Published: 11 October, 2006

While Earth's surface is mostly water, Earth is nearly all rock

Why does the earth have more water than land?

This great question emphasizes what is perhaps the Earth's most unique feature as well as its most important and precious resource. Quite simply, what makes our planet so uniquely suitable to life is that we have a sizable amount of water in liquid form at the surface. More than 70% of the surface area of the planet today is covered by water. Though this makes it seems like the planet is mostly water, in fact it is just a thin film on the surface of a stony planet. Just 0.5% of the planet by weight is water. Because water runs downhill, it covers the topographically lowest parts of the solid Earth, the ocean basins. So, one might ask: why are the ocean basins so low and so large?

The ocean basins are so low because they are made of rocks that are much denser than the rocks of the continents. Just as a block of dense hardwood floats lower in the water than a block of styrofoam, the rocks of the ocean basins (rich in magnesium and iron and relatively poor in silica) "float" lower on the viscous interior of the earth than the continents, which have more light minerals composed of silica and aluminum. The ocean basins are large in comparison to the continents because they are constantly being renewed at the "mid-ocean" ridges (which are really only in the middle in the case of the Atlantic Ocean), and consumed at the great oceanic trenches; nowhere on Earth is there oceanic crust which is older than about 200 million years. In contrast, the continents, with rocks as old as 4,000 million years (nearly as old as the Earth, itself) have slowly become differentiated from the oceans and accumulated material over time during mountain building. You can think of the ocean basins as a type of conveyor belt and the continents as a pile of stuff that gets scraped off the conveyor belt as it goes around the rollers at the end of its loop.

The percentage of the Earth's surface that is covered by water is not constant but changes through time. Even today, water not only fills the ocean basins but laps up on the edges of the continents. During the last glacial maximum, about 25,000 years ago, sea level was about 120 m (almost 400 ft.) lower than it is today, because the water was locked up in continental ice caps, and New York City was 150 km from the ocean! Sea level has been steadily rising since then. Most common consensus among scientists is that sea level may rise as much as 1 m by the end of this century, though a recent report in Science Magazine suggested that much of the Greenland ice cap may melt, causing perhaps as much as 5 m of sea level rise. If this latter scenario were to come true, it would truly be catastrophic for humankind. During most of the Earth's history, there were no glaciers at all and shallow seas covered most of the central part of North America as well as large parts of other continents. For example, the bedrock of the Ithaca area was deposited in a shallow sea 380 million years ago.