The effect of deglaciation on mantle melting beneath Iceland

Abstract

The spreading ridge on Iceland shows large variations in eruption rate over the last 10,000 years. An increase of about 30 times the steady state value, between 10,000 and 8000 years ago, coincides with the disappearance of ice at the end of the last ice age. We examine the possibility that deglaciation caused this increase by modeling the effect on melt generation of the removal of an axisymmetric ice sheet from a spreading ridge. Our calculations take into account the influence of both a nonhydrostatic stress field and viscous flow. The results show that the average melting rate is increased by about 30 times its steady state value when a 2-kin-thick ice sheet melts in 1000 years. The effect of the glacial cycle of loading and unloading is to cause a nonlinear modulation in the melt production. The factor of 30 increase in melt generation during the 1-kyr unloading can occur only because melt generation is reduced for about 60 kyr after the ice load is applied. The total volume of magma that can be produced from the deglaciation of Iceland is about 3100 km 3. The concentrations of the light rare earth elements in the melt produced by deglaciation near the center of the ice sheet are about 15% less than those in melts produced by steady state melting. Transport time for the melt to reach the surface is not yet well constrained but is likely to be less than • 1 kyr.Though the observations suggest an association between deglaciation and melt production, it is not obvious that the removal of about 2 km of ice can generate the volumes of melt involved. It is, however, easy to show that such an association is plausible. Removal of 2 km of ice is equivalent to moving the whole melting column upward by about 0.6 km. Since the melt generation rate during isentropic decompression is about 0.3% km -•, ice removal will increase the melt fraction by about 0.2%, producing a layer of magma 200 m thick from a melting zone that is 100 km thick. The Reykjanes Peninsula has an area of about 40 x 20 km, giving a total melt volume of 160 km 3 on deglaciation. This simple calculation shows that there is no obvious physical problem in accounting for melt production by deglaciation.The total volume of magma caused by deglaciation cannot, however, exceed the normal output of magma from a spreading ridge over the time interval between glacial advance and retreat. This is because the onset of glaciation increases the pressure in the underlying mantle, reducing the extent of melting. When the pressure is released during deglaciation, the production rate of melt is increased. Over the period of glacial advance and retreat there can be no change/ in the mean melt production rate.Deglaciation of Iceland has previously been proposed by several authors as the cause of increased melt pro-21,815

Feedback between deglaciation, volcanism, and atmospheric CO2

“…The deglacial magma production on Iceland has been estimated at 3000 km 3 (Jull and Mckenzie, 1996;Maclennan et al, 2002), or~10% of the total increase which we estimate, but we are more concerned with continental magmatism. Crisp (1984) estimates that the ratio of subsurface to surface emplacement is~10:1 for continental magmatism, and this leads us to expect the equivalent of 2 m of tephra being emplaced across a 100 km swath and along 25,000 km of convergent margin, where we have accounted for tephra being about half the density of the original melt.…”

“…Of these, the clearest demonstration comes from Iceland, where dates from lava flows (Sigvaldason et al, 1992), sulphate concentrations in Greenland ice cores (Zielinksi et al, 1997), and table mountains (Licciardi et al, 2007) are all consistent with increased volcanism during or following deglaciation. The effect on Iceland has also been modeled to result from decompression melting of the mantle caused by removal of an approximately 2 km thick ice sheet during deglaciation (Jull and Mckenzie, 1996;Maclennan et al, 2002). Furthermore, volcanic Earth and Planetary Science Letters 286 (2009) [479][480][481][482][483][484][485][486][487][488][489][490][491] eruptions are more likely to occur when the confining pressure associated with magma reservoirs and fluid transport in confining rock is decreased by ice removal.…”

Climate effects on volcanism: influence on magmatic systems of loading and unloading from ice mass variations, with examples from Iceland

“…Following Jull & McKenzie (1996), we assume isentropic decompression melting and that the substantive derivative of melt fraction and pressure are related in the following way:…”

“…Pressure decrease at depth leading to mantle melting or flexure of the crust creating 'easier' pathways for magma to reach the surface have been suggested as the cause. Modelling of the pressure release effect shows how a large volume of magma was generated (Jull & McKenzie 1996).…”

Climate forcing of volcano lateral collapse: evidence from Mount Etna, Sicily

“…Zielinski et al 1997), attributable to ice unloading in glaciated volcanic terrains (e.g. Jull & McKenzie 1996;Licciardi et al 2007;Carrivick et al 2009), to ocean loading associated with global sea-level rise (McGuire et al 1997a) and to increased precipitation arising from warmer, wetter conditions (Capra 2006).…”