The picture of a splitting, spreading crust agrees with an idea proposed over fifty years ago by the German scientist, Alfred Wegener. He argued that the continents originally were joined in a single mass and later drifted apart. The eastern

The rift in the Atlantic ridge emerges in plain view in Iceland, where it forms a steep, canyon-like gap across the island. All of Iceland’s recent volcanic eruptions have occurred along this gap, as have nearly all of its Earthquakes. Deep

A century ago, the shape of the ocean bottom was a mystery. Scientists lacked a good way to investigate it. They could only measure depths with a weighted line, and this was done in just a few places, because lowering and raising the line took a

The whole thickness of rock down to the Moho is called the Earth’s crust. Scientists have a fair idea of its structure. On the continents, the outermost material is a skin mostly of sedimentary rocks, worn through here and there. Under this skin

Oil prospectors have a method called ‘Seismic shooting’ in which they use small artificial Earthquakes. In the area they have chosen to explore, the prospectors bury a charge of dynamite. They plant detectors called geophones at various

An Earthquake sends both types of waves zinging through the Earth. At great distances the small, repaid vibrations cannot be felt, but the seismograph detects them. In principle, the seismograph has the pendulum hanging like a gate from a

On September 1st, 1923, an Earthquake struck the city of Tokyo. Houses collapsed and cooking fires set the wreckage ablaze. Families took refuge in a public square. While they were huddled there, a wind swept the fire over them and their

When a scientists has taken a measurement at a certain place, he compares it with the strength of gravity expected at that latitude. This theoretical value of gravity is obtained by calculation. But before making the comparison, he corrects his

When a pendulum was developed for use on board ship, scientists found that the ocean basins have the strongest gravity on Earth. Why? Here, too, gravity is affected by the structure of the crust. Its thickness under the oceans has been measured

Floating Crust: Two centuries ago, scientists noticed a curious thing about the behavior of a pendulum clock. When such a clock was taken towards the equator it ran slower. When it was taken away from the equator it ran faster. Why? Gravity and