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Syn-eruptive crystallisation can drastically increase magma viscosity, with profound implications for conduit dynamics, lava emplacement and volcanic hazards. There is growing evidence that crystallisation is not only cooling-driven, but can also occur almost isothermally during decompression-induced degassing on ascent from depth. Here we review field and experimental evidence for degassing-driven crystallisation in a range of magma compositions. We then present new results showing, for the first time, experimental evidence for this process in basaltic magma. Our experiments use simultaneous thermogravimetric analysis and differential scanning calorimetry coupled with mass spectrometry (TGA-DSC-MS) to monitor degassing patterns and thermal events during heating and cooling of porphyritic basaltic samples from Mt. Etna, Italy. The partly degassed samples, which contained 0.39–0.81 wt.% total volatiles in the glass fraction, were subjected to two cycles of heating from ambient to 1250 °C. On the first heating, TGA data show that 30–60% of the total volatiles degassed slowly at more...

Magma degassing can trigger crystal growth by increasing the magma liquidus temperature. As crystallization greatly increases magma viscosity, this process can strongly influence eruptive dynamics. We use a microscope and heated stage to obtain the first direct observations of degassing-driven crystal growth in natural basaltic melts at magmatic temperatures. Samples from Mount Etna, Italy (0.39 wt% H2O), and Kilauea volcano, Hawaii (0.18 wt% H2O) were heated in air at 1 bar, and held isothermally for 0.5–17 h between 1190 °C and 1270 °C, before cooling to solidus temperatures. On heating, bubble growth at >900 °C indicated volatile exsolution. In the hydrous Etna sample, isothermal conditions produced numerous new plagioclase crystals that grew to ≤160 μm at maximum rates of 5.2–18 × 10−6 cm s−1. Growth rates and crystal morphologies (tabular to spherulitic) depended on dwell temperature. Growth slowed dramatically after 20 min as equilibrium was approached. In the H2O-poor Kilauea sample, few new crystals appeared; they grew at maximum rates of 1.7–6.5 × 10−6 cm s−1. On cooling, crystal nucleation and growth were strongly influenced by preexisting crystal textures, highlighting the importance of studying natural samples. Our results document rapid crystal growth triggered by melt devolatilization when the H2O content of the glass is sufficiently high. Resultant swift, substantial changes in magma texture are a key control on lava rheology at Etna and elsewhere. more...

Basaltic 'a'ā lava flows often demonstrate compound morphology, consisting of many juxtaposed and superposed flow units. Following observations made during the 2001 eruption of Mt. Etna, Sicily, we examine the processes that can result from the superposition of flow units when the underlying units are sufficiently young to have immature crusts and deformable cores. During this eruption, we observed that the emplacement of new surface flow units may reactivate older, underlying units by squeezing the still-hot flow core away from the site of loading. Here, we illustrate three different styles of reactivation that depend on the time elapsed between the emplacement of the two flow units, hence the rheological contrast between them. For relatively long time intervals (2 to 15 days), and consequently significant rheological contrasts, superposition can pressurise the underlying flow unit, leading to crustal rupture and the subsequent extrusion of a small volume of high yield strength lava. Following shorter intervals (1 to 2 days), the increased pressure caused by superposition can result in renewed, slow advance of the underlying immature flow unit front. On timescales of < 1 day, where there is little rheological contrast between the two units, the thin intervening crust can be disrupted during superposition, allowing mixing of the flow cores, large-scale reactivation of both units, and widespread channel drainage. This mechanism may explain the presence of drained channels in flows that are known to have been cooling-limited, contrary to the usual interpretation of drainage as an indicator of volume-limited behaviour. Because the remobilisation of previously stagnant lava can occur swiftly and unexpectedly, it may pose a significant hazard during the emplacement of compound flows. Constant monitoring of flow development to identify areas where superposition is occurring is therefore recommended, as this may allow potentially hazardous rapid drainage events to be forecast. Reactivation processes should also be borne in mind when reconstructing the emplacement of old lava flow fields, as failure to recognise their effects may result in the misinterpretation of features such as drained channels. more...

Long-lived basaltic eruptions often produce structurally complex, compound `a`ā flow fields. Here we reconstruct the development of a compound flow field emplaced during the 2001 eruption of Mt. Etna (Italy). Following an initial phase of cooling-limited advance, the reactivation of stationary flows by superposition of new units caused significant channel drainage. Later, blockages in the channel and effusion rate variations resulted in breaching events that produced two new major flow branches. We also examined small-scale, late-stage ‘squeeze-up’ extrusions that were widespread in the flow field. We classified these as ‘flows’, ‘tumuli’ or ‘spines’ on the basis of their morphology, which depended on the rheology, extrusion rate and cooling history of the lava. Squeeze-up flows were produced when the lava was fluid enough to drain away from the source bocca, but fragmented to produce blade-like features that differed markedly from `a`ā clinker. As activity waned, increased cooling and degassing led to lava arriving at boccas with a higher yield strength. In many cases this was unable to flow after extrusion, and laterally extensive, near-vertical sheets of lava developed. These are considered to be exogenous forms of tumuli. In the highest yield strength cases, near-solid lava was extruded from the flow core as a result of ramping, forming spines. The morphology and location of the squeeze-ups provides insight into the flow rheology at the time of their formation. Because they represent the final stages of activity of the flow, they may also help to refine estimates of the most advanced rheological states in which lava can be considered to flow. Our observations suggest that real-time monitoring of compound flow field evolution may allow complex processes such as channel breaching and bocca formation to be forecast. In addition, documenting the occurrence and morphology of squeeze-ups may allow us to determine whether there is any risk of a stalled flow front being reactivated. This will therefore enhance our ability to track and assess hazard posed by lava flow emplacement. more...

[1] Surface structures on 'a'ā and blocky lavas reflect the internal flow dynamics during emplacement and also influence the dynamics of developing flows. To investigate the effects of brittle, clinkery 'a'ā flow crusts on flow dynamics and surface structures, we conducted sand and silicone laboratory experiments that simulated the advance of lava into a preexisting channelized flow with a surface crust. Experiments carried out with relatively thin crusts produced apparently ductile surface deformation structures, while thick crusts behaved dominantly in a brittle manner. Increased crustal thickness led to increased strength under compression but favored more disruption under tension, as the flow core welled up through tensile fractures, entraining crustal material. At lava flow fronts, upwelling and entrainment would increase heat losses by radiation and advection, respectively, resulting in a positive-feedback cooling loop. Fracturing caused heterogeneous crustal distribution near the flow front, which resulted in lobate flow advance, despite the absence of the viscoelastic layer that has previously been inferred as the primary control on flow advance and lobe formation. We therefore conclude that the influence of a purely brittle crust on the dynamics and surface morphologies of lava flows is more significant than often thought. All of the surface structures produced in the experiments have been observed on lavas or glaciers and many also on landslides and debris flows, suggesting the results can assist in the understanding of a range of natural flows. more...

Regular topographic surveys of active lava flows could provide significant insight into the development of flow fields, but data of sufficient accuracy, spatial extent and repeat frequency to quantify the processes involved have yet to be acquired. Here, we report results from the use of a new very-long-range terrestrial laser scanner (TLS) on active lavas at Mount Etna, Sicily. The scanner proved capable of providing useful topographic data from volcanic terrain at ranges up to ∼3500 m, with laser returns from ash-covered slopes as well as from lava. Despite very low effusion rates ( more...