Chapter 15
Chapter 15: Restless Realm: Oceans and Coasts
Feature Articles
The Human Angle: Can We Drink The Ocean?
by Stephen Marshak
“Water, water, everywhere, /Nor any drop to drink”—Samuel Taylor Coleridge’s famous lament (in The Rime of the Ancient Mariner), echoed by many a castaway languishing on a raft, holds true on a global basis too. With so much ocean water around, why do we constantly hear of water shortages? Simply because we can’t drink seawater or use it for industrial or agricultural purposes. Seawater, as we have seen, contains about 3.5% salt, while drinking water cannot contain more than 0.05%. We can, however, extract drinking water from seawater by distilling it. A distillation plant, or desalinization plant, consists simply of a furnace that boils seawater. Only freshwater goes into the steam, leaving the salt behind; the plant then transforms the steam back into water by cooling it in a coil of glass tubing. But while the method is simple, the cost is high, for it takes a lot of energy to boil water. As a result, the water obtained from a desalinization plant costs about ten times more than fresh-water pumped out of the ground. Consumers can justify the cost of distilled drinking water only in places like the Netherlands Antilles, a group of desert islands north of Venezuela, which completely lack natural freshwater supplies and cannot receive water by pipeline. Because of the cost of desalinization, some Middle Eastern nations have considered towing huge icebergs from Antarctica up to the Persian Gulf, since water leaves salt behind when it freezes. But most of the glacier would melt before it even reached its destination, and the cost of towing it would be prohibitive.
The Rest of the Story: Tides and the Length of the Day
by Stephen Marshak
Because of friction in the water of the ocean, the sublunar bulge can’t quite keep up with the movement of the Moon across the Earth. In fact, it lags behind the Moon by about 1degree. The Moon exerts a slight pull on the side of the bulge. This effect, along with the twice-daily collision between the tidal bulges and the edges of continents, acts like a brake that gradually slows the Earth’s spin. As a result, throughout Earth history the length of a day has steadily increased by a rate or about 2% per hundred million years (0.002 seconds per century). Since the slowing of Earth’s spin has no effect on the planet’s rotation around the sun, however, the length of a year remains unchanged.
This slowdown may not seem like much, but remember that the Earth has been around for 4.6 billion years, and given long periods of time, the slowdown adds up. In the middle Devonian period, about 375 million years ago, days were about 21.9 hours long, and there were about 400 days in a year. This estimate has been confirmed by counting daily growth rings in fossil corals and clams.
As a spinning ice skater stretches out his arms, he slows down. Similarly, as the spinning Earth slows in its orbit, the Moon moves farther away, at about 4 cm per year (4 km every million years). In middle Devonian time, the Moon was about 1,500 km closer, and at the beginning of the Archean (3.9 Ga), it may have been over 15,000 km (about 10%) closer. This change would mean that the tidal reach was much larger earlier in Earth history, as has been confirmed by studying characteristics or ancient strata deposited in the intertidal zone.