Since the late 1960s when John Dewey and a few other geologists began to apply plate-tectonic ideas to palaeogeography, most of us when asked to name an ancient ocean would have blurted out ‘Iapetus’. Yet, another Palaeozoic ocean, the Rheic Ocean, left a far more profound mark on the Palaeozoic world: its closure around the end of the Palaeozoic Era united all the continents in Wegener’s Pangaea supercontinent, and threw up a vast mountain belt at the suture. The earlier evolution of the Rheic Ocean involved the spalling of a series of microcontinental slivers from the flank of the earlier Gondwana supercontinent. Damien Nance and Ulf Linneman review the fascinating story of the Rheic Ocean in a nicely succinct way (R.D. Nance & Linnemann, U. 2008. The Rheic Ocean: Origin, evolution and significance. GSA Today, v. 18 (December 2008m issue), p. 4-12).
Readers might remember with some pain the 1995 film Waterworld, starring Kevin Costner: an actor so wooden he could not sink. That was based on the unlikely scenario that if all the ice caps melted the continents would be drowned entirely. In fact that global melting would raise sea level by a mere 67 m. A far higher sea-level rise took place during the Cretaceous, arguably because fast sea-floor spreading and subduction created a larger volume of ‘warm’ and so less-dense ocean lithosphere than there is now. The volume of the ocean basins shrank as a result, displacing ocean water onto low-lying areas of the continents. Something more dramatic has been suggested for the Archaean Earth (Flament, N. et al. 2008. A case for late-Archaean continental emergence from thermal evolution models and hypsometry. Earth and Planetary Science Letters, v. 275, p. 326-336). The starting point for the discussion by Flament and his Australian and French colleagues from the universities of Sydney and Lyon is that the reason for the present hypsometric distribution of surface elevations between ocean floor and continents is cooling of the Earth that has changed the isostatic balance between oceanic and continental lithosphere. That progressively sharpens the topographic contrast thereby increasing continental freeboard. Archaean times involved a hotter mantle due mainly to greater radiogenic heat production. Flament et al. argue that would have lessened the rigidity of continental lithosphere, so reducing the ability of the crust to thicken, whereas ocean floor would have had a higher relative elevation, so reducing ocean basin volume. As in the Cretaceous oceans would have flooded continents, but to a far greater extent, so that as little as 3% of the Earth surface was land.