Overview

During strombolian eruptions, large bubbles of exsolved magmatic gas, with sizes of meters or more, intermittently burst at the magma surface, spraying magma clots over distances of tens to hundreds of meters. This style of activity results from low magma viscosities allowing gas bubbles to move through the liquid magma phase. Relatively small bubbles rise and coalesce into bubbles with diameters similar to that of the conduit, at which point they are called gas slugs. This coalescence is responsible for converting the continuous degassing processes at depth into the observed intermittent surface activity, and may be controlled by the decompression expansion of the bubbles or by portions of non-vertical conduit geometry. Models of strombolian systems cover the bubble coalescence phase (slug generation), the slug ascent, and finally slug burst and the ejection of pyroclasts. A wide range of geophysical measurements, notably from Stromboli and Erebus volcanoes, are available to test these subsurface models. Nevertheless, key questions, such as the degree to which the activity is controlled by the geometry of the conduit, remain.

Introduction

At volcanoes with relatively low-viscosity magmas such as basalt and basaltic andesite, large bubbles (with sizes of meters or greater) of exsolved gas can ascend rapidly through the melt and burst energetically at the surface, producing sprays of molten pyroclasts (Fig. 6.1). This type of intermittent explosion, in which a limited amount of magma is erupted with a relatively significant mass of gas, is known as “strombolian” after the characteristic activity at Stromboli volcano, where several such events usually occur every hour.