History of Ocean Basins

Abstract

For purposes of discussion certain simplifying assumptions are made as to initial conditions o n the Earth soon after its formation. It is postulated that it had little in the way of an atmosphere or oceans and that the constituents for these were derived by leakage from the interior of the Earth in the course of geologic time. Heating by short-lived radio nuclides caused partial melting and a single-cell convective overturn within the Earth which segregated an iron core, produced the primordial continents, and gave the Earth its bilateral asymmetry. Mid-ocean ridges have high heat flow, and many of them have median rifts and show lower seismic velocities than do the common oceanic areas. They are interpreted as representing the rising limbs of mantle-convection cells. The topographic elevation is related to thermal expansion, and the lower seismic velocities both to higher than normal temperatures and microfracturing. Convective flow comes right through to the surface, and the oceanic crust is formed by hydration of mantle material starting at a level 5 k m below the sea floor. The water to produce serpentine of the oceanic crust comes from the mantle at a rate consistent with a gradual evolution of ocean water over 4 aeons. Ocean ridges are ephemeral features as are the convection cells that produce them. A n ancient trans-Pacific ridge from the Marianas Islands to Chile started to disappear 100 m illion years ago. Its trace is now evident only in a belt of atolls and guyots which have subsided 1–2 km. N o indications of older generations of oceanic ridges are found. This, coupled with the small thickness of sediments on the ocean floor and comparatively small number of volcanic seamounts, suggests an age for allthe ocean floor of not more than several times 10⁸ years. The Mid-Atlantic Ridge is truly median because each side of the convecting cell is moving away from the crest at the same velocity, ca. 1 cm/yr. A more acceptable mechanism is derived for continental drift whereby continents ride passively o n convecting mantle instead of having top low through oceanic crust. Finally, the depth of the M discontinuity under continents is related to the depth of the oceans. Early in the Earth's history, when it is assumed there wasmuch less sea water, the continental plates must have been much thinner.