Geochemistry

Less Ice, More Melting

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Science  06 Dec 2002:
Vol. 298, Issue 5600, pp. 1849
DOI: 10.1126/science.298.5600.1849b

Reducing pressure on a hot system tends to enhance melting; this decompression-induced melting has been proposed as a major pathway for producing magma in the mantle, which in turn leads to increased volcanism. An elegant test of this coupling, allowing inferences about the mechanism, is possible in regions where volcanism is abundant and where a rapid drop in pressure occurred when the major ice sheets melted during the last deglaciation. An ideal locale is Iceland, the most volcanically active region at high latitudes. Here, major ice sheets that were a kilometer thick melted abruptly after about 12,800 years ago, leading to a local decrease in pressure of 100 bars or more.

Maclennan et al. have compiled a history of the volume and composition of well-dated volcanic rocks. This record shows that after deglaciation, eruption rates increased markedly for less than 2000 years, then dropped by more than an order of magnitude. Magmas were richer in MgO, indicating higher melting rates in the mantle beneath Iceland. The timing of the peak in volcanic activity and its composition imply that melting was extensive enough that, in the mantle, magmas were flowing in channels and fractures. — BH

Geochem. Geophys. Geosyst.3, 10.1029/2001GC000282 (2002).

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