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[Volcanic eruptions generate continuous or episodic tremor, which can provide unrivaled information about activity changes during eruption. However, the wealth of information in episodic tremor patterns is often not harvested and transitions between patterns remain obscure. The 2021 Geldingadalir eruption, Iceland, is an exceptional case, where the lava effusion caused continuous tremor, and 8698 tremor episodes spanning two orders of magnitude in duration and repose. Based on seismometer and video camera data, we associate several‐minute‐long, symmetrical episodes with an open vent system, where lava remains in the crater bowl during repose, connected to a shallow magma compartment. Ramp‐shaped episodes, lasting several hours, are associated with a temporary closure of the vent system, where no lava remains in the crater bowl during repose and more time is required to resume effusion. The transition from continuous to episodic effusion is related to the cumulative time spent in effusion and repose, and to external factors like crater wall collapses. The range of observed eruption styles and shifts between them, took place at unchanged magma supply rate. This underpins the importance of processes, geometry and evolution of the shallow conduit with time.] Plain Language Summary: Volcanic eruptions can provide unrivaled information about the subsurface structure and processes driving it. Often effusion happens continuous over a time span. However, sometimes the effusion starts and stops and is hence episodic. During the 2021 Geldingadalir eruption, Iceland the lava flowed continuously from the vent but later stopped 8698 times. Some of these episodes where a few minutes long, while others lasted several hours. We interpret several‐minute‐long episodes as an open system, where lava remains in the crater during repose. This system can restart fast and easy. During several hour‐long episodes, no lava remains in the crater during repose. The system needs more time to reopen the vent. The system also reacts depending on the time it is in repose or effusion. And finally, external factors like crater wall collapses modify the episodic pattern. The most interesting feature is the range of eruption styles and transitions between them, that occurred at constant magma inflow rate and are hence caused by the shallow conduit. Key Points: The 2021 Geldingadalir eruption, Iceland, featured 8698 tremor episodes of minute to week durationAn open vent system with lava residing in the crater during repose featured minute‐long lava effusion with bell‐ or rectangle‐shaped tremorA semi‐closed vent system with no lava residing in the crater featured hour‐long lava effusion with ramp‐shaped tremor [ABSTRACT FROM AUTHOR]

Abstract: The 2500 BP Hverfjall eruption in northern Iceland produced pyroclastic and effusive deposits of widely different characteristics along the length of the eruptive fissure. The southern half of the fissure was located in a shallow lake, whereas the northern part of the fissure extended onto dry land. This specific setting, with overlapping periods of activity at the different vents, resulted in various mingling features between fine-grained phreatomagmatic deposits and lava flows. Here we reconstruct the course of events during this eruption based on field observations and granulometric analyses of the resulting deposits. The eruption can be divided into three main phases depending on the main depositional characteristics and vent locations. The initial phase is dominated by phreatomagmatic fall deposits which are attributed to an overall high eruption rate in a shallow lacustrine setting. The second phases involved opening of two new vents on dry ground, and deposition of scoria and lava flows. The third and final phase of the eruption is associated with a lowering of the eruption rate in the southernmost vent, with a shift in the activity from continuous uprush and fall deposits to discrete explosions and emplacement of base surges. These surges display features consistent with drying up with increasing distance from the vent, suggesting that their dynamics changed during emplacement. Most wet surges were channelized within a preexisting graben structure close to the vent, but some more dilute (i.e., dry) surges were able to flow over this obstacle and continued to flow for more than 5km away from the vent and 100m uphill before stopping. [Copyright &y& Elsevier]

Roughness can be used to characterize the morphologies of a lava flow. It can be used to identify lava flow features, provide insight into eruption conditions, and link roughness pattern across a lava flow to emplacement conditions. In this study, we use both the topographic position index (TPI) and the one-dimensional Hurst exponent (H) to derive lava flow unit roughness on the 2014–2015 lava field at Holuhraun using both airborne LiDAR and photogrammetric datasets. The roughness assessment was acquired from four lava flow features: (1) spiny lava, (2) lava pond, (3) blocky surface, and (4) inflated channel. The TPI patterns on spiny lava and inflated channels show that the intermediate TPI values correspond to a small surficial slope indicating a flat and smooth surface. Lava pond is characterized by low to high TPI values and forms a wave-like pattern. Meanwhile, irregular transitions patterns from low to high TPI values indicate a rough surface that is found in blocky surface and flow margins. The surface roughness of these lava features falls within the H range of 0.30 ± 0.05 to 0.76 ± 0.04. The roughest surface is the blocky, and inflated lava flows appear to be the smoothest surface among these four lava units. In general, the Hurst exponent values in the 2014–2015 lava field at Holuhraun has a strong tendency in 0.5, both TPI and Hurst exponent successfully derive quantitative flow roughness. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract It is well established that rootless cones and associated deposits are the result of explosive interactions between lava flows and environmental water, but there is substantial uncertainty about the dynamics of rootless eruptions, particularly the relative importance of lava supply, water availability and the conditions under which they meet. Here we present a case study of the Younger Laxá Lava in NE Iceland, and the >6500 rootless cones that it created. Critically, this long (63 km) lava flow interacted with water along its length, from flow through/around a large lake (Mývatn), down a narrow river gorge (Laxárdalur) and across a broad glacial valley with wetlands and rivers (Aðaldalur). Using high-resolution digital terrain models and aerial photographs, we map the flow surface morphology, and classify, measure and analyse the rootless cone type, size and spatial distribution in the context of both lava and water availability. We find that rootless cone size is controlled by the combined availability of lava and water: large rootless cones require sustained, high volumes of lava (related to proximity to vent) and water (e.g., Lake Mývatn), whereas limited supplies of lava (with distance from the vent and from the centre of the flow) and water (particularly in dammed river valleys) build smaller cones. Where we infer that where water was distributed in sediment, the lava–water interaction style changed to low-energy and distributed bubble bursts that created >3000 hornitos in the lower reaches of Aðaldalur. The distribution of rootless cones around Mývatn also defines the pre-eruption extent of the lake and suggests substantial lake level changes during the course of the eruption. By looking at the variation in rootless cone type and size in the context of the parent lava flow and the local environment, we explain how the availability of water and local mass flow rate of lava affect the dynamics of rootless eruptions. Highlights • Environmental conditions and lava supply affect the type and size of rootless cones formed. • Large cones require sustained supplies of lava and water e.g. near Lake Mývatn. • Limited lava and water supplies build smaller cones e.g. on flow margins. • Reduced supply of lava & water leads to low-energy bubble bursts, creating hornitos. • Cones show pre-eruption extent of Mývatn & imply syn -eruptive lake level change. [ABSTRACT FROM AUTHOR]

Lava that erupted during the 2014–2015 Holuhraun eruption in Iceland flowed into a proglacial river system, resulting in aqueous cooling of the lava and an ephemeral hydrothermal system. We carried out a monitoring study of this system from 2015 to 2018 to document the cooling of the lava over this time, using thermocouple measurements and data-logging sensors. The heat loss rate from advection through this hydrothermal system in August 2015 was ~5.5 × 108 W; since eruption, aqueous cooling likely accounted for ~1% of the total heat loss from the lava. This estimate excludes steam losses from fumaroles as well as any groundwater that was not released to the surface, and thus is a lower bound. Near the terminus of the flow, advection of heat by flowing water may have locally accounted for tens of percent of the total cooling of that part of the flow. Our data quantify the importance of water cooling for this lava flow and can be compared with models to better understand lava–water interactions more generally. We also provide detailed methods for simple, low-cost monitoring of similar instances in the future. • Monitoring the cooling of the lava erupted during 2014–2015 at Holuhraun, Iceland, quantifies advection of heat by interaction with flowing water. • Aqueous Advection accounted for ~1% of the total heat loss from the lava but was likely much more important in local areas. • These methods enable simple, low-cost monitoring of aqueous cooling of lava. [ABSTRACT FROM AUTHOR]