Journal of Volcanology and Geothermal Research 134 (2004) 77–107 www.elsevier.com/locate/jvolgeores A discussion of the mechanisms of explosive basaltic eruptions Elisabeth A. Parfitt Department of Environmental Science, Lancaster University, Lancaster, LA1 4YQ, UK Received 3 February 2003; accepted 16 January 2004 Abstract Two contrasting models of the dynamics of explosive basaltic eruptions are in current usage. These are referred to as the rise speed dependent (RSD) model and the collapsing foam (CF) model. The basic assumptions of each model are examined, and it is found that neither model is flawed in any fundamental way. The models are then compared as to how well they reproduce observed Strombolian, Hawaiian and transitional eruptive behaviour. It is shown that the models do not differ greatly in their treatment of Strombolian eruptions. The models of Hawaiian eruptions are, however, very different from each other. A detailed examination of the 1983–1986 Pu’u ‘O’o eruption finds that the CF model is inconsistent with observed activity in a number of important aspects. By contrast, the RSD model is consistent with the observed activity. The issues raised in the application of the CF model to this eruption draw into doubt its validity as a model of Hawaiian activity. Transitional eruptions have only been examined using the RSD model and it is shown that the RSD model is able to successfully reproduce this kind of activity too. The ultimate conclusion of this study is that fundamental problems exist in the application of the CF model to real eruptions. D 2004 Elsevier B.V. All rights reserved. Keywords: basaltic; explosive; eruption; strombolian; hawaiian; foam; separated flow 1. Introduction almost certainly to eruptions on Mars (Wilson and Head, 1983, 1994). The presence of dissolved gas Basaltic volcanism is the dominant mode of vol- within basaltic magma results in explosive volcanic canic activity on Earth, the Moon, Mars and Venus activity unless the exsolution of the gas from the (e.g., Cattermole, 1989; Head et al., 1992; Wilson and magma is suppressed (as in sufficiently deep sea-floor Head, 1994). On Earth, >80% of the annual volcanic volcanism—Head and Wilson, 2003) or the gas is lost output is basaltic with >70% of this occurring beneath from the magma prior to eruption. Although explosive the Earth’s oceans (Crisp, 1984). Basaltic eruptions basaltic eruptions are generally much less violent than are frequently described as effusive because they their more silicic counterparts they are, nonetheless, commonly generate lava flows. While the term ‘‘ef- explosive and need to be considered as part of a fusive’’ is appropriate for basaltic eruptions in which continuum of explosive activity that embraces not the lava oozes passively from the vent, it is a mis- only the familiar explosive basaltic eruption styles— leading term when applied to the majority of subaerial Hawaiian and Strombolian—but includes sub-Plinian, eruptions on Earth, to eruptions on the Moon and Plinian, ultra-Plinian and ignimbrite-forming events. Our understanding of the mechanisms of explosive basaltic eruptions has advanced considerably during E-mail address: [email protected] (E.A. Parfitt). the past f30 years due to the collection and analysis 0377-0273/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2004.01.002 78 E.A. Parfitt / Journal of Volcanology and Geothermal Research 134 (2004) 77–107 of new field data (e.g., Heiken, 1972, 1978; Walker, al., 1986; Bertagnini et al., 1990). Though rare exam- 1973; McGetchin et al., 1974; Self et al., 1974; Self, ples of sub-Plinian and Plinian basaltic activity do 1976; Williams, 1983; Walker et al., 1984; Houghton occur (Self, 1976; Williams, 1983; Walker et al., and Schmincke, 1989; Carracedo et al., 1992; Thor- 1984), explosive basaltic eruptions resulting from darson and Self, 1993; Parfitt, 1998), volcano moni- the exsolution of magmatic gases alone (rather than toring (e.g., Richter et al., 1970; Chouet et al., 1974; hydromagmatic activity) generally exhibit Hawaiian Blackburn et al., 1976; Swanson et al., 1979; Wolfe et or Strombolian styles, or behaviour which exhibits al., 1987, 1988; Neuberg et al., 1994; Vergniolle and characteristics of both end-member styles. Brandeis, 1994, 1996; Ripepe, 1996;Vergniolle et al., 1996; Hort and Seyfried, 1998; Chouet et al., 1999), 2.1. Hawaiian activity laboratory studies (e.g., Jaupart and Vergniolle, 1988; Mangan et al., 1993; Mangan and Cashman, 1996; The term ‘‘Hawaiian’’ is used to denote eruptions Zimanowski et al., 1997; Seyfried and Freundt, 2000) that are continuous in character and that generate lava and through mathematical modelling (Sparks, 1978; fountains (Fig. 1), typically tens to hundreds of metres Wilson, 1980; Wilson and Head, 1981; Stothers et al., in height (though they can occasionally exceed 1 km 1986; Vergniolle and Jaupart, 1986; Head and Wilson, in height: Wolff and Sumner, 2000). As the term 1987; Jaupart and Vergniolle, 1988; Woods, 1993; suggests, this type of activity is characteristic of the Parfitt and Wilson, 1995, 1999). It is now widely volcanoes of the Hawaiian chain but it is commonly accepted that Strombolian eruptions result from the seen on other basaltic volcanoes, e.g., Eldfell (Self et formation and bursting of a gas pocket close to the al., 1974), Hekla (Thorarinsson and Sigvaldason, surface (e.g., Blackburn et al., 1976; Wilson, 1980; 1972), Etna (Bertagnini et al., 1990) and Miyakejima Vergniolle and Brandeis, 1994, 1996), though some (Aramaki et al., 1986). Hawaiian eruptions have details of the mechanism are still disputed and are typical durations of hours to days, during which time discussed below. In the case of the dynamics of a lava fountain of fairly constant height may play Hawaiian eruptions, however, a curious situation above the vent (e.g., Wolfe et al., 1988). The lava exists in which two very different models have been fountain ejects clots of magma ranging in size from developed that are both in common usage. I refer to millimetres to about a metre in diameter into the air at 1 these models as the rise speed dependent (RSD) speeds of typically f100 m sÀ (Wilson and Head, model (Wilson, 1980; Wilson and Head, 1981; Head 1981). The majority of the erupted material lands and Wilson, 1987; Fagents and Wilson, 1993; Parfitt and Wilson, 1994, 1999; Parfitt et al., 1995) and the collapsing foam (CF) model (Vergniolle and Jaupart, 1986, 1990; Jaupart and Vergniolle, 1988, 1989; Vergniolle, 1996). The aims of this paper are to review both models of explosive basaltic eruptions, and to present an in- depth examination of the models of Hawaiian activity in which the assumptions and predictions of each model are compared with a wide range of geophysical and observational data from recent eruptions. 2. Styles of explosive basaltic eruption Volcanologists have had many opportunities to observe and monitor explosive basaltic eruptions Fig. 1. Photograph of a lava fountain at the Pu’u ‘O’o vent. The (e.g., Richter et al., 1970; Blackburn et al., 1976; fountain is f400 m in height. (Photograph taken by Lionel Wilson, Swanson et al., 1979; Fedotov et al., 1983; Aramaki et August 1984). E.A. Parfitt / Journal of Volcanology and Geothermal Research 134 (2004) 77–107 79 while still incandescent, and accumulation and coa- produced a number of lava flows, the longest of which lescence of these hot clots generates rootless lava reached a length of 27 km (Lockwood et al., 1987). flows (Head and Wilson, 1989). These flows are Much material falling from the outer edges of the typically still fluid enough to flow many kilometres fountain cools sufficiently during flight that, though it to tens of kilometres from the vent. For example, a 21- deforms on landing and is hot enough to weld to the day-long Hawaiian eruption at Mauna Loa in 1984 material around it, is not hot enough to form rootless Fig. 2. Hot clots of magma accumulate around vents forming spatter ramparts/cones. (a) A section of a spatter rampart formed during the April 1982 eruption of Kilauea. Individual clots have flattened and flowed upon landing. Each clast is f0.2 m is diameter and is welded to those above and below them. (Photograph taken by the author). (b) The spatter cone and down-wind tephra blanket formed during the 1959 Kilauea Iki eruption. Close-up the cone is formed of welded clasts like those in (a). The figure is standing in a collapse pit within the down-wind tephra blanket. Here, at a distance of f0.5 km from the vent, the deposit is composed of centimetre-scale clasts and is unwelded. (Photograph taken by the author, May 1996). 80 E.A. Parfitt / Journal of Volcanology and Geothermal Research 134 (2004) 77–107 lava flows and instead accumulates as a spatter cone around the erupting vent (Fig. 2; Head and Wilson, 1989). Some even cooler material can accumulate to form a loose cinder cone, and a small proportion of the erupted material is carried upwards in a convective plume above the fountain and is deposited downwind forming a tephra blanket (Fig. 2b, Parfitt, 1998). 2.2. Strombolian activity Strombolian activity takes its name from the fre- quent, small-scale, transient explosions exhibited by Stromboli, a volcano which forms one of the Aeolian Islands north of Sicily. Whereas the term ‘‘Hawaiian’’ is well-defined and used in a fairly restricted way, the term ‘‘Strombolian’’ has been used to denote a wide range of activity, and, thus, caution must be used in understanding individual usage of the term. The term ‘‘Strombolian’’ is most commonly used (and is used here) to denote the relatively mild explosions that occur from the accumulation of gas beneath the cooled upper surface of a magma column (e.g., Black- burn et al., 1976; Wilson, 1980).