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7.2.2.1. Expansion of the ocean floor

The expansion of the ocean floor

Hess postulated, in the early 1960s, the hypothesis of the expansion of the ocean floor. He claimed that the earth's crust was forming in the ridges and that it should be disappearing elsewhere. He suggested that the newly created oceanic crust was moving away from the ridges, and that millions of years later, it would descend into the ocean trenches. According to Hess, the Atlantic Ocean was expanding while the Pacific Ocean was shrinking.

Therefore, the age of the oceanic crust is very modern near the ridges, but increases progressively as we move away from the ridge, being older near the marine trenches.

For this reason also, the thickness of marine sediments is greater in areas far from the ridge, since they have had more time to settle.

This, coupled with evidence of symmetrical magnetic bands on either side of the ridges indicating magnetic inversions, would support the idea that the newer crust is closer to the ridge and the older crust further away.

Therefore, as proposed by Hess, the oceanic crust is formed in the oceanic ridges from materials from the mantle and would accumulate on both sides of the ridge, growing the oceanic crust. The destruction of said crust would take place in the oceanic trenches, where subduction takes place, introducing the crust inside the Earth.

Stages of ocean formation

The stages of the ocean formation process are as follows:
  • An updraft from the mantle bulges the crust until it fractures and magma can escape, creating a large valley with volcanic activity. Currently this situation occurs in the  Rift Valley (Africa).
  • When continental rupture occurs, new oceanic crust forms, appearing a ridge between the two blocks. This case can be seen, today, in the Red Sea.
  • The materials that will form the new oceanic crust will continually leave the ridge and will push the existing crust, expanding the ocean floor and displacing the continents. This is the situation in the Atlantic Ocean today.

But if the oceanic lithosphere is created on the ridges, they need to be destroyed elsewhere. In fact, the oldest lithosphere is only 180 million years old, while we can find 3800 m.y. rocks in the continental lithosphere. In the subduction zones is where the destruction of the oceanic lithosphere occurs, which will return to the mantle.