's mantle is a plastic solid in which convection currents trigger the rise of hot rocks in the mantle to rise to the surface. With the pressure and the heat these rocks melt producing magma. When they reach the surface, it becomes lava which then cools to become rock. Here, the colder, denser material sinks while the warmer, less dense material rises. When the convection current hits the closest part of the mantle next to the surface (the lithosphere) the lava spread outward in both directions from the mid-ocean ridges pushing along the plates and the continents. This process is known as sea-floor spreading, named by the marine geologist Robert Dietz.
The theory of sea-floor spreading explains only half of the puzzle. Wegener was on the right track in thinking that the energy was connected to magnetic forces. In the 1960s, magnetic surveys were done to measure the magnetism of rocks across the ocean ridges. Scientist found that rocks on the ocean floor were magnetized in alternate directions in a series of bands parallel to the ridge. This occurs when lava from the mantle cools into rock on the ocean floor and is magnetized in the direction of the magnetic field at the time it occurs. This is possible because rocks carry iron, which is a metal.
These two processes define the layout for the plate tectonic theory. The earth is divided into plates and as the asthenosphere moves, everything that sits on top of it moves along with it including the lithosphere, the ocean floor, and the continents.
graphics by www.usgs.gov