Floating islands

In the 1930s, Professor Vening Meinesz performed highly precise measurements of gravity around the world while floating in a submarine. These measurements contained the seeds of his explanation for how volcanic islands float on the earth’s mantle.

Not many people were amazed by this, but geologists were: how does the earth’s crust, including high mountains in some places, stay floating on the malleable upper part of the earth’s mantle (the asthenosphere)? There is apparently a balance between the weight of the earth’s crust and the upward force that the crust experiences from the mantle on which it floats. This ‘isostasy’ is comparable to the way in which the peak of an iceberg protrudes above the water due to the large mass under the water, which produces upward pressure – according to Archimedes’ principle.

At that time, two explanations were in circulation. One assumed the existance of columns of rock floating on a higher density mantle. Beneath a rock mountain, there should be a particularly deep root buried in the earth’s mantle to produce the additional upward pressure needed. This was the model developed by British geologist Georgy Airy in 1855.The other theory, which had been developed a year earlier by John Henry Pratt, assumed that differences in the density of rock columns would lead to higher or lower elevations and that all columns must be buried in the earth’s mantle to approximately the same depth. A comparison could be drawn with rising dough: the lower the density, the higher the mass rises. Slightly less than 80 years later (1931), Vening Meinesz presented his contrasting model, which corresponds better to modern satellite measurements. He was particularly fascinated by volcanic islands, because the earth’s crust is so thin in these areas. The notion of deep roots of rock did not fit.
Vening Meinesz’s gravity measurements from the submarine revealed that the distortion of the gravitational field around such islands could extend for hundreds of kilometres. This is also the basis of his explanation of isostasy: islands remain afloat not only due to upward pressure, but also because of the support of the earth’s crust in the area. The crust bends downward slightly, which distributes the burden of the island. This isostasy model provided an explanation for the characteristic course of the gravitational field around Hawaii: on approach, gravity first reduces (less mass under the boat due to the downward bend in the earth’s crust), followed by a local gravitational peak (a large amount of mass at the top of the volcano).

The TU Delft Library recently published a website on Vening Meinesz’s 1934 expedition with the submarine KXVII from Den Helder to Surabaya.



Illustratie: Stephan Timmers

Illustratie: Stephan Timmers


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