Icelandic Volcanoes: Take Two

I have recently come across a controversial article that has tested the validity of my previous appraisal relating the ability of melting ice caps to generate magma production deep below the earth’s crust. This new intelligence is in fact a direct response to the article that first sparked my interest in this topic.

The author of the retort, Steven Goddard credits that as stated in the original article, a direct relationship exists between melting point and pressure in magmas. Simply put – ‘as pressure increases, so does the melting point’. The phase diagram of basaltic magma below depicts this temperature/pressure link. Clearly, this shows that between 100 - 0 km depth, the melting point drops at least 300 ^oC (roughly 3^oC/km). Since ice is close to one third as dense as basaltic magma, it can be assumed that a removal of 1 km of ice would reduce the melting point by around 1^oC.

Goddard affixes as paper from the Carnegie Geophysical Institute, which produced an empirical measurement of the relationship for one basaltic mineral – diopiside. Where Tm is the melting point (^oC) and P is the pressure (atmospheres) the following relationship was discovered:

Tm = 1391.5 + 0.01297 * P

With this, Goddard is able to calculate that since one atmosphere of pressure equals roughly 10 metres of ice – the melting point would be increased by about 0.0013^oC with every additional metre of ice. Similarly, a loss of 100 metres would incur a decrease in melting point by one tenth of a degree. Interestingly, the peak thickness of Icelandic ice is only 500 metres, therefore a complete loss of the ice caps would cause an alteration in melting point by only around 0.5^oC (Bourgeois et al., 1998).

Conclusively, it does not seem plausible that this tiny reduction in melting point could stir up volcanic activity at such depths to generate major eruptions. It is unlikely that glaciers melting could substantially modify the rate at which magma is produced, which ultimately controls when an eruption takes place. Finally, Goddard concludes that a depletion of ice coverage would minimise the creation of steam and ash (ash forms when magma is cooled and fractured by steam). It therefore seems sensible to assume that a loss of glaciers would change the nature of Icelandic eruptions. Without large steam/ash cloud emissions, volcanoes would behave more like those in Hawaii and potentially be less destructive.