Some Fallout from Icelandic Volcanoes

A story on the Scientific American website proclaims “Ice cap thaw may awaken Icelandic volcanoes.” “A thaw of Iceland’s ice caps in coming decades caused by climate change may trigger more volcanic eruptions by removing a vast weight and freeing magma from deep below ground, scientists said on Friday.”

This contention is based on the fact that rocks melt at a lower temperature under lower pressure. The question is, will removal of some or all of the ice cap in Iceland result in a pressure load decrease that will make a significant difference in melting temperature and therefore produce more magma?

The relationship between pressure and melting temperature of basaltic lava, the type in Iceland, was estimated by the Carnegie Geophysical Laboratory to be:

Tm = 1391.5 + 0.01297 * P

Tm is the melting temperature in degrees Centigrade, and P is the pressure in bars (or atmospheres, 1 atmosphere = 1.01 bars or about 14.7 pounds per square inch at sea level). We need be concerned only with the last term of that formula which says that a one atmosphere change in pressure results in a 0.013º C change in melting point (rounding the number to two significant figures).

The pressure of one atmosphere is about equivalent to the pressure or weight of 10 meters of ice, so one meter of ice would result in a temperature change of about 0.0013º C. The thickest ice in Iceland is about 500 meters. Complete removal of that ice would lower the melting point of rock about 0.65 ºC, not very significant considering the base melting point is nearly 1400 ºC. The earth’s normal geothermal gradient (the change in temperature with depth), is about 20- to 30º C around most of the planet, and about 40º C at tectonically active spots like Iceland. The removal of 500 meters of ice giving a temperature difference of 0.65 ºC is the equivalent of a depth difference of about 16 meters. Big deal. It is very unlikely that this small pressure difference would stimulate additional volcanic activity.

Ash is formed when magma is rapidly cooled and fractured by steam. Removal of the surficial water source could result in less ash formation and make the Icelandic volcanoes behave more like the Hawaiian volcanoes.


Change of Melting Point of Diopside with Pressure, by Hatten S. Yoder Jr., Geophysical Laboratory, Carnegie Institution of Washington, © 1952 The University of Chicago Press.

Another type of fallout:

The banning of air travel in Europe, which is causing economic chaos, was based on predicted damage to aircraft from ash clouds. The ban was based on an advisory from the British Meteorological Office’s London Volcanic Ash Advisory Centre. That organization based their advisory on a computer program originally developed to monitor nuclear fallout. The computer model predicted dangerous concentrations of ash at various places around Europe. The Met office failed to send up any weather balloons to check actual conditions. Because there is a valid possibility that flying through an ash cloud can damage aircraft, the major airlines made test flights in the last few days to check conditions and found no danger. This is another example showing that relying on computer modeling, rather than real data, can cause unnecessary economic loss and concern.

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