There seem to be about as many answers to this question as there are, well, combinations of seas and oceans. The New Grolier Multimedia Encyclopedia [1993], for example, says that in the Age of Discovery, the seven navigable seas of the world were the Atlantic, Pacific, Arctic, and Indian Oceans, as well as the Mediterranean Sea, the Caribbean Sea, and the Gulf of Mexico.
Not so says Don Groves in The Oceans [John Wiley, 1989]. He lists the seven ancient seas as the Mediterranean, the Red, the Black, the Adriatic, and the Caspian, plus the Persian Gulf and the Indian Ocean. And Robert Hendrickson in The Ocean Almanac [Doubleday, 1984] maintains yet another compendium. He agrees with Groves on the Mediterranean and Red Seas, the Indian Ocean, and the Persian Gulf, but adds the China Sea and the West and East African Seas to top off his list.
One less confusing note: Both Groves and Hendrickson agree on a list of the modern seven seas: The North and South Atlantic, the North and South Pacific, the Arctic and Antarctic, and the Indian.
The Gulf Stream is a jetlike current of warm ocean water that meanders northeastward from the Straits of Florida to the coast of northwestern Europe. Off Florida, where the current is strongest, it has been affecting sailors since the Spanish explorer Ponce de Le\227n first noticed it in the early 1500s.
Several factors work together to generate the stream. In the equatorial Atlantic Ocean, the prevailing winds blow west (toward the Americas); but farther north, they blow east (toward Europe). The combined effect creates a clockwise circulation of water in the North Atlantic. The so-called Coriolis effect - a rightward deflection of the water in the north Atlantic due to Earth's rotation and the curvature of the planet's surface - makes the northward flow narrower and swifter than that of the water heading southward. What happens is that the Coriolis forces water to build up in the center of the ocean; this water mass then drifts west, squeezing the northward current into a narrow area along the U.S. coast. Hence the Gulf Stream.
(Why the water mass drifts west and not some other direction appears to be a question that even oceanographers have trouble explaining. When we asked one researcher, he gave a nervous chuckle and mumbled something to the effect that he'd explain it if we had time for an hour-long lecture on geophysics. We passed on his offer.)
In the narrow Straits of Florida, the Gulf Stream reaches a maximum speed of four knots, and its transport encompasses a volume of 1.1 billion cubic feet of water per second - or approximately 1,800 times that of the Mississippi River. But the Stream diffuses as it continues eastward. The water returning southward - called the Canaries current - plods at mere hundredths of a knot.
When Earth was still young, its atmosphere contained a nasty mix of hydrogen chloride, hydrogen bromide, and other noxious emissions from volcanoes. Some of these gases dissolved in the primitive ocean, making it salty, oceanographers believe.
Today, however, most of the salt in the oceans comes from the continual rinsing of the earth. Rain falling on the land dissolves the salts in eroding rocks, and these salts are carried down the rivers and out to sea. The salts accumulate in the ocean as water evaporates to form clouds. The oceans are getting saltier every day, but the rate of increase is so slow that it is virtually immeasurable.
Ocean water is currently about 3.5 percent salt. If the oceans dried up, enough salt would be left behind to build a 180-mile-tall, one- mile-thick wall around the equator. More than 90 percent of that salt would be sodium chloride, or ordinary table salt.
The ocean bottom temperature is more or less the same everywhere in the world, generally 33 F to 36 F. Why? Because cold water is heavier than warm water. Like spilt molasses, cold arctic and antarctic water slowly spreads out underneath warmer surface waters around the globe, eventually reaching the equator.
Tristan da Cunha, a 38-square-mile volcanic outpost, is the remotest inhabited island in the world, according to the Guinness Book of Records. It's situated 1,510 miles southwest of its nearest neighbor, St. Helena, and 1,950 miles west of Africa. Discovered by the Portuguese admiral of the same name in 1506, and settled in 1810, the isle belongs to Great Britain and has a few hundred residents.
Coming in a close second - and often wrongly cited as the most distant land - is Easter Island, located 1,260 miles east of its nearest neighbor, Pitcairn Island, and 2,300 miles west of South America.
The mountainous 64-square-mile island was settled around the 5th century, supposedly by people who were lost at sea. They had no contact with the outside world for more than a millennium, giving them plenty of time to construct more than 1,000 enormous stone figures, called moai, for which the island is most famous.
On Easter Sunday, 1722, however, Dutch settlers moved in and gave the island its name. Today, 2,000 people inhabit the Chilean territory. They share one paved road, a small airport, and a few hours of television per day.
On December 16, 1994, a lad from Key Largo nicknamed "Pipin" (his real name is Francisco Ferreras), rode a lead sled 417 feet down, then shot back to the surface 2 minutes, 24.45 seconds later to break his own world record for deep-diving on a lungful of air. Afterward, Pipin was pretty nonchalant about the whole thing, telling reporters, "It's just one more step on the way to 500 feet."
Had he been wearing standard scuba gear, Pipin could descend to about 350 feet before suffering "rapture of the deep," a narcotic like stupor caused by compressed nitrogen. If Pipin worked for a commercial diving outfit that serviced oil rigs, he'd spend weeks in pressurized nodules so he could work at a depth of 1,000 feet. Some test dives using oxygen mixed with various exotic gases have enabled people to survive at 2,400 feet, but that seems about the limit for unprotected humans.
A book conveniently titled Do Fishes Get Thirsty? [Franklin Watts, 1991] provides the answer to this question. It explains that fish living in the ocean need to drink a lot to avoid shriveling up like prunes.
"Water flows in and out of a fish's body through a process called osmosis," the authors explain. "In osmosis, water moves from where there is less dissolved salt to where there is more. Since seawater is saltier than the liquids in a fish's body, water inside the fish is constantly flowing out." The fish drinks to replace the lost water.
First, tidal waves have nothing to do with tides. Tidal waves - here's where you wish you hadn't napped in physics class - are the dissipation of energy in a viscous fluid over an inclined plane.
To translate this into plain English: The energy source usually is an under-sea earthquake (although it could also be a meteor strike or under-sea explosion); the viscous fluid is the ocean; and the inclined plane is the ocean floor sloping up toward land.
Let's take the case of an earthquake. When the earth thrusts up and down, it also moves all water above it up and down. This generates a huge wave traveling outward in a series of concentric rings. In deep water, most of the tidal wave, or tsunami, remains hidden beneath the surface. But as the tsunami moves toward more shallow water, its enormous energy is forced to the surface.
What makes a tidal wave so destructive is not so much its height (it can reach more than 100 feet), but its speed and tremendous volume of water. In the open ocean, tsunami are hundreds of miles wide and travel at jetliner speeds. Near land they slow down to mere freeway speeds, but even a five-foot tsunami can dump harborfuls of water on a unlucky seaside town.
Ocean Planet Exhibition Floorplan
gene carl feldman (gene@seawifs.gsfc.nasa.gov) (301) 286-9428