Linking lava morphologies to effusion rates for the 2014–2015 Holuhraun lava flow field, Iceland

Determining the parameters that control fissure-fed lava morphologies is critical for reconstructing the complex emplacement histories of eruptions on Earth and other planetary bodies. We used a geomorphological map of the 2014–2015 Holuhraun lava flow field, in combination with new constraints on lava emplacement chronology and two independently derived time-averaged discharge rate (TADR) data sets, to analyze correlations between lava morphology and effusion rate. Results show that lava morphologies are dominantly controlled by effusion rate at the vent during the early phases of the eruption and by lava transport processes as the system evolves. Initially, TADR and its variance, which reflect pulsation in the lava supply rate from the vent, directly affect lava emplacement styles. However, as the eruption progresses, the lava transport system exerts a stronger control with channels and ponds that can either dampen variation in local effusion rate or create surges during sudden drainage events. The Holuhraun eruption predominantly produced rubbly lava in its earlier eruption phases and transitioned into the production of spiny lava toward the end of the eruption. However, a drop of TADR during the first phase of the eruption correlates with a decrease in rubbly lava formation and an increase in spiny lava production. This suggests that a change in effusion rate caused the observed transition in lava type. Our findings show that rubbly lava is formed under relatively high local effusion rates with pulsating supply conditions, whereas spiny lava is formed under lower local effusion rates and steadier supply.

The 1845–46 and 1766–68 eruptions at Hekla volcano: new lava volume estimates, historical accounts and emplacement dynamics

We use new remote sensing data, historical reports, petrology and estimates of viscosity based on geochemical data to illuminate the lava emplacement flow-lines and vent structure changes of the summit ridge of Hekla during the large eruptions of 1845–46 and 1766–68. Based on the planimetric method we estimate the bulk volumes of these eruptions close to 0.4 km3 and 0.7 km3, respectively. However, comparison with volume estimates from the well-recorded 1947–48 eruption, indicates that the planimetric method appears to underestimate the lava bulk volumes by 40–60%. Hence, the true bulk volumes are more likely 0.5–0.6 km3 and 1.0–1.2 km3, respectively. Estimated melt viscosity averages for the 1766–68 eruption amount to 2.5 x10**2 Pa s (pre-eruptive) and 2.5x10**3 Pa s (degassed), and for the 1845–46 eruption 2.2x10**2 Pa s (pre-eruptive) and 1.9x10**3 Pa s (degassed). Pre-eruptive magmas are about one order of magnitude more fluid than degassed magmas. In the 1845–46 and 1947–48 eruptions, SiO2 decreased from 58–57 to 55–54 wt% agreeing with a conventional model that Hekla erupts from a large, layered magma chamber with the most evolved (silica-rich) magmas at the top. In contrast, the lava-flows from 1766–68 reveal a more complicated SiO2 trend. The lava fields emplaced in 1766 to the south have SiO2 values 54.9–56.5%, while the Hringlandahraun lava-flow that erupted from younger vents on the NE end of the Hekla ridge in March 1767 has higher SiO2 of 57.8%. This shows that the layered magma chamber model is not suitable for all lava-flows emplaced during Hekla eruptions.

Ice-mass survival for approximately 800 Myr in the tropical Kasei Valles region, Mars

<p>High obliquity excursions on Mars are hypothesised to have redistributed water from the poles to nourish mid-latitude glaciers. Evidence of this process is provided by a variety of viscous flow features—ice-rich deposits buried beneath sediment mantle—located there today, including ‘lobate debris aprons’, or LDAs. During high obliquity extremes, ice may have persisted even nearer the equator, as indicated by numerous enigmatic moat-like depressions in the tropical Kasei Valles region. Numerous depressions surround isolated mesas and demarcate the past interaction between flowing lava and what were presumably ice-rich radial flows resembling today’s LDAs, but which have long since disappeared. Little is known about ‘ghost lobate debris aprons’ (ghost LDAs), besides their spatial extent as recorded by these depressions. This collection of ghost LDAs implies tropical ice loss over an area ~100,000 km<sup>2</sup>. To constrain their history in Kasei Valles we derive model ages of different terrain types from crater counts. To constrain the volume of ice loss, we use a 2D perfect-plasticity model of ice flow to reconstruct the ghost LDA surfaces. Parametrised by the present surface topography and the range of yield stresses derived from radar interrogation of mid-latitude ice masses, the model reconstructs former ice surfaces along multiple flowlines orientated normal to ghost LDA boundaries. This reconstruction indicates between 1,300–3,300 km<sup>3</sup> of ice—similar to that present in Iceland on Earth—was lost since lava emplacement ~1.4 Ga. Dating of these depressions shows that the ghost LDAs survived for ~800 million years following lava emplacement in the Kasei Valles region before their final demise.</p>

Fossil trees, tree moulds and tree casts in the Palaeocene Mull Lava Field, NW Scotland: context, formation and implications for lava emplacement

ABSTRACTMegafossils and macrofossils of terrestrial plants (trees, leaves, fruiting bodies, etc.) are found in sedimentary and pyroclastic units interbedded with lavas in many ancient lava fields worldwide, attesting to subaerial environments of eruption and the establishment of viable plant communities during periods of volcanic quiescence. Preservation within lava is relatively rare and generally confined to the more robust woody tissues of trees, which are then revealed in the form of charcoal, mineralised tissue or as trace fossil moulds (tree moulds) and casts of igneous rock (tree casts, s.s.).In this contribution, we document several such fossil trees (s.l.), and the lavas with which they are associated, from the Palaeocene Mull Lava Field (MLF) on the Isle of Mull, NW Scotland. We present the first detailed geological account of a unique site within the Mull Plateau Lava Formation (MPLF) at Quinish in the north of the island and provide an appraisal of the famous upright fossil tree – MacCulloch's Tree – remotely located on the Ardmeanach Peninsula on the west coast of the island, and another large upright tree (the Carsaig Tree) near Malcolm's Point in the district of Brolass, SW Mull; both occurring within the earlier Staffa Lava Formation (SLF). The taphonomy of these megafossils, along with palynological and lithofacies assessments of associated strata, allows speculation of likely taxonomic affinity and the duration of hiatuses supporting the establishment of forest/woodland communities. The Ardmeanach and Carsaig specimens, because of their size and preservation as upright (? in situ) casts enveloped by spectacularly columnar-jointed basaltic lava, appear to be unique. The aspect of these trees, the thickness of the enveloping lavas and the arrangement of cooling joints adjacent to the trees, implies rapid emplacement, ponding and slow, static cooling of voluminous and highly fluid basaltic magma. The specimens from Quinish include two prostrate casts and several prostrate moulds that collectively have a preferred orientation, aligning approximately perpendicular to that of the regional Mull Dyke Swarm, the putative fissure source of the lavas, suggesting local palaeo-flow was directed towards the WSW. The Quinish Lava is an excellent example of a classic pāhoehoe (compound-braided) type, preserving some of the best examples of surface and internal features so far noted from the Hebridean Igneous Province (HIP) lava fields.These Mull megafossils are some of the oldest recorded examples, remarkably well preserved, and form a significant feature of the island's geotourism industry.