4.12 - Volcanology 101 for Seismologists

Abstract

Magmas feeding into the roots of active volcanoes contain dissolved gas. These magmas then exhibit a full range of degassing behavior, from purely passive (slow, between eruptions) to purely explosive (rapid, during explosive eruptions). Free, ‘leaky’ passive degassing reduces though does not completely eliminate explosive potential; ‘tight’ plugging of conduits for decades, centuries or longer favors gas accumulation and increases explosive potential, particularly if the final ascent of that magma is too fast to allow much of the accumulated gas to bleed off. Explosive magmatic eruptions reflect rapid expansion of gas bubbles to the point that the bubble walls and magma foam fragments (‘blows itself apart’), just as a carbonated drink forms a foam and then a fine spray when rapidly uncorked. Parallels between active volcanoes and active faults include strain accumulation (gas accumulation), stick-slip behavior (intermittent magma ascent and eruption), creep (passive degassing), and power-law magnitude frequency relations. Unique features of volcanoes include high-temperature gases and often pronounced precursory unrest before eruptions. Unrest is especially pronounced when a magma conduit has become plugged since the previous eruption, forcing magma to pressurize and break a new path to the surface before it can erupt.