Lava Viscosity Research Articles

Exceptional mobility of an advancing rhyolitic obsidian flow at Cordón Caulle volcano in Chile

The emplacement mechanisms of rhyolitic lava flows are enigmatic and, despite high lava viscosities and low inferred effusion rates, can result in remarkably, laterally extensive (>30 km) flow fields. Here we present the first observations of an active, extensive rhyolitic lava flow field from the 2011-2012 eruption at Cordón Caulle, Chile. We combine high-resolution four-dimensional flow front models, created using automated photo reconstruction techniques, with sequential satellite imagery. Late-stage evolution greatly extended the compound lava flow field, with localized extrusion from stalled, ~35 m-thick flow margins creating >80 breakout lobes. In January 2013, flow front advance continued ~3.6 km from the vent, despite detectable lava supply ceasing 6-8 months earlier. This illustrates how efficient thermal insulation by the lava carapace promotes prolonged within-flow horizontal lava transport, boosting the extent of the flow. The unexpected similarities with compound basaltic lava flow fields point towards a unifying model of lava emplacement.

The influence of temperature‐dependent viscosity on lava flow dynamics

AbstractMany studies have investigated how channels and tubes form within basaltic flows. The partitioning of a broad lava flow into narrow regions of enhanced flow velocity (which we refer to as preferred pathways) can affect lava emplacement, and allow flows to reach great lengths on Earth and other planets. In this study, we investigate the role that a dynamic instability, driven by large changes in viscosity due to small changes in temperature, can play in the formation and propagation of low‐viscosity preferred pathways. We use a fluid dynamics‐based model of laminar lava flow emplacement that examines this process exclusively. Analysis of this model shows that preferred pathways will initiate when the temperature dependence of the lava's viscosity is sufficiently strong. If a preferred pathway develops, it will form and stabilize over a distance that reflects the competition between the cooling rate through the flow's surfaces and the flow velocity within the preferred pathway. Using velocity and cooling rates of typical basaltic flows on Earth, this model is able to produce preferred pathways within lava flows with reasonable dimensions and timescales. The model also fails to produce such pathways in situations where tubes and channels are not found in nature (e.g., in basalt sheet flows and silicic flows < 1 km in length). These results are sufficiently positive to demonstrate that this thermodynamic instability may play a role in the formation and propagation of preferred pathways within some flows, and thus merit further investigation.

Viscosity of andesitic lava and its implications for possible drain-back processes in the 2011 eruption of the Shinmoedake volcano, Japan

The 850 m diameter crater of the Shinmoedake volcano was filled by andesitic lava after three subplinian eruptions on 26-27 January 2011. We analyzed blocks thrown from the lava-filled crater by subsequent Vulcanian explosions to estimate the lava’s viscosity and evaluate the possibility of drain-back processes in the crater. Petrographic work on the ejecta, including bulk and glass chemistry, phenocryst and microlite modes, and the water content of the glass enabled us to estimate the bulk viscosity of the lava to be 109.8(+15 − 12) Pa s. The conduit radius is constrained to 4.5 to 6 m by the eruption rate of preceding subplinian eruptions (450–740 m3/s dense rock equivalent). We estimate the simple drain-back rate of the lava to be 3 × 10−2 ~ 2 × 10−5 m3/s. At this rate, less than 1 percent of the total amount of the effused lava could drain back within 100 days. Synthetic aperture radar (SAR) observations did not reveal evidence of drain-back after the eruption, possibly because the chamber was sustained, at least in part by repressurization and refilling as observed by global navigation satellite system (GNSS) measurements of the volcano. This study showed that degassing and crystallization of the andesitic magma during emplacement increased magma viscosity by more than five orders of magnitude, prohibiting drain-back of the lava that filled the crater after the emplacement.

Temporal variation of the Shinmoe-dake crater in the 2011 eruption revealed by spaceborne SAR observations

SAR observations to investigate the 2011 Shinmoe-dake eruption were carried out using several SAR satellites. We detected temporal variations of the Shinmoe-dake crater in the SAR images. A convex shape appeared in the PALSAR image acquired on January 27, but it is unclear whether this shape indicates a lava dome. A larger convex shape that does indicate a lava dome could be identified in SAR images acquired after that, and it seems to have grown progressively. Estimating topography so that the simulated intensity image corresponds to the acquired one, we estimated that the lava dome grew from the morning of January 29 until January 31 with a constant effusion rate of 88.7 m3/sec. The lava volume of 15 million m3 estimated from the TerraSAR-X image of February 1 was consistent with that of airborne SAR observation within 20% of its volume. From the estimated lava effusion rate and lava-covered area, we estimated that lava viscosity was less than 2.1 GPa∙s, suggesting the potential to form a lava flow. Furthermore, we corrected the foreshortening distortion using the estimated topography and suggested that the lava effusion point was around the crater lake.

Switching from seismic to seismo-acoustic harmonic tremor at a transition of eruptive activity during the Shinmoe-dake 2011 eruption

This paper reports a sequence of harmonic tremor observed during the 2011 eruption of Shinmoe-dake volcano, Kyushu, Japan. The main eruptive activity started with subplinian eruptions, followed by lava effusion. Harmonic tremor was observed as seismic waves during the final stage of the effusive eruption. The tremor observed at this stage had unclear and fluctuating harmonic modes. In the atmosphere, however, many impulsive acoustic waves indicating small surface explosions were observed. When effusion stopped and explosive degassing began, harmonic tremor was observed as acoustic waves in the air and in the seismic data, and the harmonic modes became clearer and more stable. This transition in the character of the harmonic tremor coincided with rapid deflation of the lava that had accumulated in the crater. Based on these observations, and laboratory experiments reproducing the features of the wave fields, it is concluded that the harmonic tremor sequence at Shinmoe-dake was generated by gas flowing through channels in the gradually solidifying lava. Comparing our results with the few cases of similar transition observed at other volcanoes, we expect that the transition indicates changes in magma rheology and degassing conditions in the crater, and therefore of changes in eruptive activity. Key words: Harmonic tremor, infrasound, lava deflation, degassing, lava viscosity, bubbles.

Evolution of crust- and core-dominated lava flows using scaling analysis

We present a simple tool to evaluate the dominant dynamical regime of a lava flow and to estimate the order of magnitude of the main rheological parameter (viscosity or yield strength) controlling the length of the lava flow with time. We consider three dynamical regimes: a Newtonian viscous regime, a yield strength-dominated regime and a crust-dominated regime. For each of these regimes, we present a scaling analysis to derive relationships between front position and time, emitted volume, slope, width of the flow and rheological properties. We apply the resulting equations to published data from eruptions of 10 lava flows with a range of compositions and conditions. Comparisons of the fits of the models to the data reveal that short-lived, high effusion rate eruptions are dominated by the internal viscosity of the lava, whereas low effusion rate or long-lived eruptions are dominated by the yield strength in the growing crust. Finally, blocky lavas with very high initial crystal contents are dominated by the internal yield strength. The evolution of some flows can be approximated with only two viscosity values: an early low lava viscosity stage and a later higher viscosity stage. The increase in viscosity is attributed to the initial disequilibrium conditions of the magma at the vent with further degassing and cooling triggering crystallisation of the lava flow. For yield strength-dominated flows yield strength is always within an order of magnitude of 105 Pa. This study provides a practical framework for predicting the evolution of the length of lava flows from estimates of the crystal content of the erupting lava and its effusion rate.

Rheology of arc dacite lavas: experimental determination at low strain rates

Andesitic–dacitic volcanoes exhibit a large variety of eruption styles, including explosive eruptions, endogenous and exogenous dome growth, and kilometer-long lava flows. The rheology of these lavas can be investigated through field observations of flow and dome morphology, but this approach integrates the properties of lava over a wide range of temperatures. Another approach is through laboratory experiments; however, previous studies have used higher shear stresses and strain rates than are appropriate to lava flows. We measured the apparent viscosity of several lavas from Santiaguito and Bezymianny volcanoes by uniaxial compression, between 1,109 and 1,315 K, at low shear stress (0.085 to 0.42 MPa), low strain rate (between 1.1 × 10−8 and 1.9 × 10−5 s−1), and up to 43.7 % total deformation. The results show a strong variability of the apparent viscosity between different samples, which can be ascribed to differences in initial porosity and crystallinity. Deformation occurs primarily by compaction, with some cracking and/or vesicle coalescence. Our experiments yield apparent viscosities more than 1 order of magnitude lower than predicted by models based on experiments at higher strain rates. At lava flow conditions, no evidence of a yield strength is observed, and the apparent viscosity is best approached by a strain rate- and temperature-dependent power law equation. The best fit for Santiaguito lava, for temperatures between 1,164 and 1,226 K and strain rates lower than 1.8 × 10−4 s−1, is $$ \log {\eta_{\text{app}}} = - 0.738 + 9.24 \times {10^3}{/}T(K) - 0.654 \cdot \log \dot{\varepsilon } $$ where η app is apparent viscosity and $$ \dot{\varepsilon } $$ is strain rate. This equation also reproduced 45 data for a sample from Bezymianny with a root mean square deviation of 0.19 log unit Pa s. Applying the rheological model to lava flow conditions at Santiaguito yields calculated apparent viscosities that are in reasonable agreement with field observations and suggests that internal shear heating may be significant ongoing heat source within these flows, enabling highly viscous lava to travel long distances.

DOWNFLOW code and LIDAR technology for lava flow analysis and hazard assessment at Mount Etna

The use of a lava-flow simulation (DOWNFLOW) probabilistic code and airborne light detection and ranging (LIDAR) technology are combined to analyze the emplacement of compound lava flow fields at Mount Etna (Sicily, Italy). The goal was to assess the hazard posed by lava flows. The LIDAR-derived time series acquired during the 2006 Mount Etna eruption records the changing topography of an active lava-flow field. These short-time-interval, high-resolution topographic surveys provide a detailed quantitative picture of the topographic changes. The results highlight how the flow field evolves as a number of narrow (5-15 m wide) disjointed flow units that are fed simultaneously by uneven lava pulses that advance within formed channels. These flow units have widely ranging advance velocities (3-90 m/h). Overflows, bifurcations and braiding are also clearly displayed. In such a complex scenario, the suitability of deterministic codes for lava-flow simulation can be hampered by the fundamental difficulty of measuring the flow parameters (e.g. the lava discharge rate, or the lava viscosity of a single flow unit). However, the DOWNFLOW probabilistic code approaches this point statistically and needs no direct knowledge of flow parameters. DOWNFLOW intrinsically accounts for complexities and perturbations of lava flows by randomly varying the pre-eruption topography. This DOWNFLOW code is systematically applied here over Mount Etna, to derive a lava-flow hazard map based on: (i) the topography of the volcano; (ii) the probability density function for vent opening; and (iii) a law for the expected lava-flow length for all of the computational vents considered. Changes in the hazard due to the recent morphological evolution of Mount Etna have also been addressed.

Three dimensional modelling of lava flow using Smoothed Particle Hydrodynamics

The paper describes the use of the grid-free Smoothed Particle Hydrodynamics (SPH) method to investigate lava flow from volcanic eruptions using real three dimensional topography in the form of Digital Terrain Models (DTM). Heat transfer resulting from conduction and radiation and solidification of the lava modelled via a variable viscosity are coupled to the fluid flow solution. Simulations show that the run-out distance and the nature of the lava flow are affected significantly by the lava viscosity and that this is dependent on the scale of the volcanic eruption, with solidification effects strongest on the smallest scale. SPH appears to be a highly effective technique for predicting lava flow with very good representations of the fluid free surface, close interaction with the complex topography, easy inclusion of the thermal and solidification effects leading to very plausible flow predictions. The pile-up of the lava at the front as it solidifies and the subsequent deceleration of the flow are easily modelled by SPH.

A hybrid model for leveed lava flows: Implications for eruption styles on Mars

Many channelized lava flows on the plains of Mars have substantial embanking margins and levees inferred to have been stationary while the central channel was active. Levee formation can be attributed to two end‐member processes during emplacement: construction during passage of the flow front and growth along the entire length of the flow while it is active. It is shown here that the amount of lava that can be deposited by the flow front alone is limited. Estimates of the levee volume for many Mars plains flows exceed this limit and must have formed by processes that continued after the passage of the front. Experimental studies of analogous laboratory flows also indicate a combination of both modes of emplacement. A model that combines both modes of levee formation is presented, including a method for estimating volumetric flow rate, eruption duration, and viscosity. Six lava flows on the plains of the Tharsis volcanic province are used as illustrative examples. Crustal thicknesses for the six flows examined range from 9 to 23 m. Estimated emplacement times required to cool crusts of these thicknesses range from 1 year to 10 years. Corresponding viscosities are on the order of 105–106 Pa s. Effusion rates range from 25 to 840 m3 s−1 and are all within the range of terrestrial observations. Therefore, the large leveed plains flows on Mars are not dramatically different in eruption rate or lava viscosity from large terrestrial analogs.

Shape, rheology and emplacement times of small martian shield volcanoes

This paper focuses on the shape of 31 shield volcanoes of small to intermediate size (a few tens of kilometers in diameter) in 5 regions of Mars: Tempe Terra, Syria Planum, Pavonis Mons, Arsia Mons and central Elysium Planitia. A model for the shape of these shield volcanoes is applied, based upon the concept of porous flow of an unconfined aquifer proposed by Lacey et al. [Lacey, A., Ockendon, J.R., Turcotte, D.L., 1981. On the geometrical form of volcanoes, Earth Planet. Sci. Lett., 54(1), 139–143] and developed by Turcotte and Schubert [Turcotte, D., Schubert, G., 2002. Geodynamics. Cambridge University Press, second edition]. The agreement between the topographic profiles and the theoretical shape suggests that the model is appropriate and can be used to derive the product of flow rate and viscosity ( Qµ). Estimates of Qµ are found to be homogeneous within a given volcanic region, but show large differences from one region to another, values generally decreasing with age (from oldest to youngest). Intrinsically, the method cannot separate the flow rate from the viscosity, but independent observations on individual lava flows in Syria Planum and central Elysium Planitia suggest that this evolution may be explained by decreasing magma viscosity with age. Independent constraints on the viscosity are used to interpret Q variations between volcanic regions. Except for Tempe Terra, and within error bars, relative effusion rates are essentially independent of volcano volumes, suggesting that the size of a shield volcano is principally controlled by the duration of volcanic activity. The specificity of central Elysium Planitia is emphasized, with highest flow rates associated with lowest lava viscosity. However, the lower viscosities at central Elysium Planitia cannot totally explain low Qµ values. Considering the equation of magma flow through a dike from a magma chamber at depth, to the shield, we suggest that lower volatile contents and/or deeper magma chambers may contribute to the decrease of Qµ values with time. Finally, the emplacement times of the 31 shield volcanoes were calculated using a calibration based upon terrestrial volcanoes Mauna Loa, Eldborgir, and Skjalbreiður. Emplacement times of martian volcanoes calculated in this way range from a few hundred thousand years (Tempe Terra) to a few tens of thousands of years (central Elysium Planitia). Despite the simplicity of the model, the quality of the fits and the internal consistency of the results are reasonable, encouraging development of more advanced models linking shield volcano shape to rheological properties of lavas and their characteristic time-scales of emplacement.

Fissure eruptions in Tharsis, Mars: Implications for eruption conditions and magma sources

The Tharsis region has been the focus of many studies of volcanism on Mars. Not only are the largest volcanoes on the planet located here, but also many of the youngest volcanic features are found within Tharsis. Comparatively little attention has been given to understanding the origin of the smaller volcanoes within Tharsis, and these smaller structures may provide important information on both the regional tectonics of the area, and the availability of magma from the shallow or deep crust. We have identified six separate, but physically close, vent systems in eastern Tharsis just to the east of the volcano Jovis Tholus. These vents are typically linear fissures a few to ~ 20 km in length that have built small shields rising to ~ 50–85 m above the level of the surrounding topography. The maximum length of individual lava flows from these fissures is ~ 30 km, but more typical lengths are 15–20 km. Mapping of the vent complexes reveals that smooth material, interpreted to be spatter from fire fountaining, is located on the rims of some of the fissures. This spatter then formed either short (< 5 km) flows or merged close to the fissure to form the longer flows. There appears to have been some temporal evolution of the flow field, as the oldest parts of the basement at each center are built from a series of compound flows that cannot be subdivided into individual flows. We find evidence of frozen lava ponds and channelized flows from central vents. Photoclinometric profiles are constructed across one of the longer flows to confirm an unusual attribute, originally identified by Mouginis-Mark and Christensen [Mouginis-Mark, P.J., Christensen, P.R., 2005. New observations of volcanic features on Mars from the THEMIS instrument. J. Geophys. Res. 110: E8, doi: 10.1029/2005JE002421], of the flows from the central vent complex: they are all < 5 m thick, which is a factor of ~ 8–15 thinner than flows elsewhere on Mars. To investigate the unusual eruption conditions (limited total volume of the construct and of individual flows) we model the lava rheology, the duration of emplacement, the subsurface conditions that may have led to these eruptions, and the volatile content of the magma. We find lava viscosities and yield strengths to be ~ 100 Pa s and ~ 100 Pa, respectively, eruption rates to be ~ 5000 m 3 s − 1, flow speeds to be 1–2 m s − 1, durations of emplacement of individual flow units to be ~ 5 h, and the equivalent magma water content to be 0.1–0.2 mass%. These eruption conditions are consistent with a wide range of possible depths of the magma reservoirs feeding the eruptions. Small vents such as the ones studied here have also been identified in other parts of eastern Tharsis, so that the eruptions described here may characterize a common style of volcanism on Mars that can only be identified now that image spatial resolution in the range 1–20 m/pixel is available. It is therefore possible that additional searches of other volcanic areas (Syrtis Planum, Elysium Planitia, and Hesperia Planum) may also show greater diversity of activity than is currently accepted.

Rheological properties of dome lavas: Case study of Unzen volcano

Transitions between effusive and explosive styles of lava dome eruptions are likely accompanied by changes in lava rheology. The common presence of crystals in dome lavas produces a complex non-Newtonian rheology. Thus models of such complex rheology are essential for volcanic eruption models. Here, we have measured the rheology of natural Unzen lavas with a compressive uniaxial press operating at stresses between 1 and 70 MPa and temperatures between 940 and 1010 °C. Crystal-rich Unzen lavas are characterised by two essential rheological features which produce non-Newtonian effects. The first is an instantaneous response of the apparent viscosity to applied stress which requires that the magma be described as a visco-elastic fluid that exhibits shear-thinning. The second effect takes the form of a time-dependence of the viscosity at moderate to high stress (≥ 10 MPa). In this regime, the apparent viscosity slowly decreases as increasing fracturing of the phenocrysts and the groundmass occurs. Fragmentation of crystals and alignment of crystal fragments are observed to produce flow banding-effects which in turn lower the apparent viscosity of natural dome lavas. Ultimately, deformation may lead to complete rupture of the lava if the stress is sufficient. Cracking thus stands as an important process in natural dome lava rheology. The ubiquitous non-Newtonian rheology of dome lavas, observed experimentally here, needs to be adequately treated in order to generate appropriate eruption models.

A self‐replication model for long channelized lava flows on the Mars plains

A model is presented for channelized lava flows emplaced by a self‐replicating, levee‐building process over long distances on the plains of Mars. Such flows may exhibit morphologic evidence of stagnation, overspills, and upstream breakouts. However, these processes do not inhibit the formation and persistence of a prominent central channel that can often be traced for more than 100 km. The two central assumptions of the self‐replication model are (1) the flow advances at the average upstream velocity of the molten core and (2) the fraction of the lava that travels faster than the average upstream velocity forms stationary margins in the advancing distal zone to preserve the self‐replication process. For an exemplary 300 km long flow north of Pavonis Mons, the model indicates that ∼8 m of crust must have formed during emplacement, as determined from the channel and levee dimensions. When combined with independent thermal dynamic estimates for the crustal growth rate, relatively narrow constraints are obtained for the flow rate (2250 m3 s−1), emplacement duration (600 d), and the lava viscosity of the molten interior (106 Pa s). Minor, transient overspills and breakouts increase the emplacement time by only a factor of 2. The primary difference between the prodigious channelized Martian flows and their smaller terrestrial counterparts is that high volumetric flow rates must have persisted for many hundreds of days on Mars, in contrast to a few hours or days on Earth.

Some first-order observations on magma transfer from mantle wedge to upper crust at volcanic arcs

Abstract The viscosity of lavas erupted at volcanic arcs varies over orders of magnitude. A comparison of the relative abundance of viscous lava dome eruptions indicates that the average viscosity of arc lavas also varies considerably between arcs. It is shown that, for continental or transitional arcs with little within-arc crustal deformation and without underlying slab windows or tears, average lava viscosity is anticorrelated with average surface heat flux. The latter may be influenced by crustal thickness and crustal magma throughput. To constrain the relative contributions of these parameters, variations of average lava viscosity with average crustal thickness and plate convergence rate are assessed. While crustal thickness appears to have little effect on average lava viscosity, a good anticorrelation exists between average lava viscosity and plate convergence rate, with the exception of two arcs that show significant intra-arc crustal deformation. If plate convergence rate is a good proxy of the rate of melt generation within the mantle wedge, these first-order observations indicate that, where the rate of mantle melting is high, crustal magma throughput is rapid and efficient, resulting in low-viscosity melts migrating through a hot overriding crust; in contrast, where the rate of mantle melting is low, crustal magma transfer is slow and inefficient, resulting in high-viscosity melts that may frequently stall within a cool overriding crust prior to eruption. Uranium series geochemical evidence from dome lavas is presented and lends support to this interpretation. Finally, some explanations are offered for the observed average viscosity variations of arcs with underlying slab windows or tears and/or significant intra-arc crustal deformation.

The transition from endogenous to exogenous growth of lava domes with the development of shear bands

The transition from an endogenous to an exogenous regime of lava dome growth must be achieved by the formation of discontinuities within the dome. Such transitions (and vice versa) are an important characteristic of most long-lived lava domes and often coincide with significant changes in the dynamics of magma supply and lava dome collapse events. For the purpose of this paper, following recent experimental and observational evidence, we assume that such a transition occurs when shear bands are generated. A model for the formation of shear bands, and therefore the growth transition within a dome and coupled conduit domain is presented. Shear bands are most likely to initiate at the junction of the conduit and base of the dome, where the shear stress experienced between new lava entering the dome and existing lava is greatest. Stress accumulation within shear bands is likely to lead to brittle shear, resulting in the formation of fractures. Finite element modelling of lava flow shows that such shear bands only develop for certain extrusion rates and lava viscosities. Similarly, the growth regime of the lava dome will depend upon the extrusion rate and viscosity within the conduit, which is largely controlled by volatile loss and the growth of crystals in the upper part of the conduit. We consider a simplified rheology during lava dome growth considering isothermal conditions with crystal growth. The development of shear bands in the conduit is explored with a numerical model parameterized with values appropriate for Soufrière Hills Volcano, Montserrat. During October to December 1996 this lava dome forming eruption experienced a transition from endogenous to exogenous growth as it grew in height by about 90 m. Modelling indicates that the observed fall in magma extrusion rate from about 2.0 m 3 s − 1 to 0.5 m 3 s − 1 , as a result of the increased pressure head from the dome and the evolution in viscosity, could have subsequently changed the dome growth regime due to the development of shear bands. Our models provide insight into the shear stress fields possible within the conduit and the shear stresses required for shear band development.

Lunar domes in Mare Undarum: Spectral and morphometric properties, eruption conditions, and mode of emplacement

In this study we examine five lunar domes in Mare Undarum. Four domes termed Condorcet 1–4 are located between the craters Condorcet P and Dubiago, immediately east of Dubiago V and W. The fifth dome, termed Dubiago 3, is located about 35 km further south. The region under study is situated in a major trough concentric to the Crisium basin. The domes Condorcet 1–3 are aligned radially with respect to the Crisium basin. Similar dome configurations aligned radial to major impact basins are known from other lunar mare dome fields. The spectral signature of the domes derived from Clementine UV/VIS imagery reveals that they consist of basaltic lava with a low TiO 2 content below 2 wt% and with a FeO content around 10 wt%. Three examined domes exhibit highland components in their soils, which we attribute to lateral mixing between the material in the mare ponds and the surrounding highland material due to random impacts. All five domes have moderate diameters between 10 and 12 km. Condorcet 1–3 are similar to effusive domes of intermediate flank slope between 1 ∘ and 2 ∘ like those situated in the Hortensius/Milichius/T. Mayer region, while Condorcet 4 has an exceptionally steep flank slope of 2 . 8 ∘ and a large volume. With its low flank slope of 0 . 9 ∘ , the dome Dubiago 3 is morphometrically very similar to a known intrusive dome in the west of Mare Serenitatis. Hence, this structure is possibly of intrusive origin, but with the available data an effusive origin cannot be ruled out. Based on a rheologic model, we infer the physical conditions under which the domes were formed (lava viscosity, effusion rate, magma rise speed) as well as the geometries of the feeder dikes.

Formation of lunar mare domes along crustal fractures: Rheologic conditions, dimensions of feeder dikes, and the role of magma evolution

In this study we examine a set of lunar mare domes located in the Hortensius/Milichius/T. Mayer region and in northern Mare Tranquillitatis with respect to their formation along crustal fractures, their rheologic properties, the dimensions of their feeder dikes, and the importance of magma evolution processes during dome formation. Many of these domes display elongated summit vents oriented radially with respect to major impact basins, and several dome locations are also aligned in these preferential directions. Analysis of Clementine UV/VIS and Lunar Prospector gamma ray spectrometer data reveals that the examined mare domes formed from low-Si basaltic lavas of high FeO and low to moderate TiO 2 content. Based on their morphometric properties (diameter, height, volume) obtained by photoclinometric and shape from shading analysis of telescopic CCD images, we derive rheologic quantities (lava viscosity during eruption, effusion rate, duration of the effusion process, magma rise speed) and the dimensions of the feeder dikes. We establish three rheologic groups characterised by specific combinations of rheologic properties and dike dimensions, where the most relevant discriminative parameter is the lava viscosity η. The first group is characterised by 10 4 Pa s < η < 10 6 Pa s and contains the domes with elongated vents in the Milichius/T. Mayer region and two similar domes in northern Mare Tranquillitatis. The second group with 10 2 Pa s < η < 10 4 Pa s comprises the very low aligned domes in northern Mare Tranquillitatis, and the third group with 10 6 Pa s < η < 10 8 Pa s the relatively steep domes near Hortensius and in the T. Mayer region. The inferred dike dimensions in comparison to lunar crustal thickness data indicate that the source regions of the feeder dikes are situated within the upper crust for six of the domes in northern Mare Tranquillitatis, while they are likely to be located in the lower crust and in the upper mantle for the other examined domes. By comparing the time scale of magma ascent with the time scale on which heat is conducted from the magma into the host rock, we find evidence that the importance of magma evolution processes during ascent such as cooling and crystallisation increases with lava viscosity. We conclude that different degrees of evolution of initially fluid basaltic magma are able to explain the broad range of lava viscosities inferred for the examined mare domes. The spectral data reveal that differences in TiO 2 content may additionally account for the systematic difference in lava viscosity between the two examined lunar regions. We show that the described mechanisms are likely to be valid also for other lunar mare domes situated near Cauchy and Arago, regarded for comparison. On the other hand, we find for the Gruithuisen and Mairan highland domes that despite their inferred high lava viscosities of 10 8 – 10 9 Pa s , no significant magma cooling in the dike occurred during ascent, supporting previous findings that the highland domes were formed during a specific phase of non-mare volcanism by highly silicic viscous lavas.

Non-Newtonian rheological law for highly crystalline dome lavas

Volcanic eruption models are hampered by the lack of multiphase magmatic flow laws. Most rheological models estimate the viscosity of multiphase lavas via the Einstein-Roscoe equation, but this simplification cannot be used for high crystallinity and it does not consider the non-Newtonian strain-rate dependence of viscosity. We carried out parallel plate experiments on natural samples to simulate multiphase lava deformation under various stresses and strain rates. Multiphase lavas exhibit an important component of shear thinning, and appear to invalidate the adequacy of Einstein-Roscoe–based formulations for highly crystalline lava rheology. The remarkable singular dependence of viscosity (η) on strain rate (γ) yields a novel universal rheology law at eruptive temperatures ( T ), i.e., log η = −0.993 + 8974/ T −0.543·log γ Our work reveals the importance of considering microcracking and viscous dissipation at very high strain rate (>10 −3 s −1 ), explaining the occurrence of seismic swarms along the conduit margins, and consequently supporting plug-like magma ascent models.

Evaluation of lava effusion rate and viscosity from other flow parameters

The effusion rate of lava from a volcanic vent is an important quantity controlling the length of the ensuing lava flow. The ability to evaluate the effusion rate is therefore of importance to estimate the hazard connected with the flow. In the present paper we provide formulae giving the effusion rate and the lava viscosity for two‐ and three‐dimensional (3‐D) Newtonian and Bingham flows. In order to keep the difference between the actual lava effusion rate and the model estimation within 10%, the 3‐D model must be used if the ratio between the width and the thickness of the lava flow does not exceed the value 13. The Bingham model must be used if the ratio between the plug thickness and the thickness of the lava flow exceeds the value 0.07. An analogous consideration can be made for viscosity.