Subaqueous basaltic magmatic explosions trigger phreatomagmatism: A case study from Askja, Iceland

Highlights

• Textural investigation of subaqueous basaltic effusion to explosion transition

• Example of subaqueous magmatic explosions triggering phreatomagmatism

• Evaluation of textures for identifying basaltic fragmentation mechanisms

Abstract

Sequences of basaltic pillow lavas that transition upward with systematic gradation from pillow fragment breccias to fluidal bomb-bearing breccias to bomb-bearing lapilli tuffs are common at Askja volcano, Iceland. Based on the detailed textural investigation of three of these sequences, we argue that they record temporally continuous transitions from effusive to explosive products that were erupted from, and deposited at or near a single subaqueous vent. The recognition of such sequences is important as they provide evidence for controls on the onset of explosive activity in subaqueous environments. Such investigations are complicated by the interplay of magmatic gas expansion, and phreatomagmatic and mechanical granulation fragmentation mechanisms in the subaqueous eruptive environment.

All of the sequences studied at Askja have textural, componentry and sedimentological characteristics suggestive of a close genetic and spatial relationship between the pillow lavas and all of the overlying glassy clastic deposits. The identification of magma fragmentation signatures in pyroclasts was accomplished through detailed textural studies of pyroclasts within the full range of grain sizes of a given deposit i.e. bomb/blocks, lapilli and fine ash. These textural characteristics were compared and evaluated as discriminators of fragmentation in pyroclastic deposits. The presence of angular vitric clasts within the breccia and lapilli tuff displaying fragile glassy projections indicates little or no post-depositional textural modification. A shift in vesicle and clast textures between the pillow lavas and the large concentration of fluidal bombs in the breccia indicates that the phreatomagmatic explosions were initially triggered by magmatic vesiculation. The initial magmatic gas expansion may have been triggered by depressurization caused by the drainage of the ice-confined lake surrounding Askja. The fuel coolant interactions (FCIs) of the more efficient phreatomagmatic explosions were enabled by the increase in the surface area to volume ratio of the fluidal bombs in the water, producing a premix of magma and water. The onset and increasing influence of phreatomagmatic fragmentation is preserved in the presence of very fine blocky ash particles and diminished presence of larger particles such as fluidal bombs. The textural, sedimentological and environmental characteristics of these deposits suggest that phreatomagmatic explosions can be triggered by initial magmatic gas expansion, but that it is likely one of many mechanisms for triggering such explosions.

Introduction

The question of what controls the transition from basaltic effusive to explosive activity in water-rich environments has been a matter of debate over the last 20 years (Wohletz, 1986, Houghton and Schmincke, 1989, Houghton and Nairn, 1991, White, 1996a, Zimanowski et al., 1997, Wohletz, 2002, Wohletz, 2003, Zimanowski et al., 2003, Mastin et al., 2004). The answer to this question has important implications for modeling volcanic hazards, such as the potential for explosions or the grain size distribution, height and duration of ash plumes in wet environments. Submarine exploration technology has advanced our ability to describe subaqueous explosive volcanic deposits from the submarine environment (Clague and Davis, 2003, Clague et al., 2003, Eissen et al., 2003, Clague et al., 2009). However, the study of explosively generated deposits formed in an ice-confined (glaciovolcanic) environment offers much more accessible deposits that are commonly well-preserved in three-dimensions. We argue that such centers preserve sequences that record in situ transitions from effusive to explosive activity at a single vent. The detailed textural and stratigraphic study of such sequences offers the best opportunity for understanding the onset of explosive activity and the interplay of fragmentation mechanisms in natural subaqueous settings.

The focus of this study is three ca. 30 m thick, glass-rich, incipiently palagonitized basaltic pyroclastic deposits that directly overlie pillow lavas from Askja volcano, Iceland (Fig. 1). All three clastic sequences are massive but display a systematic and continuous fining upward from pillow-fragment breccia to fluidly-shaped bomb-bearing breccia and then into vitric lapilli tuff. Ostensibly similar sequences of pillow lavas overlain by breccias, containing pillow fragments, and capped by vitric lapilli tuffs, have been described from many areas, including ophiolite sequences (Carlisle, 1963), Archean basalt provinces (Dimroth et al., 1978), ocean island settings (Fujibayashi and Sakai, 2003) and several glaciovolcanic sequences (Jones, 1970, Werner and Schmincke, 1999, Skilling, 2009). Such sequences could clearly be derived through many processes. Common interpretations for these sequences can be divided into those where the clastic deposits were (1) erupted from a different vent from that which produced the lavas, (2) produced by the same vent that produced the lavas, but not as part of a temporally continuous eruption, or (3) erupted from the same vent as lavas and were part of a continuous eruption. Mechanisms that produce unrelated sequences of pillow lavas and clastic deposits include deposition of density currents from nearby vents or post-emplacement collapse of deposits on top of the pillows. Similar sequences produced from the same vent without continuous eruption include flow related collapse (autoclastic breccias), intrusion of pillowed dikes into clastic deposits, or explosive activity instigated under already solidified lava. Based on a detailed textural analysis of the sequences from the Austurfjöll massif of Askja, we argue that the deposits were produced from the same vent over a brief eruption that transitioned from effusive activity to magmatic explosive, and then to phreatomagmatic explosive eruptive activity.

The interpretation of such clastic deposits relies heavily on the distinction of the influence of different fragmentation mechanisms in the formation of subaqueous basaltic pyroclasts along the contact between facies within the transitional sequences. Evidence for the mechanisms of fragmentation, transportation and deposition is preserved in the textures of subaqueous pyroclasts over the full range of grain sizes, including bombs, lapilli and fine ash, though most previous research has focused on fine ash textures (Heiken, 1971, Mattox and Mangan, 1997, Büttner et al., 1999, De Rosa, 1999, Dellino et al., 2001, Dellino and Liotino, 2002, Ersoy et al., 2006, Durig et al., 2012b). These textural data can then be used to make inferences about the controls of the onset of subaqueous explosive activity and specifically phreatomagmatic activity. In this study we present data on textural characteristics of fine ash to block-sized clasts that could be used to help distinguish phreatomagmatic from magmatic fragmentation, and argue that the phreatomagmatic eruptions in our study area were generated following an initial mingling (premixing) of magma and water driven by magmatic fragmentation. It is not clear how important or common initial mingling by magmatic fragmentation might be in basaltic phreatomagmatic sequences elsewhere, and this is probably only one mechanism for instigating such eruptions. The uniquely dynamic nature of the ice-confined lakes may play a role in the initiation of magmatic gas expansion through depressurization caused by lake drainage. Nevertheless, similar textural investigations may be used to identify the mechanisms during the onset of explosivity in other basaltic phreatomagmatic systems.

Basaltic glaciovolcanic systems under thick ice (> 400 m) typically evolve into ice-confined lacustrine centers (Allen et al., 1982, Gudmundsson et al., 1997, Werner and Schmincke, 1999, Gudmundsson, 2003). Within the ice-confined lake the water level may change rapidly and repeatedly through time (Gudmundsson et al., 1997, Bjornsson, 2002, Höskuldsson et al., 2006, Smellie et al., 2008). Simplified models of such centers include initial subaqueously emplaced effusive products, dominated by pillowed lavas, followed by a shift towards more explosive activity with deposition of glassy fragmental deposits (Jones, 1970, Allen, 1980, Moore et al., 1995, Werner et al., 1996). Within this model it is assumed that there is a decreasing fragmentation and dispersal of subaqueous eruptions as confining pressure or water depth increases, particularly with depths greater than 400 m (Allen, 1980, Clague et al., 2003, Zimanowski and Büttner, 2003). However, investigations of submarine basaltic deposits are revealing the presence of explosively derived deposits at depths up to 3 km (Clague and Davis, 2003, Fujibayashi and Sakai, 2003, Wohletz, 2003, Sohn et al., 2008, Portner et al., 2010, Schipper and White, 2010, Schipper et al., 2010a, Helo et al., 2011, Schipper et al., 2011a). This uncertainty about the importance of controls other than confining pressure (White, 1996a, Mastin et al., 2009, Schipper et al., 2011b) on the triggering of subaqueous explosions emphasizes the importance for detailed studies of natural deposits that may record the onset of basaltic explosions in water.

Askja is one of the largest and best-exposed formerly ice-confined volcanoes on Earth. Most research to date has been on its Holocene (ice-free) evolution. It comprises a complex of basaltic glaciovolcanic massifs that are dominated by pillow lavas and subaqueously emplaced vitric lapilli tuff deposits. These massifs are cut by at least three calderas and surrounded by Holocene subaerial lava flows (Fig. 1). The greatest volume of glaciovolcanic deposits at Askja is the eastern mountain massif, Austurfjöll, which is truncated by the two youngest calderas. Austurfjöll has been described briefly by Brown et al., 1994, Sigvaldason, 1968, Sigvaldason, 2002 and more recently in detail by Graettinger et al. (2012).

Austurfjöll is incised on its eastern side by large gullies that extend up to 3 km into the massif. The vertical exposure within the gullies is between 10 and 100 m. These exposures are dominated by pillow lava sheets, lava breccias and vitric lapilli tuffs. Three of these gullies contain well-exposed sequences that display gradual transitions up-section between effusive pillow lavas at their base, to an upward-fining pillow fragment and bomb-bearing breccia and vitric lapilli tuff sequence. The sequences have lateral continuities of tens of meters and can be traced in multiple directions. The three gullies from north to south are named Drekagil, Nautagil and Rosagil (Fig. 1).