The ocean basins

The ocean covers about 70% of Earth's surface, and so has a total area of about 3.61 X 1014 km2. Currently, about two-thirds of Earth's land area is in the Northern Hemisphere, so that about 57% of the ocean is in the Southern Hemisphere, 43% in the Northern; the Northern Hemisphere itself is 61% ocean, and the Southern Hemisphere is about 80% ocean. The ocean's average depth is about 3.7 km, but there are deep trenches where the depth reaches about 10 km. The volume of the ocean is approximately 1.3 X 1018 m3 and, given that the average density of seawater is about 1.03 X 103 kg/m3, the total mass of the ocean is about 1.4 X 1021 kg, or 1,400,000,000,000,000,000 metric tons. In comparison, the mass of the atmosphere is about 5 X 1018 kg, about 300 times less: the air at the surface weighs about 1,000 times less than seawater, but the effective vertical extent of the atmosphere is a few times greater than that of the ocean.

The ocean basins have changed over time as the continents have moved and deformed as a consequence of the convection in Earth's mantle that leads to the movement of the tectonic plates and so of the continents themselves, all taking place on a timescale of tens to hundreds of millions of years. The ocean itself has existed for a long time, essentially because the water once formed has nowhere to go: the loss of water vapor to space is negligible because nearly all the water vapor is concentrated at the lowest levels of the atmosphere, and the stratosphere and the upper atmosphere are extremely dry. It is believed that the ocean has in fact existed in roughly the sense that we know it now for perhaps some 3.8 Ga (3.8 billion years), since the beginning of the Archaean era, when Earth cooled sufficiently for land masses to form and water to condense. The water itself had its origins both in volca-nism and degassing from Earth's interior, and in collisions with extraterrestrial bodies—probably mainly small icy protoplanets (moonlike bodies) and comets. Such collisions were fortunately much more common in this stage in the evolution of the solar system than they are now. Since that time, the ocean basins have certainly come and gone many times. As continents move significantly on a timescale of tens to hundreds of millions of years, one can envision several distinct continent-ocean configurations over Earth's history, and some of the more recent ones are illustrated in figure 2.1 since the breakup of the "supercontinent" Pangea some 200 million years ago. Reconstruction of the configuration naturally becomes increasingly difficult and so more prone to error the further back one goes in time, but it is believed that there may have been a number of supercontinents over Earth's history, perhaps each a few hundred million years apart.

Jurassic, 150 Myr Cretaceous, 65 Myr

Figure 2.1. Schematic of the configuration of the oceans and continents over the past 225 million years, since the breakup of the supercontinent Pangea. Source: Adapted from USGS (http://pubs .usgs.gov/gip/dynamic/historical.html).

Jurassic, 150 Myr Cretaceous, 65 Myr

Figure 2.1. Schematic of the configuration of the oceans and continents over the past 225 million years, since the breakup of the supercontinent Pangea. Source: Adapted from USGS (http://pubs .usgs.gov/gip/dynamic/historical.html).

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