The Askja volcano eruption in 1875 - where did all the water come from? (Invited)

Eyewitness records observed a distinct change from wet to dry explosive activity that occurred in March 1875 at Askja volcano, Iceland. This rhyodacite eruption of the Askja volcano is the third largest silicic explosive event since settlement in Iceland. It was part of a larger volcano-tectonic episode that took place in the North Volcanic Zone from 1874 to 1876. This eruption is unique in that it features abrupt and gradual shifts between wet and dry explosive activity. It consisted of an initial and abrupt shift from dry subplinian to wet phreatoplinian activity followed by a gradual shift from wet phreatoplinian with pyroclastic density currents to a dry Plinian phase. However, the source of water that caused this shift in eruptive style is not clear - a possible source could be groundwater stored in the Askja caldera, which is bounded by steep-sided hyaloclastic mountains and filled by Holocene and historical (fractured) lava flows. To analyse the transient groundwater dynamics during the March 1875 eruption of the Askja volcano, we developed a flow model that integrates geological observations and numerical discrete fracture techniques to compute effective permeabilities of the fractured lava flows. The eruptive mixture from the vents is thought to induce a venturi effect that removes groundwater from the caldera. Based on this model we propose that the water necessary to maintain the phreatoplinian phase was stored within the lava pile of the Askja caldera and that the rapid groundwater flow through the fractured lavas provided enough water to drive the hour-long phreatoplinian eruption. The decline in groundwater table caused the progressive dry-out of the wet phase and affected the dry character of the following Plinian activity. This study is one of the first that demonstrates, using direct numerical simulations, how the availability of groundwater and the geology of the volcanic edifice can strongly affect the style of a volcanic eruption.