Crumbling of dacite dome lava and generation of pyroclastic flows at Unzen volcano

Abstract

RECENT modelling of volcanic eruptions has shown that the efficiency of subsurface degassing of magmas determines whether magma erupts explosively or effuses quietly1,2. Slow uprise of magma is often accompanied by effective degassing, leading to the extrusion of lava flows and domes. Although lava dome extrusion is one of the less explosive modes of eruption, it is often accompanied by explosive pyroclastic activities3–5. The 1991 eruption of Unzen volcano provided an opportunity to observe at close range several types of small-scale pyroclastic flow (glowing avalanches) originating from lava domes. Most of the pyroclastic flows are of Merapi type, caused by blocks falling from a collapsing dome; others are of Pelean type originating in an explosion from the side of a dome. The lavas apparently show variable degrees of self-explosivity. We suggest that variable degrees of degassing of the magma produced a wide range of excess pore pressures in the extruded lava domes, resulting in both Merapi-type and Pelean-type pyroclastic flows from the domes.