Breakout evolution

The growth and inflation of breakouts is inferred to have occurred through a combination of continued magma supply and vesiculation, with the smaller, morphologically simpler breakouts forming from less mobile areas of the lava flow (i.e. the lateral margins rather than the fronts; Fig. 4.1, 4.8). Small dome-like breakouts, some with a drained amphitheatre structure in the adjacent leveé (Fig. 4.6A, 4.12A, 4.13D) indicate a limited, locally-sourced magma supply. In these breakouts, vesiculation caused by decompression after extrusion from the main channel could have played a significant role in the overall inflation, with the observed vesicularities (15 – 30 volume%, Table 4.1) representing a potential contribution of order metres to inflation (or a lateral advance of order tens of metres). Decompression-driven vesiculation of breakout lava likely contributed to the formation of their scoriaceous and glassy surfaces (Fig. 4.9), which are similar to the surface carapaces observed at lava domes (Anderson and Fink, 1990). At Cordón Caulle, this glassy surface grades into the lighter, grey, breakout interior that contains abundant globulites (Fig. 4.10 A). Given that globulites form due to devitrification below the glass transition temperature

(Schipper et al., 2015), their growth is unlikely to have driven vesiculation within breakouts and they probably had negligible influence on breakout development.

As breakout inflation progresses, causing increased fracturing and upturning of surface slabs (Tuffen et al., 2013), we infer an evolution from domed to petaloid, rubbly then cleft-split morphologies (Fig. 4.8, 4.13). The striated surfaces of the resulting clefts mark repeated transitions from brittle fracturing to ductile tearing during the cleft opening (Forbes et al., 2012; Forbes et al., 2014). However, clefts do not have the centimetre-scale striations and convex sides typical of crease structures commonly observed at other silicic lava flows and domes, which suggests that the simultaneous upwelling and deformation of core lava associated with crease structure formation did not occur (c.f. Anderson and Fink, 1990; Anderson and Fink, 1992). We therefore propose that the formation of clefts within cleft-split breakouts is similar to that of “lava- inflation clefts” observed in basaltic pahoehoe tumuli, where a surface crust is buckled upwards by an increased core pressure (Walker, 1991). Nevertheless, in contrast to basaltic systems, we consider that late-stage vesiculation at Cordón Caulle provided an important contribution to this inflation.

The sustained advance of breakouts further promotes carapace brecciation, and breakouts that are active over timescales of up to months require the establishment of extended preferential thermal pathways to maintain a continued lava supply. In one of the longer rubbly breakouts, the observation of platy fractures suggests that sustained breakouts may begin to develop microstructure characteristics comparable with those of the main channel. With such characteristics not observed in smaller breakouts, it is likely that the presence of platy fractures reflects processes that occur only towards the end of the emplacement of long breakouts or substantial channels.

The surface morphology of a breakout may also reflect the rate at which it grows, just as the transition from pahoehoe to 'a'ā lava can be driven by increased effusion and strain rates (Pinkerton and Sparks, 1976; Rowland and Walker, 1990; Cashman et al., 1999; Hon, 2003). In analogue experiments of silicic lava dome formation, low effusion rates (relative to cooling rate) aided surface crust formation and favoured of spiny or platy morphologies (Fink and Griffiths, 1998), whereas higher effusion rates created smoother and lower profiles (Fink and Griffiths, 1998). Thus, we propose that the larger volume rubbly and brecciated breakouts (Table 4.2) formed at higher supply rates than domed and petaloid breakouts, with higher supply rates facilitated by efficient preferential thermal pathways. Future work should attempt to systematically relate breakout properties (i.e. volume, effusion rate, rheology) to breakout morphologies and the transitions between different morphological types. Such work would likely combine analysis of breakout types, and comparisons to features generated in analogue experiments (e.g. Griffiths and Fink, 1993; Griffiths and Fink, 1997; Fink and Griffiths, 1998).

← Figure 4.13: Cross section schematic of breakout formation. (A) The flow front stalls due to the strength of the cooled surface crust. (B) Material continues to be supplied to the flow front leading to inflation. Vesiculation of the flow core, partly driven by ongoing anhydrous crystallisation, applies pressure to the flow crust, causing it to fracture. (C) The crust fails and a breakout forms. As the breakout extrudes, some vesicles are sheared and partially collapse as volatiles escape from the lava. The drop in pressure during breakout formation leads to further vesiculation of the breakout surface. (D) As the breakout extrudes, sections of the main channel surface drained to form an amphitheatre behind the breakout. Continued supply to the breakout, coupled with core vesiculation, leads to inflation and upturning of surface slabs. Globulites form in the breakout core below the quenched surface crust, these have limited impact on the breakout development. The crystallisation history of the lava flow is shown in Appendix 8C.

Finally, it is likely that breakouts did not all evolve independently, particularly those linked by preferential thermal pathways. For example, the onset of the formation of the large northeast breakout coincided with a decreased rate of new breakout initiation from the main northern lava flow (Fig. 4.1, Appendix 8A.18). This suggests that the supply of lava towards the northeast breakout reduced lava supply to the northern lava flow. Nevertheless, the drainage (or partial drainage) of preferential thermal pathways enabled continued formation and evolution (e.g. fracture growth) of some breakouts even after vent effusion had ceased (Fig. 4.3). The rubbly northeast breakout ultimately attained a final length of ~2 km and it represents an example of the challenge for hazard forecasting that late-stage breakouts present.