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 by seismic shooting.

A depth charge explodes, and compression waves from the blast go down through the water, through sediments on the bottom, and through the crust.

Echoing from the Moho, the waves are recorded on board ship. Their travel time, which is short, shows that the mantle is not far down.

In some areas it is only three miles beneath the ocean floor. This helps to explain the strong gravity of the ocean basins.

There the heavy, dense rock of the mantle lies near the earth’s surface, increasing the pull of gravity.

In addition, the rock of the ocean floor itself is heavier and denser than average rocks of the continents.

The Crust in Balance:

Regions of heavy rock tend to be lowlands. Regions of light rock are highlands.

This does not mean that the mantle is liquid. But very likely it is Plastic. In this state a material does not break when some force works upon it over a long period of time.

Instead, it yields by a slow, oozy sort of flowing. Tar is an example of such material. A block of tar appears solid.

It will break when struck with a hammer. But if the hammer is laid on top of the tar and rest there awhile, it gradually sinks in.

Mountain blocks, being lighter than the mantle, rest on it as iceberg float on the sea. Icebergs extend down into the water, which buoys them up; mountains extend down into the mantle, which buoys them up.

As the mountains erode away and become lighter, the pressure of the mantle keeps buoying them up.

The same thing happens to the continents. They are always wearing down as their rocks crumble and rivers carry the fragments to the sea.

This has been going on for ages, yet the continents are still high and the ocean basins low.

Gravity and the buoyant force of the mantle work together to hold them so. Thus, our planet keeps the great surface features-land and sea-which it has had for thousands of millions of years.

The Ocean