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Mountain Roots

Sir George Everest

Q: When did we first find out that mountains had roots and that these mountains stabilised the earth?

From Mr Akhlaaq Choudry

Reply by Dr Ted Nield

Thank you for your inquiry about mountain roots, which I can answer with the following modified extract from my book Supercontinent – 10 billion years in the life of our planet (Granta).

It all began in India, when the British Raj was keen to reinforce its dominion by surveying the Empress’s possessions with the most modern techniques then available. The Great Trigonometrical Survey, as it was called, was meant to take 6 years and took about 60. During this time mapmakers crisscrossed India using two methods to determine their position, one providing a check on the other. The first method fixed positions on the ground like a sailor at sea, using the stars, the horizon and a sextant. The other was the process known as triangulation, whereby each point on the ground is fixed relative to another by measuring the intervening distance, and taking the compass bearing from each triangulation point to two others. The rest is trigonometry.

When, during the mapping of the Gangetic Plain south of the Himalayas in the 1850s, these two methods were found to give widely differing results, the mapmakers found themselves in a spot of bother. It all came to a head over the difference in latitude between the towns of Kalianpur and Kaliana. These were supposed to be 370 miles apart. But their latitude measurements, determined using the two methods, differed by 550 feet. This did not much please India’s Surveyor General, Colonel (later Sir) George Everest.

Astronomical measurement depended on the use of a plumb bob to level the instrument before readings were taken, and Everest had the idea that the extra gravitational attraction of the Himalayas might have been pulling the plumb away from true vertical. The Archdeacon of Calcutta, John Pratt, who happened to be a Cambridge-educated mathematician, was recruited to examine the conundrum; but his first results singularly failed to make things clearer. When Pratt compensated the astronomical readings for the expected extra gravitational attraction exerted by the mass of mountains that he could see, the observed discrepancy turned out to be much smaller than it should have been. The mountains were exerting less of a pull on the plumb bob than they should have done. It was as though they were hollow.

When Pratt continued correcting readings taken in places near to the coast, the reverse was true. The ocean, despite its thick covering of less-dense water, seemed to be pulling the plumb bob much more than it should have done. Pratt and the mapmakers were on the verge of one of the most fruitful discoveries in all geology. The Archdeacon wrote a paper for the Royal Society.

One of the things that makes science scientific is the fact that reputable journals will not publish anything before receiving the comments of one or more expert referees to whom they send every paper that comes their way. It is a process called peer review, and despite its occasional shortcomings, it remains a cornerstone of reliable science.

It fell to George Biddell Airy, the Astronomer Royal, to review Pratt’s paper for the Royal Society. And it was he who came up with the geologically more correct explanation of these puzzling gravity anomalies.

The Airy model of isostasy. Mountains are tall because they are deep (the crust under mountains is therefore very much thicker than under oceans). Their visible mass is balanced at depth by roots extending down into the Earth. Mountains, Airy said, exert less gravitational pull than they should do because they have roots. Their less dense material extends down into the planet, in whose denser interior they float like icebergs in water. Continental masses, Airy said, stand high above the ocean floor because they are buoyant; in their case, floating in a substrate of denser rock. They stand proud, but only because they have much larger roots below. Mountains are higher than plains for the same reason that big icebergs stand taller than small ones.

The ocean floor, on the other hand, is made of more dense rock. To change the analogy from ice to wood, if continents are light, like balsa wood and stand high in the water, ocean floor is like mahogany or teak ­ so dense that it floats, but only just. Hence despite all that water on top of them, the oceans still exert more gravitational attraction than scientists had expected.

Later on, it was also discovered that if you make a graph of the Earth’s crustal elevation against the total area lying at that level, on this broad scale (at which small ups and downs can be neglected) the crust only has two basic levels. Continents are almost everywhere a few hundred metres above present sea level, and ocean basins are almost everywhere four to five kilometres below it. Sure, the continents have the odd mountain that’s very high, and the oceans have the odd trench that’s very deep. But basically, nearly all land is at one level, and nearly all ocean floor is at another.

This is so because ocean crust has its characteristic density and is the same everywhere (basalt), while continental crust is lighter, and sits higher. And finally, by one of the greatest coincidences of all, there’s just enough water in the ocean basins to fill them - so nearly all continent is also land, and nearly all ocean floor is under several kilometres of water.

This principle is called “isostasy”, but it is really no more than Archimedes’ Principle applied to rocks, which contrary to all intuition, are all floating. Continents, despite what everyone thought they knew, and despite all the legends and myths, simply cannot sink. True, if you freight the land with thick ice sheets, then the extra mass of ice will gradually cause material underneath slowly to flow away. But when the ice melts, the deep, hot rock will flow back, and the land will rise again.

Although it took its time, the idea of isostasy, of the buoyant balance of light and dense rock types, and the knowledge that, given time, the Earth is indeed soft to the touch, was what ultimately paved the way for a true understanding of how supercontinents form and disperse. They do it by moving sideways.