Emplacing a cooling-limited rhyolite lava flow: similarities with basaltic lava flows

Accurate forecasts of lava flow length rely on estimates of eruption andmagma properties and, potentially more challengingly, on an understanding of the relative influence of characteristics such as the apparent viscosity, the yield strength of the flow core, or the strength of the lava’s surface crust. For basaltic lavas, the relatively high frequency of eruptions has resulted in numerous opportunities to test emplacement models on such low silica lava flows. However, the flow of high silica lava is much less well understood due to the paucity of contemporary events and, if observations of flow length change are used to constrain straightforward models of lava advance, remaining uncertainties can limit the insight gained. Here, for the first time, we incorporatemorphological observations from during and after flow field evolution to improve model constraints and reduce uncertainties. After demonstrating the approach on a basaltic lava flow (Mt. Etna 2001), we apply it to the 2011–2012 Cordón Caulle rhyolite lava flow, where unprecedented observations and syn-emplacement satellite imagery of an advancing silica-rich lava flow have indicated an important influence from the lava flow’s crust on flow emplacement. Our results show that an initial phase of viscosity-controlled advance at Cordón Caulle was followed by later crustal control, accompanied by formation of flow surface folds and large-scale crustal fractures. Where the lava was unconstrained by topography, the cooled crust ultimately halted advance of the main flow and led to the formation of breakouts from the flow front and margins, influencing the footprint of the lava, its advance rate, and the duration of flow advance. Highly similar behavior occurred in the 2001 Etna basaltic lava flow. In our comparison of these two cases, we find close similarities between the processes controlling the advance of a crystal-poor rhyolite and a basaltic lava flow, suggesting common controlling mechanisms that transcend the profound rheological and compositional differences of the lavas

Diffuse degassing at Longonot volcano, Kenya: implications for CO2 flux in continental rifts

Magma movement, fault structures and hydrothermal systems influence volatile emissions at rift volcanoes. Longonot is a Quaternary caldera volcano located in the southern Kenyan Rift, where regional extension controls recent shallow magma ascent. Here we report the results of a soil carbon dioxide (CO2) survey in the vicinity of Longonot volcano, as well as fumarolic gas compositions and carbon isotope data. The total non-biogenic CO2 degassing is estimated at < 300 kg d− 1, and is largely controlled by crater faults and fractures close to the summit. Thus, recent volcanic structures, rather than regional tectonics, control fluid pathways and degassing. Fumarolic gases are characterised by a narrow range in carbon isotope ratios (δ13C), from − 4.7‰ to − 6.4‰ (vs. PDB) suggesting a magmatic origin with minor contributions from biogenic CO2. Comparison with other degassing measurements in the East African Rift shows that records of historical eruptions or unrest do not correspond directly to the magnitude of CO2 flux from volcanic centres, which may instead reflect the current size and characteristics of the subsurface magma reservoir. Interestingly, the integrated CO2 flux from faulted rift basins is reported to be an order of magnitude higher than that from any of the volcanic centres for which CO2 surveys have so far been reporte

The origin and evolution of breakouts in a cooling-limited rhyolite lava flow

Understanding lava flow processes is important for interpreting existing lavas and for hazard assessments. Although substantial progress has been made for basaltic lavas our understanding of silicic lava flows has seen limited recent advance. In particular, the formation of lava flow breakouts, which represent a characteristic process in cooling-limited basaltic lavas, but has not been described in established models of rhyolite emplacement. Using data from the 2011–2012 rhyolite eruption of Puyehue-Cordón Caulle, Chile, we develop the first conceptual framework to classify breakout types in silicic lavas, and to describe the processes involved in their progressive growth, inflation, and morphological change. By integrating multi-scale satellite, field, and textural data from Cordón Caulle, we interpret breakout formation to be driven by a combination of pressure increase (from local vesiculation in the lava flow core, as well as from continued supply via extended thermally preferential pathways) and a weakening of the surface crust through lateral spreading and fracturing. Small breakouts, potentially resulting more from local vesiculation than from continued magma supply, show a domed morphology, developing into petaloid as inflation increasingly fractures the surface crust. Continued growth and fracturing results in a rubbly morphology, with the most inflated breakouts developing into a cleft-split morphology, reminiscent of tumulus inflation structures seen in basalts. These distinct morphological classes result from the evolving relative contributions of continued breakout advance and inflation. The extended nature of some breakouts highlights the role of lava supply under a stationary crust, a process ubiquitous in inflating basalt lava flows that reflects the presence of thermally preferential pathways. Textural analyses of the Cordón Caulle breakouts also emphasize the importance of late-stage volatile exsolution and vesiculation within the lava flow. Although breakouts occur across the compositional spectrum of lava flows, the greater magma viscosity is likely to make late-stage vesiculation much more important for breakout development in silicic lavas than in basalts. Such late-stage vesiculation has direct implications for hazards previously recognized from silicic lava flows, enhancing the likelihood of flow front collapse, and explosive decompression of the lava core

Active rifting, magmatism and volcanism in the Afar Depression, Ethiopia

The Afar Depression forms a topographic low in north-eastern Ethiopia at the triple junction of the Red Sea, Gulf of Aden and Main Ethiopian Rifts at the northern end of the East African Rift system. This setting is one of few locations where active continental breakup and the transition to oceanic crust can be observed on land and serves as a unique natural laboratory to understand the processes involved (Makris and Ginzburg. 1987). The margins of the Afar Depression are marked by the Ethiopian plateau to the west, Somali plateau to the southeast and Danakil Highlands to the northeast. Since the emplacement of the Ethiopian Trap Basalts ~ 31-29 Ma, the Arabian plate has been moving away from the Nubian plate to form the Red Sea (Ebinger et al., 1993; Hofmann et al., 1997; Wolfenden et al., 2005). The inland continuation of the Red Sea extension is displayed in en-echelon ~ 60 km-long discrete magmatic rift segments which are foci for volcanic and tectonic activity: Erta’Ale, Tat’Ale, Alayta and Dabbahu-Manda Hararo (Acocella et al., 2008; Ebinger and Casey, 2001; Hayward and Ebinger, 1996). The Dabbahu-Manda Hararo magmatic segment is the southernmost segment to display the north-west – south-east orientation of the Red Sea Rift in Afar and therefore marks the propagating tip of the Red Sea Rif

Development of a volcanic hazard assessment methodology in low-data environments: Ascension Island, South Atlantic

Knowledge of the character, frequency, magnitude and impacts of previous eruptions from historical records and geological data is in many cases insufficient to enable comprehensive volcanic hazard and impact assessments. Volcanic islands often have only sparse geological data due to a combination of poor exposure, poor deposit preservation, remote location, small physical size of the island, or a lack of resources to carry out the required fieldwork. Inhabitants of small isolated islands, such as the UK Overseas Territories in the South Atlantic, are exposed to multiple natural hazards and the effects of climate change, leading to increasing risks. However, there are limited options for early warning or timely self-evacuation in the event of a crisis. We present the development of a volcanic hazard assessment methodology for low data environments using an analysis carried out for Ascension Island in the South Atlantic as a case study. Using a combined approach of the available geological data, co-development of scenarios with the Ascension Island Government, expert elicitation, consideration of uncertainties, and the application of an eruption analogue to parameterise hazard models, we have carried out probabilistic vent-opening and tephra fall hazard analyses, as well as lava flow modelling. The probabilistic hazard maps form the evidence base to enable discussions with stakeholders on potential future volcanic activity and impacts