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Sea level is the average height of the ocean surface relative to land. This has changed throughout Earth’s history in response to climate change and the movement of tectonic plates. Eighty million years ago, as a result of an expansion of mid-ocean ridges (see pp.264–265) that caused displacement of water upward, sea level was 820–1,150 ft (250–350 m) higher than today. At the peak of the last I..

The first oceans that formed on Earth some 4 billion years ago contained little salt but were weakly acidic as a result of gases released by volcanic activity. But today, as a result of dissolution of minerals from the land and the seafloor, the oceans contain over 5.5 trillion tons (5 trillion tonnes) of salts from nearly 100 different elements. The concentration of salts in the oceans is betwe..