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Many volcanoes worldwide still have poorly resolved eruption histories, with the date of the last eruption often undetermined. One such example is Ascension Island, where the timing of the last eruption, and consequently, the activity status of the volcano, is unclear. Here, we use the 40Ar/39Ar dating technique to resolve ages of the three youngest lava flows on the island, which are hawaiites and mugearite with 1.5 – 1.9 wt% K2O. In dating these lavas, we provide the first evidence of Holocene volcanic activity on Ascension (0.51 ± 0.18 ka; 0.55 ± 0.12 ka; 1.64 ± 0.37 ka), determining that it should be classed as an active volcanic system. In addition, we demonstrate that the 40Ar/39Ar method can reproducibly date mafic lava flows younger than 1 ka, decreasing the gap between recorded history and geological dating. These results offer new prospects for determining patterns of late-Holocene volcanic activity; critical for accurate volcanic hazard assessment.

Hawaiian basaltic eruptions commonly initiate as a fissure, producing fountains, spattering, and clastogenic lava flows. Most fissures rapidly localize to form a small number of eruptive vents, the location of which may influence the subsequent distribution of lava flows and associated hazards. We present results from a detailed field investigation of the proximal deposits of episode 1 of the 1969 fissure eruption of Mauna Ulu, Kīlauea, Hawai‘i. Exceptional preservation of the deposits allows us to reconstruct vent-proximal lava drainage patterns and to assess the role that drainage played in constraining vent localization. Through detailed field mapping, including measurements of the height and internal depth of lava tree moulds, we reconstruct high-resolution topographic maps of the pre-eruption ground surface, the lava high-stand surface and the post-eruption ground surface. We calculate the difference in elevation between pairs of maps to estimate the lava inundation depth and lava drainage depth over the field area and along different segments of fissure. Aerial photographs collected during episode 1 of the eruption allow us to locate those parts of the fissure that are no longer exposed at the surface. By comparing with the inundation and drainage maps, we find that fissure segments that were inundated with lava to greater depths (typically 1–6 m) during the eruption later became foci of lava drainage back into the fissure (internal drain-back). We infer that, in these areas, lava ponding over the fissure suppressed discharge of magma, thereby favouring drain-back and stagnation. By contrast, segments with relatively shallow inundation (typically less than ~ 1 m), such as where the fissure intersects pre-eruptive topographic highs, or where flow away from the vent (outflow) was efficient, are often associated with sub-circular vent geometries in the post-eruption ground surface. We infer that these parts of the fissure became localization points for ongoing magma ascent and discharge. We conclude that lava inundation and drainage processes in basaltic fissure eruptions can play an important role in controlling their localization and longevity.

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 − 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 reported.

Calderas are formed by the collapse of large magma reservoirs and are commonly elliptical in map view. The orientation of elliptical calderas is often used as an indicator of the local stress regime; but, in some rift settings, pre-existing structural trends may also influence the orientation. We investigated whether either of these two mechanisms controls the orientation of calderas in the Kenyan Rift. Satellite-based mapping was used to identify the rift border faults, intra-rift faults and orientation of the calderas to measure the stress orientations and pre-existing structural trends and to determine the extensional regime at each volcano. We found that extension in northern Kenya is orthogonal, whereas that in southern Kenya is oblique. Elliptical calderas in northern Kenya are orientated NW–SE, aligned with pre-existing structures and perpendicular to recent rift faults. In southern Kenya, the calderas are aligned NE–SW and lie oblique to recent rift faults, but are aligned with pre-existing structures. We conclude that, in oblique continental rifts, pre-existing structures control the development of elongated magma reservoirs. Our results highlight the structural control of magmatism at different crustal levels, where pre-existing structures control the storage and orientation of deeper magma reservoirs and the local stress regime controls intra-rift faulting and shallow magmatism. Supplementary material: Details of the Standard Deviation Ellipse function and statistical methods are available at http://www.geolsoc.org.uk/SUP18849 .

1. An introduction to global volcanic hazard and risk S. C. Loughlin, C. Vye-Brown, R. S. J. Sparks, S. K. Brown, J. Barclay, E. Calder, E. Cottrell, G. Jolly, J.-C. Komorowski, C. Mandeville, C. Newhall, J. Palma, S. Potter, G. Valentine, B. Baptie, J. Biggs, H. S. Crosweller, E. Ilyinskaya, C. Kilburn, K. Mee and M. Pritchard 2. Global volcanic hazard and risk S. K. Brown, S. C. Loughlin, R. S. J. Sparks, C. Vye-Brown, J. Barclay, E. Calder, E. Cottrell, G. Jolly, J.-C. Komorowski, C. Mandeville, C. Newhall, J. Palma, S. Potter, G. Valentine, B. Baptie, J. Biggs, H. S. Crosweller, E. Ilyinskaya, C. Kilburn, K. Mee and M. Pritchard 3. Volcanic ash fall hazard and risk S. F. Jenkins, T. M. Wilson, C. Magill, V. Miller, C. Stewart, R. Blong, W. Marzocchi, M. Boulton, C. Bonadonna and A. Costa 4. Populations around Holocene volcanoes and development of a Population Exposure Index S. K. Brown, M. R. Auker and R. S. J. Sparks 5. An integrated approach to Determining Volcanic Risk in Auckland, New Zealand: the multidisciplinary DEVORA project N. I. Deligne, J. M. Lindsay and E. Smid 6. Tephra fall hazard for the Neapolitan area W. Marzocchi, J. Selva, A. Costa, L. Sandri, R. Tonini and G. Macedonio 7. Eruptions and lahars of Mount Pinatubo, 1991-2000 C. G. Newhall and R. Solidum 8. Improving crisis decision-making at times of uncertain volcanic unrest (Guadeloupe, 1976) J.-C. Komorowski, T. Hincks, R. S. J. Sparks, W. Aspinall and CASAVA ANR project consortium 9. Forecasting the November 2010 eruption of Merapi, Indonesia J. Pallister and Surono 10. The importance of communication in hazard zone areas: case study during and after 2010 Merapi eruption, Indonesia S. Andreastuti, J. Subandriyo, S. Sumarti and D. Sayudi 11. Nyiragongo (Democratic Republic of Congo), January 2002: a major eruption in the midst of a complex humanitarian emergency J.-C. Komorowski and K. Karume 12. Volcanic ash fall impacts T. M. Wilson, S. F. Jenkins and C. Stewart 13. Health impacts of volcanic eruptions C. Horwell, P. Baxter and R. Kamanyire 14. Volcanoes and the aviation industry P. W. Webley 15. The role of volcano observatories in risk reduction G. Jolly 16. Developing effective communication tools for volcanic hazards in New Zealand, using social science G. Leonard and S. Potter 17. Volcano monitoring from space M. Poland 18. Volcanic unrest and short-term forecasting capacity J. Gottsmann 19. Global monitoring capacity: development of the Global Volcano Research and Monitoring Institutions Database and analysis of monitoring in Latin America N. Ortiz Guerrero, S. K. Brown, H. Delgado Granados and C. Lombana Criollo 20. Volcanic hazard maps E. Calder, K. Wagner and S. E. Ogburn 21. Risk assessment case history: the Soufriere Hills Volcano, Montserrat W. Aspinall and G. Wadge 22. Development of a new global Volcanic Hazard Index (VHI) M. R. Auker, R. S. J. Sparks, S. F. Jenkins, S. K. Brown, W. Aspinall, N. I. Deligne, G. Jolly, S. C. Loughlin, W. Marzocchi, C. G. Newhall and J. L. Palma 23. Global distribution of volcanic threat S. K. Brown, R. S. J. Sparks and S. F. Jenkins 24. Scientific communication of uncertainty during volcanic emergencies J. Marti 25. Volcano Disaster Assistance Program: preventing volcanic crises from becoming disasters and advancing science diplomacy J. Pallister 26. Communities coping with uncertainty and reducing their risk: the collaborative monitoring and management of volcanic activity with the Vigias of Tungurahua J. Stone, J. Barclay, P. Ramon, P. Mothes and STREVA.

Nili Patera is a 50 km diameter caldera at the center of the Syrtis Major Planum volcanic province. The caldera is unique among Martian volcanic terrains in hosting: (i) evidence of both effusive and explosive volcanism, (ii) hydrothermal silica, and (iii) compositional diversity from olivine-rich basalts to silica-enriched units. We have produced a new geological map using three mosaicked 18 m/pixel Context Camera digital elevation models, supplemented by Compact Remote Imaging Spectrometer for Mars Hyperspectral data. The map contextualizes these discoveries, formulating a stratigraphy in which Nili Patera formed by trapdoor collapse into a volcanotectonic depression. The distinctive bright floor of Nili Patera formed either as part of a felsic pluton, exposed during caldera formation, or as remnants of welded ignimbrite(s) associated with caldera formation—both scenarios deriving from melting in the Noachian highland basement. After caldera collapse, there were five magmatic episodes: (1) a basaltic unit in the caldera's north, (2) a silica-enriched unit and the associated Nili Tholus cone, (3) an intrusive event, forming a ~300 m high elliptical dome; (4) an extrusive basaltic unit, emplaced from small cones in the east; and (5) an extreme olivine-bearing unit, formed on the western caldera ring fault. The mapping, together with evidence for hydrated materials, implies magmatic interaction with subsurface volatiles. This, in an area of elevated geothermal gradient, presents a possible habitable environment (sampled by the hydrothermal deposits). Additionally, similarities to other highland volcanoes imply similar mechanisms and thus astrobiological potential within those edifices.

The physical features and morphologies of collections of lava bodies emplaced during single eruptions (known as flow fields) can be used to understand flood basalt emplacement mechanisms. Characteristics and internal features of lava lobes and whole flow field morphologies result from the forward propagation, radial spread, and cooling of individual lobes and are used as a tool to understand the architecture of extensive flood basalt lavas. The features of three flood basalt flow fields from the Columbia River Basalt Group are presented, including the Palouse Falls flow field, a small (8,890 km2, ∼190 km3) unit by common flood basalt proportions, and visualized in three dimensions. The architecture of the Palouse Falls flow field is compared to the complex Ginkgo and more extensive Sand Hollow flow fields to investigate the degree to which simple emplacement models represent the style, as well as the spatial and temporal developments, of flow fields. Evidence from each flow field supports emplacement by inflation as the predominant mechanism producing thick lobes. Inflation enables existing lobes to transmit lava to form new lobes, thus extending the advance and spread of lava flow fields. Minimum emplacement timescales calculated for each flow field are 19.3 years for Palouse Falls, 8.3 years for Ginkgo, and 16.9 years for Sand Hollow. Simple flow fields can be traced from vent to distal areas and an emplacement sequence visualized, but those with multiple-layered lobes present a degree of complexity that make lava pathways and emplacement sequences more difficult to identify.