talks to Dr. Jeff Karson of Syracuse University who explains the conditions that produce the ash rich explosive eruptions generally associated with subduction zones and not the mid-oceanic ridge setting of Iceland.
He identifies two factors:
1) Unlike most mid-ocean ridge settings the magma underneath Iceland have a silica rich rhyolitic component along with the copious basalt. Silicic magmas which form rhyolites contains more dissolved volatiles that fizz out of solution as the magma ascends and the pressure on it is released. Hence rhyolitic volcanism tends to be explosive.
2) The ice cover over Iceland. As hot magma comes in contact with ice, it vaporizes it and produces explosive steam, contributing along with other volatiles to a violent eruption.
Why is there rhyolite at a mid-oceanic setting which is usually dominated by basalts?
Two possible answers: One is that it forms through fractional crystallization of a tholeiitic magma. Some geochemical work on the Iceland igneous province suggest though that this might not be the dominant mechanism at work. Instead, partial melting of older crust may be at work. Iceland differs in this respect with Hawai where fractional crystallization of basalts do not produce rhyolite. In Iceland tectonics keeps bringing to depth earlier extrusives. Ascending magmas interact with this earlier formed crust which is heterogenous in character being made up of basalt and containing silicic segregations from earlier cycles of solidification and this interaction likely generates rhyolitic composition magmas.