Who is credited with seafloor spreading

It was possible, he said, that molten magma from beneath the earth's crust could ooze up between the plates in the Great Global Rift. As this hot magma cooled in the ocean water, it would expand and push the plates on either side of it -- North and South America to the west and Eurasia and Africa to the east.

This way, the Atlantic Ocean would get wider but the coastlines of the landmasses would not change dramatically. If, as Georges LeMaitre suggested for visualizing the early universe, you play the "film" of this phenomenon backwards, the continents come closer together until Brazil fits right into the Gulf of Guinea. Hess proved Wegener's basic idea right and clarifed the mechanism that broke the once-joined continents into the seven with which we are familiar.

In addition he found that the deepest parts of the oceans were very close to continental margins in the Pacific with Ocean Trenches extending down to depths of over 11 km in the case of the Marianas Trench off the coast of Japan.

This created new seafloor which then spread away from the ridge in both directions. The ocean ridge was thermally expanded and consequently higher than the ocean floor further away. As spreading continued, the older ocean floor cooled and subsided to the level of the abyssal plain which is approximately 4 km deep.

At their crests, they had V-shaped central valleys with steep faults on either side. This evidence led early marine geologists to deduce that the mid-ocean ridges were formed by seafloor volcanoes.

When these volcanoes erupted, they spewed out lava that cooled and solidified to become new seafloor. So depending on when seafloor rocks were formed, their particles are aligned in either one direction or the other, and they are said to have either positive or negative magnetic anomalies.

The stripes ran parallel to the mid-ocean ridges and extended out hundreds of miles on either side of them. This seafloor spreading hypothesis had been proposed a few years earlier by Harry Hess, a petrologist at Princeton University, and Robert Dietz, an oceanographer in the US Coast and Geodetic Survey the federal department that made maps of the oceans and US coastlines.

Hess went on to say that as the ocean crust spreads and cools over millions of years, it becomes denser and eventually sinks down into oceanic trenches, or subduction zones, a long way from where it forms at the mid-ocean ridge crest.

As ocean crust descends toward the hot mantle, it melts and becomes recycled into the mantle. Volcanoes and earthquakes are common in subduction zones, which often occur at the edges, or margins, of continents. The Rim of Fire, which is named for its volcanoes and earthquakes, is created by a series of subduction zones along the coastlines surrounding the Pacific Ocean-from western South and Central America to the Aleutian Islands in Alaska, down the western Pacific, from Japan and the Philippines, all the way to Indonesia and New Zealand.