The ocean floor

We have better maps of the Moon than of our own sea floor. Light dies out within a few hundred metres of the surface, and the average depth of the ocean is 3,682 m. Almost everything we know about the floor comes from sound, sent down from ships and timed as it bounces back.

What that sound has found is a landscape as dramatic as anything on land, and quite different from it. It has the longest mountain range on Earth, plains flatter than any field, and trenches deep enough to swallow the highest mountains.

A mountain range 65,000 km long

Running through every ocean, like the seam on a tennis ball, is a chain of underwater volcanic mountains called the mid-ocean ridge. It is nearly 65,000 km long, and more than 90% of it is under water. On average its crest is about 2,500 m down. In a few places it rises above the waves: Iceland sits right on top of it.

The ridge is where plates pull apart. Hot mantle rises, melts a little and wells up as basalt lava, which sets into new sea floor. Along the slow Mid-Atlantic Ridge the two sides separate by about 2 to 5 cm a year, and a rift valley as deep and wide as the Grand Canyon runs down its middle. The East Pacific Rise spreads three times as fast or more, at 6 to 16 cm a year, and has a smooth crest instead.

The woman who found the rift

In 1952 Marie Tharp, working at Columbia University in New York, was turning thousands of depth soundings across the Atlantic into a picture of the sea floor. She noticed a deep notch running along the top of the ridge in profile after profile. It was a rift valley, and it suggested the sea floor was being pulled apart. Her colleague Bruce Heezen at first dismissed it, until it turned out that the earthquakes in the Atlantic lined up along that same valley.

Tharp and Heezen published their first map of the North Atlantic floor in 1957. Their painted map of the whole world's ocean floor came out in 1977. For years Heezen got most of the credit, but Tharp has since been recognised as one of the great mapmakers of the 20th century.

Why the sea floor is young

New sea floor made at the ridges moves slowly away on either side, like two conveyor belts. Where an ocean plate meets another plate, the heavier one bends and sinks back into the mantle. That makes a trench, the deepest kind of place on Earth. The deepest of all is Challenger Deep in the Mariana Trench in the western Pacific, 10,935 m down.

Because the sea floor is recycled, none of the open ocean floor is very old in geological terms. The oldest parts are about 180 million years old, beside the continents in the north Atlantic and the western Pacific. Rocks on the continents, by contrast, go back four billion years.

Life without the Sun

In February 1977, scientists in the small research submarine Alvin dived to the Galápagos Rift in the eastern Pacific, looking for hot springs on the sea floor. They found them, and something nobody expected: clams, mussels and red-tipped tube worms up to two metres long, crowded round vents in the dark.

Seawater soaks into the hot rock near the ridge, is heated, and comes back out full of dissolved minerals. Microbes there live on chemicals such as hydrogen sulfide instead of sunlight, and the animals live on the microbes. That way of making food is called chemosynthesis. Some scientists think life on Earth may have begun in places like these.