Morphological complexities and hazards during the emplacement of channel-fed `a`ā lava flow fields: A study of the 2001 lower flow field on Etna

Abstract

Long-lived basaltic eruptions often produce structurally complex, compound`a`ā flow fields. Here we reconstruct the development of a compound flow field emplaced during the 2001 eruption of Mt. Etna (Italy). Following an initial phase of cooling-limited advance, the reactivation of stationary flows by superposition of new units caused significant channel drainage. Later, blockages in the channel and effusion rate variations resulted in breaching events that produced two new major flow branches. We also examined small-scale, late-stage 'squeeze-up' extrusions that were widespread in the flow field. We classified these as 'flows', 'tumuli' or 'spines' on the basis of their morphology, which depended on the rheology, extrusion rate and cooling history of the lava. Squeeze-up flows were produced when the lava was fluid enough to drain away from the source bocca, but fragmented to produce blade-like features that differed markedly from`a`ā clinker. As activity waned, increased cooling and degassing led to lava arriving at boccas with a higher yield strength. In many cases this was unable to flow after extrusion, and laterally extensive, near-vertical sheets of lava developed. These are considered to be exogenous forms of tumuli. In the highest yield strength cases, near-solid lava was extruded from the flow core as a result of ramping, forming spines. The morphology and location of the squeeze-ups provides insight into the flow rheology at the time of their formation. Because they represent the final stages of activity of the flow, they may also help to refine estimates of the most advanced rheological states in which lava can be considered to flow. Our observations suggest that real-time monitoring of compound flow field evolution may allow complex processes such as channel breaching and bocca formation to be forecast. In addition, documenting the occurrence and morphology of squeeze-ups may allow us to determine whether there is any risk of a stalled flow front being reactivated. This will therefore enhance our ability to track and assess hazard posed by lava flow emplacement.

Emplacing a Cooling-Limited Rhyolite Lava Flow: Similarities with Basaltic Lava Flows

“…The cooled crust acted to retard the lava flows, ultimately halting them before breakouts formed at the flow front and along flow margins (Behncke and Neri, 2003;Coltelli et al, 2007;Applegarth et al, 2010;Tuffen et al, 2013).…”

“…The 2001 flow deposit is almost entirely covered by loose clinker (rubble), making it difficult to observe any surface crust. However, breakouts are common and typically found at flow margins ( Figure 3A) and fronts, together with some areas of drained channel (Applegarth et al, 2010). Most breakouts are hundreds of meters long but some are substantially longer.…”

“…Pressure builds within the flow as new material is supplied to the flow front, causing inflation and eventually breaking the crust to form a secondary flow or breakout (Calvari and Pinkerton, 1998;Blake and Bruno, 2000). This is a common process in basaltic eruptions (Walker, 1971;Kilburn and Guest, 1993;Hon et al, 1994;Calvari and Pinkerton, 1998;Applegarth et al, 2010) but has been observed rarely in high silica lava flows (Harris et al, 2004), partly due to the infrequency with which such eruptions occur.…”

Exceptional mobility of an advancing rhyolitic obsidian flow at Cordón Caulle volcano in Chile

“…Crust formation is known to be important in basaltic lavas 7 , and extensive tubefed lavas are commonly compound (consisting of multiple flow units [8][9] ). Basaltic flow fields often advance through extrusion from stalled flow fronts 8,10 .…”

The Giant Lavas of Kalkarindji: rubbly pāhoehoe lava in an ancient continental flood basalt province

“…By contrast, eruption units with brecciated flow tops and, crucially brecciated bases are termed 'a'ā flows. These are relatively uncommon in CFBP successions (Applegarth et al, 2010;Brown et al, 2011). Between these two end members, however, there is now an established and well-defined spectrum of genetically related lava types Single and Jerram, 2004;Brown et al, 21 2011; Vye- Brown et al, 2013;Duraiswami et al, 2014).…”