Preferential eruption of andesitic magmas through recharge filtering

Andesitic volcanic rocks are common in subduction zones and are argued to play an important role in the formation and evolution of the continental crust at convergent margins(1-4). Andesite formation is dominated by mixing between iron- and magnesium-rich (mafic) magmas and silica-rich (felsic) magmas(1,2,4). The abundance of andesites in many subduction zones suggests they erupt in preference to the magmas that mix to produce them(4); however, the reasons for this remain unclear. Here we use textural and geochemical analyses of andesites from Mount Hood, Oregon, to show that eruptions are closely linked with episodes of mafic recharge-the intrusion of mafic magma into a shallow felsic magma reservoir. The felsic and mafic magmas involved rarely erupt by themselves, probably because the former are too viscous and the latter too dense. Mafic recharge overcomes these barriers to eruption, and, as it also promotes efficient mixing, results in preferential eruption of mixed andesitic magmas. The abundance of andesites therefore relates to local crustal conditions and the ability of magmas to erupt. We suggest that volcanoes, such as Mount Hood, that erupt homogeneous andesitic compositions through time are those that are the most reliant on mafic recharge to initiate eruptions.