The 1984 to 1996 cyclic activity of Lascar Volcano, northern Chile: cycles of dome growth, dome subsidence, degassing and explosive eruptions

 Lascar Volcano (5592 m; 23°22'S, 67°44'W) entered a new period of vigorous activity in 1984, culminating in a major explosive eruption in April 1993. Activity since 1984 has been characterised by cyclic behaviour with recognition of four cycles up to the end of 1993. In each cycle a lava dome is extruded in the active crater, accompanied by vigorous degassing through high-temperature, high-velocity fumaroles distributed on and around the dome. The fumaroles are the source of a sustained steam plume above the volcano. The dome then subsides back into the conduit. During the subsidence phase the velocity and gas output of the fumaroles decrease, and the cycle is completed by violent explosive activity. Subsidence of both the dome and the crater floor is accommodated by movement on concentric, cylindrical or inward-dipping conical fractures. The observations are consistent with a model in which gas loss from the dome is progressively inhibited during a cycle and gas pressure increases within and below the lava dome, triggering a large explosive eruption. Factors that can lead to a decrease in gas loss include a decrease in magma permeability by foam collapse, reduction in permeability due to precipitation of hydrothermal minerals in the pores and fractures within the dome and in country rock surrounding the conduit, and closure of open fractures during subsidence of the dome and crater floor. Dome subsidence may be a consequence of reduction in magma porosity (foam collapse) as degassing occurs and pressurisation develops as the permeability of the dome and conduit system decreases. Superimposed upon this activity are small explosive events of shallow origin. These we interpret as subsidence events on the concentric fractures leading to short-term pressure increases just below the crater floor. Received: 12 December 1996 / Accepted: 6 May 1997     

Io: Volcanic thermal sources and global heat flow

We have examined thermal emission from 240 active or recently-active volcanic features on Io and quantified the magnitude and distribution of their volcanic heat flow during the Galileo epoch. We use spacecraft data and a geological map of Io to derive an estimate of the maximum possible contribution from small dark areas not detected as thermally active but which nevertheless appear to be sites of recent volcanic activity. We utilize a trend analysis to extrapolate from the smallest detectable volcanic heat sources to these smallest mapped dark areas. Including the additional heat from estimates for “outburst” eruptions and for a multitude of very small (“myriad”) hot spots, we account for ～62 × 10¹² W (～59 ± 7% of Io’s total thermal emission). Loki Patera contributes, on average, 9.6 × 10¹² W (～9.1 ± 1%). All dark paterae contribute 45.3 × 10¹² W (～43 ± 5%). Although dark flow fields cover a much larger area than dark paterae, they contribute only 5.6 × 10¹² W (～5.3 ± 0.6%). Bright paterae contribute ～2.6 × 10¹² W (～2.5 ± 0.3%). Outburst eruption phases and very small hot spots contribute no more than ～4% of Io’s total thermal emission: this is probably a maximum value. About 50% of Io’s volcanic heat flow emanates from only 1.2% of Io’s surface. Of Io’s heat flow, 41 ± 7.0% remains unaccounted for in terms of identified sources. Globally, volcanic heat flow is not uniformly distributed. Power output per unit surface area is slightly biased towards mid-latitudes, although there is a stronger bias toward the northern hemisphere when Loki Patera is included. There is a slight favoring of the northern hemisphere for outbursts where locations were well constrained. Globally, we find peaks in thermal emission at ～315°W and ～105°W (using 30° bins). There is a minimum in thermal emission at around 200°W (almost at the anti-jovian longitude) which is a significant regional difference. These peaks and troughs suggest a shift to the east from predicted global heat flow patterns resulting from tidal heating in an asthenosphere. Global volcanic heat flow is dominated by thermal emission from paterae, especially from Loki Patera (312°W, 12°N). Thermal emission from dark flows maximises between 165°W and 225°W. Finally, it is possible that a multitude of very small hot spots, smaller than the present angular resolution detection limits, and/or cooler, secondary volcanic processes involving sulphurous compounds, may be responsible for at least part of the heat flow that is not associated with known sources. Such activity should be sought out during the next mission to Io.     

Penetration modeling of ultra-high performance concrete using multiscale meshfree methods

Terminal ballistics of concrete is of extreme importance to the military and civil communities. Over the past few decades, ultra-high performance concrete (UHPC) has been developed for various applications in the design of protective structures because UHPC has an enhanced ballistic resistance over conventional strength concrete. Developing predictive numerical models of UHPC subjected to penetration is critical in understanding the material's enhanced performance. This study employs the advanced fundamental concrete (AFC) model, and it will run inside the reproducing kernel particle method (RKPM)-based code known as the nonlinear meshfree analysis program (NMAP). NMAP is advantageous for modeling impact and penetration problems that exhibit extreme deformation and material fragmentation. A comprehensive experimental study was conducted to characterize the UHPC. The investigation consisted of fracture toughness testing, the utilization of nondestructive microcomputed tomography analysis, and lastly projectile penetration shots on the UHPC targets. To improve the accuracy of the model, a new scaled damage evolution law (SDEL) is employed within the microcrack informed damage model. During the homogenized macroscopic calculation, the corresponding microscopic cell needs to be dimensionally equivalent to the mesh dimension when the partial differential equation becomes ill posed and strain softening ensues. To ensure arbitrary mesh geometry for which the homogenized stress-strain curves are derived, a size scaling law is incorporated into the homogenized tensile damage evolution law. This ensures energy-bridging equivalence of the microscopic cell to the homogenized medium irrespective of arbitrary mesh geometry. Results of numerical investigations will be compared with results of penetration experiments.     

The exceptional magnitude and intensity of the Toba eruption,sumatra: An example of the use of deep-sea tephra layers as a geological tool

The eruption of Toba (75,000 years BP), Sumatra, is the largest magnitude eruption documented from the Quaternary. The eruption produced the largest-known caldera the dimensions of which are 100 × 30 km and which is surrounded by rhyolitic ignimbrite covering an area of over 20,000 km². The associated deep-sea tephra layer is found in piston cores in the north-eastern Indian Ocean covering a minimum area of 5 × 10⁶ km². We have investigated the thickness, grain size and texture of the Toba deep-sea tephra layer in order to demonstrate the use of deep-sea tephra layers as a volcanological tool. The exceptional magnitude and intensity of the Toba eruption is demonstrated by comparison of these data with the deep-sea tephra layers associated with the eruptions of the Campanian ignimbrite, Italy and of Santorini, Greece in Minoan time. The volume of ignimbrite and distal tephra fall deposit produced in the Toba eruption are comparable, a total of at least 1000 km³ of dense rhyolitic magma. In contrast the volume of dense magma produced by the Campanian and Santorini eruptions are approximately 70 and 13 km³ respectively. Thickness versus distance data on the three deep-sea tephra layers show that eruptions of smaller magnitude than Santorini are unlikely to be preserved as distinct tephra layers in most deep-sea cores. In proximal cores all three tephra layers show two distinct units: a lower coarse-grained unit and an upper fine-grained unit. We interpret the lower unit as a plinian deposit and the upper unit as a co-ignimbrite ash-fall deposit, indicating two major eruptive phases. The Toba tephra layer is coarser both in maximum and median grain size than the Campanian and Santorini layers at a given distance from source. These data are interpreted to indicate a very high cruption column, estimated to be at least 45 km. We have applied a method for estimating the duration of the Toba eruption from the style of graded-bedding in deep-sea tephra layers. Studies of two cores yield estimates of 9 and 14 days. The eruption column height and duration estimates both indicate an average volume discharge rate of approximately 10⁶ m³/sec. The Toba eruption therefore was not only of exceptional magnitude, but also of exceptional intensity.     

The significance of vitric-enriched air-fall ashes associated with crystal-enriched ignimbrites

A distinctive type of fine-grained air-fall ash is found intimately associated with many ignimbrites. They have crystal/glass ratios systematically lower than artificially crushed pumice from the same ignimbrites. The crystal enrichment found in crystal-bearing ignimbrites indicates substantial losses of the vitric component, amounting to an average of at least 35% by weight of the original juvenile material, and this lost material is believed to occur in the ash-fall deposits. These ashes thus complement ignimbrite, and are here called “co-ignimbrite ashes”. The loss is believed to take place during ignimbrite eruptions as a result of: (1) the escape of fine ash and gas above a collapsing eruptive column; (2) the preferential entry of fine vitric ash into an upper turbulent cloud when (immediately following column collapse) the segregation of a dense pyroclastic flow from an initially highly turbulent, low-concentation density flow takes place; (3) the elutriation of fine vitric ash (generated in part within the pyroclastic flow) from the fluidised flow. Ash from all three mechanisms would be expected to rise to a great height in convective plumes and be dispersed by winds to produce extensive, vitric-enriched ash-fall deposits.The data indicate that the co-ignimbrite ashes must have volumes comparable with those of ignimbrites, and examples are given of particularly large ash-fall deposits (including some found in deep-sea cores) associated with large ignimbrites which may be of this type rather than fall-out from a preceding plinian phase as hitherto thought.     

The transport of xenoliths in magmas

Field measurements of the rheology of Hawaiian and Etnean lavas have shown that, on eruption, they behave as Bingham liquids with yield strengths in the range 70–450 N/m². Ultramafic nodules entrained in a Bingham liquid cannot settle unless the stress they impose on the liquid exceeds approximately 5–7 times its yield strength. Consequently magmas with yield strengths of 10–1000 N/m² can transport xenoliths up to 30 cm diameter without settling occurring. The size of nodules commonly observed rarely exceeds 30 cm. A review of experimental data shows that, when conditions are appropriate for settling, the terminal velocities of nodules in magmas are substantially slower if a Bingham rather than Newtonian Model is assumed.The view that large nodules imply fast rates of magma ascent is rejected. A case is presented for slow rates of ascent being more suitable for nodule transport as there is more opportunity for cooling, crystallisation and hence development of a yield strength. The relative abundance of nodules in the alkaline suite may be a consequence of their slow rates of ascent, whereas their absence in tholeiitic melts may be a consequence of rapid ascent rates. This interpretation is compatible with deductions on their relative rates of ascent based on other geological evidence.     

The fluid dynamics of a basaltic magma chamber replenished by influx of hot,dense ultrabasic magma

This paper describes a fluid dynamical investigation of the influx of hot, dense ultrabasic magma into a reservoir containing lighter, fractionated basaltic magma. This situation is compared with that which develops when hot salty water is introduced under cold fresh water. Theoretical and empirical models for salt/water systems are adapted to develop a model for magmatic systems. A feature of the model is that the ultrabasic melt does not immediately mix with the basalt, but spreads out over the floor of the chamber, forming an independent layer. A non-turbulent interface forms between this layer and the overlying magma layer across which heat and mass are transferred by the process of molecular diffusion. Both layers convect vigorously as heat is transferred to the upper layer at a rate which greatly exceeds the heat lost to the surrounding country rock. The convection continues until the two layers have almost the same temperature. The compositions of the layers remain distinct due to the low diffusivity of mass compared to heat. The temperatures of the layers as functions of time and their cooling rate depend on their viscosities, their thermal properties, the density difference between the layers and their thicknesses. For a layer of ultrabasic melt (18% MgO) a few tens of metres thick at the base of a basaltic (10% MgO) magma chamber a few kilometres thick, the temperature of the layers will become nearly identical over a period of between a few months and a few years. During this time the turbulent convective velocities in the ultrabasic layer are far larger than the settling velocity of olivines which crystallise within the layer during cooling. Olivines only settle after the two layers have nearly reached thermal equilibrium. At this stage residual basaltic melt segregates as the olivines sediment in the lower layer. Depending on its density, the released basalt can either mix convectively with the overlying basalt layer, or can continue as a separate layer. The model provides an explanation for large-scale cyclic layering in basic and ultrabasic intrusions. The model also suggests reasons for the restriction of erupted basaltic liquids to compositions with MgO<10% and the formation of some quench textures in layered igneous rocks.     

Discharge of pyroclastic flows into the sea during the 1996–1998 eruptions of the Soufrière Hills volcano, Montserrat

Explosive eruptions of the Soufrière Hills volcano on the island of Montserrat in the West Indies generated pyroclastic flows that reached the sea on the east and southwest coasts between November 1995 and July 1998. Discharge of the flows produced two pyroclastic deltas off of the Tar and White River valleys. A marine geological survey was conducted in July 1998 to study the submarine extensions of both deltas. Detailed profiles of depth and sub-bottom structure were obtained using a CHIRP II/bubble pulser system. These profiles were compared with pre-eruption bathymetric data in order to identify areas of recent deposition and erosion. Deposition off the Tar and White River valleys was thickest nearest the coastline and deltas, and extended into deeper water up to 5 km from shore. The total volume of submarine pyroclastic deposits as of July 1998 was 73×10⁶ m³ DRE. Submarine pyroclastic deposits off the Tar River valley made up more than two-thirds of the total volume (55×10⁶ m³ DRE) and covered an area of approximately 5.0 km², which included the delta. The volume of submarine pyroclastic deposits in the White River area (18×10⁶ m³ DRE) is probably underestimated due to the lack of precise pre-eruption bathymetric data in areas greater than 2 km from shore. Growth of pyroclastic deltas at the mouths of the Tar and White River valleys continued to the edge of the submarine shelf where there was a steep break in slope. In the Tar River area pyroclastic material was distributed down the steep shelf break and into deeper water at least a few kilometers from shore. The material spread out radially, forming a submarine fan, where distribution was primarily controlled by bathymetry and slope.Editorial responsibility; J. Stix