Effects of the crater on eruption column dynamics

Abstract

During explosive eruptions, a mixture of pyroclasts and volcanic gas forms a buoyant eruption column or a pyroclastic flow. We systematically investigate how the condition that separates these two eruption styles (column collapse condition) depends on crater shape and magma chamber conditions by integrating the theoretical models for conduit flow, flow inside a crater, and eruption column dynamics. The results show that previous model predictions of column collapse condition based on the relationship between magma discharge rate () and water content (nf) strongly depend on crater shape (depth D and opening angle θ). When a crater is present, the decompression and/or compression of gas‐pyroclast mixture inside and just above the crater result in two distinct types of column collapse: collapse with increasing (HM‐side collapse) and that with decreasing (LM‐side collapse). HM‐side collapse is caused by an increase in conduit radius during the waxing stage of an eruption. LM‐side collapse is associated with a decrease in magma chamber pressure during the waning stage. The value of for HM‐side collapse varies by two orders of magnitude depending on crater shape for fixed nf. This estimate also depends on assumed models for decompression into the atmosphere. The value of for LM‐side collapse is <106 kg s−1 for a shallow crater with a small opening angle, whereas it can be >108 kg s−1 when D tan θ > 102 m. These results are consistent with the field observations from the St. Helens 1980 and Pinatubo 1991 eruptions.