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 own figure in certain ways.

Suppose he is measuring gravity in the Andes Mountains, at a station a few miles above sea level.

This extra distance from the Earth’s centre lessens the pull of gravity by a certain amount, which the scientist adds to this figure to make it equivalent to a reading at sea level.

Secondly, the rock in the mountain between the station and sea level causes an extra amount of pull, which the scientist subtracts from his figure.

Now the figure is fully corrected, and can be compared with the theoretical value. Does it agree with this value? It does not.

The figure is lower; gravity is weaker than it should be. And it is considerably weaker than gravity in lowland areas of the continent.

In trying to account for this, the scientists remember that gravity depends partly on the mass- the weight or quantity of material-under the mountains.

Somehow, there is a shortage of mass. This is true of the Andes, and also of the mountains of other continents.

Why is gravity weaker in mountains than in lowlands? In pondering this question, scientists began to suspect that a great body of light rock under the mountains, as though the Earth’s crust were thicker there.

Several years ago scientists from Peru, Chile, and the United states climbed the Andes to measure the thickness of the crust beneath the mountains.

Their plan was to take advantage of powerful dynamic blasts set off daily in the copper mines of Chile and Peru.

Compression waves from a blast would probably go through the crust, bounce back from the Moho, and return to the surface.

Instruments were set up, the blasts went off, and compression waves recorded. The scientists noted how long it took the waves to echo back from the top of the mantle.

Their records showed the crust under the mountains to be some 40 miles thick. This is about twice the thickness of the crust in lowland regions.

Beautiful Mountain