Measurement and implication of "effective" viscosity for rhyolite flow emplacement

Hazardous explosive activity may sporadically accompany the extrusion of silicic lava domes. Modelling of the emplacement of silicic domes is therefore an important task for volcanic hazard assessment. Such modelling has been hampered by a lack of a sufficiently accurate rheological database for silicic lavas with crystals and vesicles. In the present study, the parallel-plate viscometry method was applied to determine the shear viscosity of five natural rhyolitic samples from a vertical section through the Ben Lomond lava dome, Taupo Volcanic Centre, New Zealand. Rheological measurements were performed at volcanologically relevant temperatures (780-950°C) and strain rates (10^-5-10^-7 s^-1). Although these samples are in the metastable state, viscosity determinations, melt composition, as well as water and crystal contents of samples were demonstrably stable during experiments. For samples containing up to 5 vol.% microlites, the composition of the melt, rather than the physical effect of suspended crystals, had greater influence on the effective viscosity of the silicic magma. Samples with 10 vol.% microlites and containing a flow banding defined by microlites show no significant orientational effects on apparent viscosity. The rheological measurements were used together with a simple cooling model to construct thermal and viscosity profiles revealing conditions during the emplacement of the Ben Lomond lava dome.     

The effect of crystallization on the rheology and dynamics of lava flows

The dynamics of a lava flow is studied by a two-dimensional model describing a viscous fluid with Bingham rheology, flowing down a slope. The temperature in the flow is calculated assuming that heat is transferred through the plug by conduction and is lost by radiation to the atmosphere at the top of the flow. Taken into account is that the increasing crystallization takes place in the flow as a consequence of cooling. The lava viscosity and yield stress are expressed as a function of crystallization degree as well as of temperature: in particular it is assumed that yield stress reaches a maximum value above the solidus temperature, according to experimental data. Dynamical variables, such as velocity and thickness of the flow, are calculated for different values of the maximum crystallization degree and the flow rate. The model shows how the lava flow dynamics is affected by cooling and crystallization. The cooling of the flow is controlled by the increase of yield stress, which produces a thicker plug and makes the heat loss slower. The increasing crystallization has two opposing effects on viscosity: it produces an increase of viscosity, but at the same time produces an increase of yield stress and hence reduces the heat loss and keeps the internal temperature high. As a consequence, lava flows are significantly affected by the dependence of yield stress on temperature and scarcely by the maximum crystallization degree.     

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.     

A laboratory model of surface crust formation and disruption on lava flows through non-uniform channels

Crust formation on basaltic lava flows dictates conditions of both flow cooling and emplacement. For this reason, flow histories are dramatically different depending on whether lava is transported through enclosed lava tubes or through open channels. Recent analog experiments in straight uniform channels (Griffiths et al. J Fluid Mech 496:33–62, 2003) have demonstrated that tube flow, dictated by a stationary surface crust, can be distinguished from a mobile crust regime, where a central solid crust is separated from channel walls by crust-free shear zones, by a simple dimensionless parameter ϑ, such that ϑ<25 produces tube flow and ϑ>25 describes the mobile crust regime. ϑ combines a previously determined parameter ψ, which describes the balance between the formation rate of surface solid and the shear strain that disrupts the solid crust, with the effects of thermal convection (described by the Rayleigh number Ra).Here we explore ways in which ϑ can be used to describe the behavior of basaltic lava channels. To do this we have extended the experimental approach to examine the effects of channel irregularities (expansions, contractions, sinuosity, and bottom roughness) on crust formation and disruption. We find that such changes affect local flow behavior and can thus change channel values of ϑ. For example, gradual widening of a channel results in a decrease in flow velocity that causes a decrease in ϑ and may allow a down-flow transition from the mobile crust to the tube regime. In contrast, narrowing of the channel causes an increase in flow velocity (increasing ϑ), thus inhibiting tube formation.We also quantify the fraction of surface covered by crust in the mobile crust regime. In shallow channels, variations in crust width (d _c) with channel width (W) are predicted to follow d _c∼W ^5/3. Analysis of channelized lava flows in Hawaii shows crustal coverage consistent with this theoretical result along gradually widening or narrowing channel reaches. An additional control on crustal coverage in both laboratory and basaltic flows is disruption of surface crust because of flow acceleration through constrictions, around bends, and over breaks in slope. Crustal breakage increases local rates of cooling and may cause local blockage of the channel, if crusts rotate and jam in narrow channel reaches. Together these observations illustrate the importance of both flow conditions and channel geometry on surface crust development and thus, by extension, on rates and mechanisms of flow cooling. Moreover, we note that this type of analysis could be easily extended through combined use of FLIR and LiDAR imaging to measure crustal coverage and channel geometry directly.Editorial responsibility: A. Harris     

基于热流变运动学模型的长白山熔岩流数值模拟

热流变运动学模型是熔岩流复杂流动过程模拟的一种方法,其通过热量系统和地形的计算来模拟熔岩流的流动速度、温度变化和流动距离等.本文以气象站期碱流岩的野外调查数据为依据,应用该模型对其进行模拟研究.通过对模拟结果的分析发现,速度变化趋势与地形坡度变化趋势一致,但随着流动过程中黏度的增大,地形影响作用逐渐减小.在热量系统中,...     

A laboratory investigation into the effects of slope on lava flow morphology

In an attempt to model the effect of slope on the dynamics of lava flow emplacement, four distinct morphologies were repeatedly produced in a series of laboratory simulations where polyethylene glycol (PEG) was extruded at a constant rate beneath cold sucrose solution onto a uniform slope which could be varied from 1° through 60°. The lowest extrusion rates and slopes, and highest cooling rates, produced flows that rapidly crusted over and advanced through bulbous toes, or pillows (similar to subaerial “toey” pahoehoe flows and to submarine pillowed flows). As extrusion rate and slope increased, and cooling rate decreased, pillowed flows gave way to rifted flows (linear zones of liquid wax separated by plates of solid crust, similar to what is observed on the surface of convecting lava lakes), then to folded flows with surface crusts buckled transversely to the flow direction, and, at the highest extrusion rates and slopes, and lowest cooling rates, to leveed flows, which solidified only at their margins. A dimensionless parameter, Ψ, primarily controlled by effusion rate, cooling rate and flow viscosity, quantifies these flow types. Increasing the underlying slope up to 30° allows the liquid wax to advance further before solidifying, with an effect similar to that of increasing the effusion rate. For example, conditions that produce rifted flows on a 10° slope result in folded flows on a 30° slope. For underlying slopes of 40°, however, this trend reverses, slightly owing to increased gravitational forces relative to the strength of the solid wax. Because of its significant influence on heat advection and the disruption of a solid crust, slope must be incorporated into any quantitative attempt to correlate eruption parameters and lava flow morphologies. These experiments and subsequent scaling incorporate key physical parameters of both an extrusion and its environment, allowing their results to be used to interpret lava flow morphologies on land, on the sea floor, and on other planets.     

Thick lava flows of Karisimbi Volcano, Rwanda: insights from SIR-C interferometric topography

 We use a digital elevation model (DEM) derived from interferometrically processed SIR-C radar data to estimate the thickness of massive trachyte lava flows on the east flank of Karisimbi Volcano, Rwanda. The flows are as long as 12 km and average 40–60 m (up to >140 m) in thickness. By calculating and subtracting a reference surface from the DEM, we derived a map of flow thickness, which we used to calculate the volume (up to 1 km³ for an individual flow, and 1.8 km³ for all the identified flows) and yield strength of several flows (23–124 kPa). Using the DEM we estimated apparent viscosity based on the spacing of large folds (1.2×10¹² to 5.5×10¹² Pa s for surface viscosity, and 7.5×10¹⁰ to 5.2×10¹¹ Pa s for interior viscosity, for a strain interval of 24 h). We use shaded-relief images of the DEM to map basic flow structures such as channels, shear zones, and surface folds, as well as flow boundaries. The flow thickness map also proves invaluable in mapping flows where flow boundaries are indistinct and poorly expressed in the radar backscatter and shaded-relief images. Received: 6 September 1997 / Accepted: 15 May 1998     

The dynamics of a channel-fed lava flow on Pico Partido volcano, Lanzarote

A short length of channel on Pico Partido volcano, Lanzarote, provides us the opportunity to examine the dynamics of lava flowing in a channel that extends over a sudden break in slope. The 1–2-m-wide, 0.5–2-m-deep channel was built during the 1730–1736 eruptions on Lanzarote and exhibits a sinuous, well-formed channel over a steep (11° slope) 100-m-long proximal section. Over-flow units comprising smooth pahoehoe sheet flow, as well as evidence on the inner channel walls for multiple (at least 11) flow levels, attest to unsteady flow in the channel. In addition, superelevation is apparent at each of the six bends along the proximal channel section. Superelevation results from banking of the lava as it moves around the bend thus causing preferential construction of the outer bank. As a result, the channel profile at each bend is asymmetric with an outer bank that is higher than the inner bank. Analysis of superelevation indicates flow velocities of ~8 m s^–1. Our analysis of the superelevation features is based on an inertia-gravity balance, which we show is appropriate, even though the down-channel flow is in laminar flow. We use a viscosity-gravity balance model, together with the velocities calculated from superelevation, to obtain viscosities in the range 25–60 Pa s (assuming that the lava behaved as a Newtonian liquid). Estimated volume fluxes are in the range 7–12 m³ s^–1. An apparent down-flow increase in derived volume flux may have resulted from variable supply or bulking up of the flow due to vesiculation. Where the channel moves over a sharp break in slope and onto slopes of ~6°, the channel becomes less well defined and widens considerably. At the break of slope, an elongate ridge extends across the channel. We speculate that this ridge was formed as a result of a reduction in velocity immediately below the break of slope to allow deposition of entrained material or accretion of lava to the channel bed as a result of a change in flow regime or depth.     

The 1997 eruption of Okmok Volcano, Alaska: a synthesis of remotely sensed imagery

Okmok Volcano, in the eastern Aleutian Islands, erupted in February and March of 1997 producing a 6-km-long lava flow and low-level ash plumes. This caldera is one of the most active in the Aleutian Arc, and is now the focus of international multidisciplinary studies. A synthesis of remotely sensed data (AirSAR, derived DEMs, Landsat MSS and ETM+ data, AVHRR, ERS, JERS, Radarsat) has given a sequence of events for the virtually unobserved 1997 eruption. Elevation data from the AirSAR sensor acquired in October 2000 over Okmok were used to create a 5-m resolution DEM mosaic of Okmok Volcano. AVHRR nighttime imagery has been analyzed between February 13 and April 11, 1997. Landsat imagery and SAR data recorded prior to and after the eruption allowed us to accurately determine the extent of the new flow. The flow was first observed on February 13 without precursory thermal anomalies. At this time, the flow was a large single lobe flowing north. According to AVHRR Band 3 and 4 radiance data and ground observations, the first lobe continued growing until mid to late March, while a second, smaller lobe began to form sometime between March 11 and 12. This is based on a jump in the thermal and volumetric flux determined from the imagery, and the physical size of the thermal anomalies. Total radiance values waned after March 26, indicating lava effusion had ended and a cooling crust was growing. The total area (8.9 km²), thickness (up to 50 m) and volume (1.54×10⁸ m³) of the new lava flow were determined by combining observations from SAR, Landsat ETM+, and AirSAR DEM data. While the first lobe of the flow ponded in a pre-eruption depression, our data suggest the second lobe was volume-limited. Remote sensing has become an integral part of the Alaska Volcano Observatory’s monitoring and hazard mitigation efforts. Studies like this allow access to remote volcanoes, and provide methods to monitor potentially dangerous ones.     

Cooling and crystallization of lava in open channels, and the transition of Pāhoehoe Lava to 'A'ā

 Samples collected from a lava channel active at Kīlauea Volcano during May 1997 are used to constrain rates of lava cooling and crystallization during early stages of flow. Lava erupted at near-liquidus temperatures (∼1150  °C) cooled and crystallized rapidly in upper parts of the channel. Glass geothermometry indicates cooling by 12–14  °C over the first 2 km of transport. At flow velocities of 1–2 m/s, this translates to cooling rates of 22–50  °C/h. Cooling rates this high can be explained by radiative cooling of a well-stirred flow, consistent with observations of non-steady flow in proximal regions of the channel. Crystallization of plagioclase and pyroxene microlites occurred in response to cooling, with crystallization rates of 20–50% per hour. Crystallization proceeded primarily by nucleation of new crystals, and nucleation rates of ∼10⁴/cm³s are similar to those measured in the 1984 open channel flow from Mauna Loa Volcano. There is no evidence for the large nucleation delays commonly assumed for plagioclase crystallization in basaltic melts, possibly a reflection of enhanced nucleation due to stirring of the flow. The transition of the flow surface morphology from pāhoehoe to 'a'ā occurred at a distance of 1.9 km from the vent. At this point, the flow was thermally stratified, with an interior temperature of ∼1137  °C and crystallinity of ∼15%, and a flow surface temperature of ∼1100  °C and crystallinity of ∼45%. 'A'ā formation initiated along channel margins, where crust was continuously disrupted, and involved tearing and clotting of the flow surface. Both observations suggest that the transition involved crossing of a rheological threshold. We suggest this threshold to be the development of a lava yield strength sufficient to prevent viscous flow of lava at the channel margin. We use this concept to propose that 'a'ā formation in open channels requires both sufficiently high strain rates for continued disruption of surface crusts and sufficient groundmass crystallinity to generate a yield strength equivalent to the imposed stress. In Hawai'i, where lava is typically microlite poor on eruption, these combined requirements help to explain two common observations on 'a'ā formation: (a) 'a'ā flow fields are generated when effusion rates are high (thus promoting crustal disruption); and (b) under most eruption conditions, lava issues from the vent as pāhoehoe and changes to 'a'ā only after flowing some distance, thus permitting sufficient crystallization. Received: 3 September 1998 / Accepted: 12 April 1999     

Volumetric characteristics of lava flows from interferometric radar and multispectral satellite data: the 1995 Fernandina and 1998 Cerro Azul eruptions in the western Galápagos

We have used a suite of remotely sensed data, numerical lava flow modeling, and field observations to determine quantitative characteristics of the 1995 Fernandina and 1998 Cerro Azul eruptions in the western Galápagos Islands. Flank lava flow areas, volumes, instantaneous effusion rates, and average effusion rates were all determined for these two eruptions, for which only limited syn-eruptive field observations are available. Using data from SPOT, TOPSAR, ERS-1, and ERS-2, we determined that the 1995 Fernandina flow covers a subaerial area of 6.5×10⁶ m² and has a subaerial dense rock equivalent (DRE) volume of 42×10⁶ m³. Field observations, ATSR satellite data, and the FLOWGO numerical model allow us to determine that the effusion rate declined exponentially from a high of ~60–200 m³ s^-1 during the first few hours to <5 m³ s^-1 prior to ceasing after 73 days, with a mean effusion rate of 4–16 m³ s^-1. Integrating the ATSR-derived, exponentially declining effusion rate over the eruption duration produces a total (subaerial + submarine) DRE volume of between 27 and 100×10⁶ m³, the range in values being due to differing assumptions about heat loss characteristics; only values in the higher part of this range are consistent with the independently derived subaerial volume. Using SPOT, TOPSAR, ERS-1, and ERS-2 data, we determine that the 1998 Cerro Azul flow is 16 km long, covers 16 km², and has a DRE volume of 54×10⁶ m³. FLOWGO produces at-vent velocity and effusion rate values of 11 m s^-1 and ~600 m³ s^-1, respectively. The velocity value agrees well with the 12 m s^-1 estimated in the field. The mean effusion rate (total DRE volume/duration) was 7–47 m³ s^-1. Dike dimensions, fissure lengths, and pressure gradients along the conduit based on magma chamber depth estimates of 3–5 km produce mean effusion rates for the two eruptions that range over nearly four orders of magnitude, the range being due to uncertainty in the magma viscosity, dike dimensions, and pressure gradient between magma chamber and vent. Although somewhat consistent with mean effusion rates from other techniques, their wide range makes them less useful. The exponentially declining effusion rates during both eruptions are consistent with release of elastic strain being the driving mechanism of the eruptions. Our results provide independent input parameters for previously published theoretical relationships between magma chamber pressurization and eruption rates that constrain chamber volumes and increases in volume prior to eruption, as well as time constants of exponential decay during the eruption. The results and theoretical relationships combine to indicate that at both volcanoes probably 25–30% of the volumetric increase in the magma chamber erupted as lava onto the surface. In both eruptions the lava flow volumes are less than 1% of the magma chamber volume.     

Ground-based thermal imaging of lava lakes at Erebus volcano,Antarctica

Mount Erebus, a large intraplate stratovolcano dominating Ross Island, Antarctica, hosts the world's only active phonolite lava lakes. The main manifestation of activity at Erebus volcano in December 2004 was as the presence of two convecting lava lakes within an inner crater. The long-lived Ray Lake, ~ 1400 m² in area, was the site of up to 10 small Strombolian eruptions per day. A new but short-lived, ~ 1000–1200 m² lake formed at Werner vent in December 2004 sourced by lava flowing from a crater formed in 1993 by a phreatic eruption. We measured the radiative heat flux from the two lakes in December 2004 using a compact infrared (IR) imaging camera. Daily thermal IR surveys from the Main Crater rim provide images of the lava lake surface temperatures and identify sites of upwelling and downwelling. The radiative heat outputs calculated for the Ray and Werner Lakes are 30–35 MW and 20 MW, respectively. We estimate that the magma flux needed to sustain the combined heat loss is ~ 250–710 kg s^− 1, that the minimum volume of the magma reservoir is 2 km³, and that the radius of the conduit feeding the Ray lake is ~ 2 m.     

Pyroclastic surges and flows from the 8–10 May 1997 explosive eruption of Bezymianny volcano, Kamchatka, Russia

The 8-10 May 1997 eruption of Bezymianny volcano began with extrusion of a crystallized plug from the vent in the upper part of the dome. Progressive gravitational collapses of the plug caused decompression of highly crystalline magma in the upper conduit, leading at 13:12 local time on 9 May to a powerful, vertical Vulcanian explosion. The dense pyroclastic mixture collapsed in boil-over style to generate a pyroclastic surge which was focused toward the southeast by the steep-walled, 1956 horseshoe-shaped crater. This surge, with a temperature <200 °C, covered an elliptical area >30 km² with deposits as much as 30 cm thick and extending 7 km from the vent. The surge deposits comprised massive to vaguely laminated, gravelly sand (Md -1.2 to 3.7J sorting 1.2 to 3J) of poorly vesiculated andesite (mean density 1.82 g cm^-3; vesicularity 30 vol%; SiO₂ content ~58.0 wt%). The deposits, with a volume of 5-15᎒⁶ m³, became finer grained and better sorted with distance; the maximal diameter of juvenile clasts decreased from 46 to 4 cm. The transport and deposition of the surge over a snowy landscape generated extensive lahars which traveled >30 km. Immediately following the surge, semi-vesiculated block-and-ash flows were emplaced as far as 4.7 km from the vent. Over time the juvenile lava in clasts of these flows became progressively less crystallized, apparently more silicic (59.0 to 59.9 wt% SiO₂) and more vesiculated (density 1.64 to 1.12 g cm^-3; vesicularity 37 to 57 vol%). At this stage the eruption showed transitional behavior, with mass divided between collapsing fountain and buoyant column. The youngest pumice-and-ash flows were accompanied by a sustained sub-Plinian eruption column ~14 km high, from which platy fallout clasts were deposited (~59.7% SiO₂; density 1.09 g cm^-3; vesicularity 58 vol%). The explosive activity lasted about 37 min and produced a total of ~0.026 km³ dense rock equivalent of magma, with an average discharge of ~1.2᎒⁴ m³ s^-1. A lava flow ~200 m long terminated the eruption. The evolutionary succession of different eruptive styles during the explosive eruption was caused by vertical gradients in crystallization and volatile content of the conduit magma, which produced significant changes in the properties of the erupting mixture.     

Lava discharge during Etna's January 2011 fire fountain tracked using MSG-SEVIRI

Etna's January 2011 eruption provided an excellent opportunity to test the ability of Meteosat Second Generation satellite's Spinning Enhanced Visible and InfraRed Imager (SEVIRI) sensor to track a short-lived effusive event. The presence of lava fountaining, the rapid expansion of lava flows, and the complexity of the resulting flow field make such events difficult to track from the ground. During the Etna's January 2011 eruption, we were able to use thermal data collected by SEVIRI every 15 min to generate a time series of the syn-eruptive heat flux. Lava discharge waxed over a ~1-h period to reach a peak that was first masked from the satellite view by a cold tephra plume and then was of sufficient intensity to saturate the 3.9-μm channel. Both problems made it impossible to estimate time-averaged lava discharge rates using the syn-eruptive heat flux curve. Therefore, through integration of data obtained by ground-based Doppler radar and thermal cameras, as well as ancillary satellite data (from Moderate Resolution Imaging Spectrometer and Advanced Very High Resolution Radiometer), we developed a method that allowed us to identify the point at which effusion stagnated, to allow definition of a lava cooling curve. This allowed retrieval of a lava volume of ~1.2 × 10⁶ m³, which, if emitted for 5 h, was erupted at a mean output rate of ~70 m³ s⁻¹. The lava volume estimated using the cooling curve method is found to be similar to the values inferred from field measurements.     

Slabby pahoehoe from the western Deccan Volcanic Province: evidence for incipient pahoehoe–aa transitions

The pahoehoe–aa transition for a flow exposed near Bodshil village from the western part of the Deccan Volcanic Province (DVP) is reported for the first time. The 1-km-long Bodshil flow issued as a small sheet from a pre-existing lobe. Near the source, the crust is characterised by numerous squeeze-ups. A number of gaping fractures, parallel to sub-parallel to the flow direction, are exposed on the surface in the medial portion of the flow. About 800 m away, the flow completely transforms to slabby pahoehoe. The terminal portion of the flow is characterised by concentrations of slabs, blocks and lava balls. The size and concentrations of the slabs and lava balls appear to increase along the length of the flow. Petrographic studies reveal a dominant hypohyaline texture. The flow core is coarse and is characterised by plagioclase set in a glassy matrix. The presence of clinopyroxene in addition to plagioclase and glass distinguishes the crust and interslab crust from the core. On the basis of mineralogy, a temperature range of 1146±15°C to 1169±15°C is inferred for the Bodshil flow. Increased vesicle deformation across the transition is discernible and an average D-value of <0.4 indicates moderate strain rates during emplacement. In light of the morphology and petrography, the cooling history and the mode of emplacement of the Bodshil flow is discussed. The flow originated as a small toe at the leading edge of a pahoehoe flow, and grew into a sheet by the mechanism of inflation. Continuous inflation caused the brittle crust to uplift and produce a network of inflation clefts that were subsequently occupied by squeeze-ups. Temporary stagnation of the flow due to cessation of lava supply or storage allowed the crust to grow and thicken. Renewed movement of the stored and cooled lava to the flow front at a fairly high volumetric rate was responsible for the initial disruption of the crust. High rates of crustal disruption induced higher rates of degassing and cooling, which resulted in rapid crystallisation of the fluid core. Increase in crystallinity lead to the onset of yield strength, and it is envisaged that at least the terminal parts of the flow behaved as a Bingham fluid. The Bodshil flow is unique to the DVP because it is the first to record slabby pahoehoe and provide evidence for the incipient transformation of basaltic lava from pahoehoe to aa.     

On the content and vertical distribution of K,Th and U in the continental crust

The content of K, Th and U in the continental crust is estimated based on the assumption that the concentration of these elements decreases with depth asA_x = A₀e^?x/D [11], withA_x andA₀ the heat production rates at depthx and at the surface, respectively. Taking the weighted mean heat production rate of the intrusive rocks of the upper crust asA₀ = 2.33 μWm^?3, that of the granulites representing the lower crust asA_x = 0.72 μWm^?3, and the mean scale heightD= 9.5km [1] the average vertical distancex = b between these intrusives and granulites is 11.2 km. Withb known and the average concentrations of K, Th and U in granulites and intrusive rocks of the upper crust the scale heights of the vertical distribution of these elements areD_K = 71km,D_Th = 9.5km,D_U = 5.8km. The knowledge of these parameters permits to calculate the average concentrations of these elements in a 33.3 km thick crust:K= 2.19%,Th= 4.43ppm,U= 0.66ppm; Th/U = 6.7 and K/U = 3.3 × 10⁴. The resulting heat flow is 23.0 mW m^?2 which is practically identical with the value deduced from heat flow measurements. Assuming that the Th/U ratio of the entire crust—including the sediments—is 3.9, the high ratio of 6.7 in the crystalline crust indicates that about 7.2 × 10¹² t U were extracted from it. All rocks with Th/U ratios <3.9 are possible sinks of this U. About half that amount is deposited in sedimentary rocks, mainly in black shales. The second important sink are the volcanic rocks of the continental margins.     

A major lava tube system from Undara Volcano,North Queensland

The Undara Volcano erupted 0.19 m.y. ago and formed lava fields covering 1,500 km² with a volume of approximately 23 km³. One of the flows extended 160 km on a gradient that averaged only 0.3°. This great length was a result of very high effusion rates, favourable topography and lava tube efficiency. The Undara lavas are rather uniform hawaiites. Lava temperatures are estimated to have been somewhat less than 1175–1220°C and viscosities greater than 10 to 30 Pa s. Long, apparently single lava tubes are well preserved in many places. They are marked by depressions, caves and long level ridges. A system of lava tubes extends for perhaps more than 100 km. The features of the lava tubes are comparable with those described elsewhere. Aligned depressions associated with caves appear to have formed contemporaneously. Most are much wider than the caves and probably represent collapsed lava ponds. The lava tubes appear to have formed by roofing over of lava channels. Close to lava tubes, the rocks developed strongly oxidised characteristics, such as oxidised olivine phenocrysts, ferric clinopyroxene and extensively developed hematite. Differentiated lava forms drips in some caves and is also oxidised.     

Growth of a young, frequently active composite cone: Ngauruhoe volcano, New Zealand

Ngauruhoe cone, in southern Taupo Volcanic Zone, New Zealand, has grown rapidly over the last 2,500 years in an alternation of effusive, strombolian, vulcanian, and sub-plinian eruptions of andesitic magma. At times growth has been 'staccato' in fashion as evidenced in the historical record. Each historical eruption typically lasted days to months, alternating with repose periods of years to decades. Major historic eruptions occurred in 1870 1949 1954-1955 and 1973-1975, encompassing wide variations in eruptive style over short timescales. The early period of cone building appears to have been dominated by a more continuous form of activity characterised by a series of numerous frequent explosive eruptions, with associated lava flows. The 2.2-km³ cone has grown in a piecemeal sectorial manner reflecting constant modification to the morphology of the summit, which has funnelled eruption products to specific sectors of the cone. Eruption rates can be calculated on several different timescales. Discharge rates averaged over individual eruptive pulses vary by two orders of magnitude (2.7-280 m³ s^-1), reflecting variations in high level magma ascent rates and processes such as degassing, which are, in turn, reflected in contrasting eruptive styles. Lower rates (e.g. 0.65 m³ s^-1) are obtained by averaging the discharge over an entire eruption lasting several months and may correspond to the ascent rate of magma batch(es) feeding the eruption. The long-term growth rate of Ngauruhoe is 0.9 km³ ky^-1. This is an average rate reflecting the long-term deep supply rate of magma to crustal reservoirs. By looking at eruption rates on these different timescales we are better able to constrain processes occurring at various depths within the plumbing system. There are few detailed studies of the growth patterns of young volcanic cones, but such data are essential in understanding the dynamics of andesitic systems. More than 60 lavas and pyroclastic units mapped on different sectors of Ngauruhoe cone have been correlated by flow chronology and their distinctive compositions into five groups. Although the cone has grown rapidly, Ngauruhoe shows little evidence for the existence of large crustal magma reservoirs and long-lived magma batches. Instead, abrupt and non-systematic changes in magma chemistry and isotopic composition between and within the five groups indicate that the volcano has an open-system, multi-process, multi-directional character and erupts small (<0.1 km³) and short-lived (10⁰-10³ years) magma batches with no simple time-composition relationships between successive batches.     

Eruption and emplacement of a basaltic welded ignimbrite during caldera formation on Gran Canaria

The 14.1 Ma old composite ignimbrite cooling unit P1 (45 km³) on Gran Canaria comprises a lower mixed rhyolite-trachyte tuff, a central rhyolite-basalt mixed tuff, and a slightly rhyolite-contaminated basaltic tuff at the top. The basaltic tuff is compositionally zoned with (a) an upward change in basalt composition to higher MgO content (4.3–5.2 wt.%), (b) variably admixed rhyolite or trachyte (commonly <5 wt.%), and (c) an upward increasing abundance of basaltic and plutonic lithic fragments and cognate cumulate fragments. The basaltic tuff is divided into three structural units: (I) the welded basaltic ignimbrite, which forms the thickest part (c. 95 vol.%) and is the main subject of the present paper; (II) poorly consolidated massive, bomb- and block-rich beds interpreted as phreatomagmatic pyroclastic flow deposits; and (III) various facies of reworked basaltic tuff. Tuff unit I is a basaltic ignimbrite rather than a lava flow because of the absence of top and bottom breccias, radial sheet-like distribution around the central Tejeda caldera, thickening in valleys but also covering higher ground, and local erosion of the underlying P1 ash. A gradual transition from dense rock in the interior to ash at the top of the basaltic ignimbrite reflects a decrease in welding; the shape of the welding profile is typical for emplacement temperatures well above the minimum welding temperature. A similar transition occurs at the base where the ignimbrite was emplaced on cold ground in distal sections. In proximal sections the base is dense where it was emplaced on hot felsic P1 tuff. The intensity of welding, especially at the base, and the presence of spherical particles and of mantled and composite particles formed by accretion and coalescence in a viscous state imply that the flow was a suspension of hot magma droplets. The flow most likely had to be density stratified and highly turbulent to prevent massive coalescence and collapse. Model calculations suggest eruption through low pyroclastic fountains (<1000 m high) with limited cooling during eruption and turbulent flow from an initial temperature of 1160°C. The large volume of 26 km³ of erupted basalt compared with only 16 km³ of the evolved P1 magmas, and the extremely high discharge rates inferred from model calculations are unusual for a basaltic eruption. It is suggested that the basaltic magma was erupted and emplaced in a fashion commonly only attributed to felsic magmas because it utilized the felsic P1 magma chamber and its ring-fissure conduits. Evolution of the entire P1 eruption was controlled by withdrawal dynamics involving magmas differing in viscosity by more than four orders of magnitude. The basaltic eruption phase was initially driven by buoyancy of the basaltic magma at chamber depth and continued degassing of felsic magma, but most of the large volume of basalt magma was driven out of the reservoir by subsidence of a c. 10 km diameter roof block, which followed a decrease in magma chamber pressure during low viscosity basaltic outflow.     

Pahoehoe and aa in Hawaii: volumetric flow rate controls the lava structure

The historical records of Kilauea and Mauna Loa volcanoes reveal that the rough-surfaced variety of basalt lava called aa forms when lava flows at a high volumetric rate (>5–10 m³/s), and the smooth-surfaced variety called pahoehoe forms at a low volumetric rate (<5–10 m³/s). This relationship is well illustrated by the 1983–1990 and 1969–1974 eruptions of Kilauea and the recent eruptions of Mauna Loa. It is also illustrated by the eruptions that produced the remarkable paired flows of Mauna Loa, in which aa formed during an initial short period of high discharge rate (associated with high fountaining) and was followed by the eruption of pahoehoe over a sustained period at a low discharge rate (with little or no fountaining). The finest examples of paired lava flows are those of 1859 and 1880–1881. We attribute aa formation to rapid and concentrated flow in open channels. There, rapid heat loss causes an increase in viscosity to a threshold value (that varies depending on the actual flow velocity) at which, when surface crust is torn by differential flow, the underlying lava is unable to move sufficiently fast to heal the tear. We attribute pahoehoe formation to the flowage of lava at a low volumetric rate, commonly in tubes that minimize heat loss. Flow units of pahoehoe are small (usually <1 m thick), move slowly, develop a chilled skin, and become virtually static before the viscosity has risen, to the threshold value. We infer that the high-discharge-rate eruptions that generate aa flows result from the rapid emptying of major or subsidiary magma chambers. Rapid near-surface vesiculation of gas-rich magma leads to eruptions with high discharge rates, high lava fountains, and fast-moving channelized flows. We also infer that long periods of sustained flow at a low discharge rate, which favor pahoehoe, result from the development of a free and unimpeded pathway from the deep plumbing system of the volcano and the separation of gases from the magma before eruption. Achievement of this condition requires one or more episodes of rapid magma excursion through the rift zone to establish a stable magma pathway.