Eruption of andesite triggered by dyke injection: contrasting cases at Karymsky Volcano, Kamchatka and Mt Katmai, Alaska

Arc volcanoes often erupt andesite that appears to have been stored in rese rvoirs at shallow depth for protracted periods. As crystal-rich andesite is close in density to upper crust, such storage may be quite stable. Petrolo gical evidence, and occasionally geological and geophysical evidence as wel l, suggests that the immediate trigger for eruption of the stored magma is injection of new magma into the reservoir, presumably through dykes rising from depth, When the dyke magma is more mafic than the stored andesite, eff usive eruption typically results. When the dyke magma is voluminous and mor e silicic, the results are catastrophic, with production of discontinuously zoned tephra deposits and caldera collapse. Contrasting end-members are il lustrated by the eruptions of Karymsky Volcano in 1996 and of Mt Katmai in 1912.