2.14 Do Mountains have Roots? Surveying the Himalayas of India with a Plumb-Bob

One of the major advances in determining the structure of mountains occurred in the 1840s when Sir George Everest, after whom Mount Everest is named, conducted the first geographical survey in India. During this survey, the distance between the towns of Kalianpur and Kaliana, located south of the Himalayan range, was measured using two different methods. One method employed the conventional surveying technique of triangulation, and the other method determined the distance astronomically. Although the two techniques should have given similar results, the astronomical calculations placed these towns nearly 150 meters closer to each other than did the triangulation survey.

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The discrepancy was attributed to the gravitational attraction exerted by the massive Himalayas on the plumb bob used for leveling the astronomical instrument. A plumb-bob is a metal weight suspended by a cord used to determine a vertical orientation. It was suggested that the deflection of the plumb bob would be greater at Kaliana than at Kallianpur because it is closer to the mountains.

A few years later, J. H. Pratt estimated the mass of the Himalayas and calculated the error that should have been caused by the gravitational influence of the mountains. To his surprise, Pratt discovered that the mountains should have produced an error three times larger than was actually observed. Simply stated, the mountains were not pulling their weight. It was as if they had a hollow central core.

A hypothesis to explain the apparent missing mass was developed by George Airy. He suggested that Earth’s lighter crustal rocks float on the denser more easily deformed mantle. Further, he correctly argued that the crust must be thicker under mountains than beneath the adjacent lowlands. In other words, mountainous terrains are supported by light crustal material that extends as roots into the denser mantle. This phenomenon is exhibited by icebergs, which are buoyed up by the weight of the displaced water.

If the Himalayas do have roots of light crustal rocks that extend far beneath them, then these mountains would exert less gravitational attraction, as Pratt had calculated. Hence, Airy’s model explained why the plumb bob was deflected much less than expected.

Seismological and gravitational studies have confirmed the existence of crustal roots under some mountain ranges. The thickness of continental crust is normally about 20 miles, but crustal thicknesses exceeding 40 miles have been determined for some mountain belts.

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