The mantle material continues to cool, and eventually begins to sink.At this point, the oceanic crust begins to be subducted in deep oceanic trenches.

Subduction is the process by which the oceanic crust is pushed against, and finally underneath, continental or oceanic crust.

Subduction zones are often marked by overlying chains of volcanic islands called island arcs.

Geologists believe sea floor spreading results from convection in the mantle and lower crust that brings hotter, less dense, and more plastic material up toward the surface; the colder, more dense rock and sediment, such as subducted crustal material, sinks toward the mantle (Figure 1).

The technologies developed in the 1940s and 1950s also permitted more detailed mapping of the ocean floor and continental margins.

A much better fit between the rifted continents is apparent when the shape of the continental slope is used instead of the continent's shoreline.

Detailed mapping of distinctive rock units that extend out to sea along the South American and African coasts and North American and British coasts has shown that they would converge perfectly if the continents could be fitted together. In the 1960s, geologist Harry Hess proposed that the sea floor was moving outward from the midoceanic ridges.

His theory of sea floor spreading maintained that new basaltic oceanic crust forms at a midoceanic ridge and is slowly pushed away on both sides toward the continents as more new crust is produced.

These convective forces tear the ocean crust apart at the midoceanic ridge, forming a rift valley marked by high‐angle faults, basaltic lavas, and high heat flows.

The Mid‐Atlantic Ridge is one of the best studied midoceanic ridges. Its 10,000‐foot‐tall mountain peaks lie about a mile below the surface of the ocean.