The ∼2 ka subplinian eruption of Montaña Blanca,Tenerife

The latest cycle of volcanism on Tenerife has involved the construction of two stratovolcanoes, Teide and Pico Viejo (PV), and numerous flank vent systems on the floor of the Las Cañadas Caldera, which has been partially infilled by magmatic products of the basanite-phonolite series. The only known substantial post-caldera explosive eruption occurred 2 ka bp from satellite vents at Montaña Blanca (MB), to the east of Teide and at PV. The MB eruption began with extrusion of 0.022 km³ of phonolite lava (unit I) from a WNW-ESE fissure system. The eruption then entered an explosive subplinian phase. Over a 7–11 hour period, 0.25 km³ (DRE) of phonolitic pumice (unit II) was deposited from a 15 km high subplinian column, dispersed to the NE by 10 m/s winds. Pyroclastic activity also occurred from vents near PV to the west of Teide. Fire-fountaining towards the end of the explosive phase formed a proximal welded spatter facies. The eruption closed with extrusion of small volume domes and lavas (0.025 km³) at both vent systems. Geochemical, petrological data and Fe-Ti oxide geothermometry indicate the eruption of a chemically and thermally stratified magma system. The most mafic and hottest (875°C) unit I magma can yield the more evolved and cooler (755–825°C) phonolites of units II and III by between 7 and 11% fractional crystallization of an assemblage dominated by alkali feldspar. Analyses of glass inclusions from phenocrysts by ion microprobe show that the pumice was derived from the water-saturated roof zone of a chamber containing 3.0–4.5 wt.% H₂O and abundant halogens (F0.35wt.%). Hotter, more mafic tephritic magma intermingled with the evolved phonolites in banded pumice, indicating the injection of mafic magma into the system during or just before eruption. Reconstruction ot the event indicates a small chamber chemically stratified by in situ (side-wall) crystallization at a depth of 3–4 km below PV. Although phonolite is the dominant product of the youngest activity of the Teide-PV system, there has been no eruption of phonolitic magma for at least 500 years from teide itself and for 2000 years from the PV system. Therefore there could be a large volume of highly evolved, volatile-rich magma accumulating in these magma systems. An eruption of fluorine-rich magma comparable with MB would have major damaging effects on the island.     

Carbon Dioxide Discharged through the Las Cañadas Aquifer, Tenerife, Canary Islands

Carbon dioxide is one of the first gases to escape the magmatic environment due to its low solubility in basaltic magmas at low pressures. The exsolved CO₂ gas migrates towards the surface through rock fractures and high permeability paths. If an aquifer is located between the magmatic environment and the surface, a fraction of the CO₂ emitted is dissolved in the aquifer. In this paper, an estimation of the water mass balance and the CO₂ budget in Las Cañadas aquifer, Tenerife, Canary Islands, is presented. Magmatic CO₂ is transported by groundwater and discharged through man-made sub-horizontal drains or galleries that exist in this island, and by the flow of groundwater discharged laterally towards other aquifers or to the ocean. In addition, the pCO₂ at the gallery mouth (or entrance) and at the gallery bottom (internal and deepest discharge point where the gallery starts) are calculated and mapped. The total CO₂ advectively transported by groundwater is estimated to range from 143 to 211 t CO₂ d^?1. Considering that the diffuse soil emission of CO₂ for the same area is 437 t d^?1, the diffuse/dissolved CO₂ flux ratio varies between 2 and 3. The high dissolved inorganic carbon content of groundwater explains the ability of this low temperature hydrothermal water to dissolve and transfer magmatic CO₂ at volcanoes, even during quiescence periods.     

Event-stratigraphy of a caldera-forming ignimbrite eruption on Tenerife: the 273 ka Poris Formation

The 273 ka Poris Formation in the Bandas del Sur Group records a complex, compositionally zoned explosive eruption at Las Cañadas caldera on Tenerife, Canary Islands. The eruption produced widespread pyroclastic density currents that devastated much of the SE of Tenerife, and deposited one of the most extensive ignimbrite sheets on the island. The sheet reaches ~ 40-m thick, and includes Plinian pumice fall layers, massive and diffuse-stratified pumiceous ignimbrite, widespread lithic breccias, and co-ignimbrite ashfall deposits. Several facies are fines-rich, and contain ash pellets and accretionary lapilli. Eight brief eruptive phases are represented within its lithostratigraphy. Phase 1 comprised a fluctuating Plinian eruption, in which column height increased and then stabilized with time and dispersed tephra over much of the southeastern part of the island. Phase 2 emplaced three geographically restricted ignimbrite flow-units and associated extensive thin co-ignimbrite ashfall layers, which contain abundant accretionary lapilli from moist co-ignimbrite ash plumes. A brief Plinian phase (Phase 3), again dispersing pumice lapilli over southeastern Tenerife, marked the onset of a large sustained pyroclastic density current (Phase 4), which then waxed (Phase 5), covering increasingly larger areas of the island, as vents widened and/or migrated along opening caldera faults. The climax of the Poris eruption (Phase 6) was marked by widespread emplacement of coarse lithic breccias, thought to record caldera subsidence. This is inferred to have disturbed the magma chamber, causing mingling and eruption of tephriphonolite magma, and it changed the proximal topography diverting the pyroclastic density current(s) down the Güimar valley (Phase 7). Phase 8 involved post-eruption erosion and sedimentary reworking, accompanied by minor down-slope sliding of ignimbrite. This was followed by slope stabilization and pedogenesis. The fines-rich lithofacies with abundant ash pellets and accretionary lapilli record agglomeration of ash in moist ash plumes. They resemble phreatomagmatic deposits, but a phreatomagmatic origin is difficult to establish because shards are of bubble-wall type, and the moisture may have arisen by condensation within ascending thermal co-ignimbrite ash plumes that contained atmospheric moisture enhanced by that derived from the evaporation of seawater where the hot pyroclastic currents crossed the coast. Ash pellets formed in co-ignimbrite ash-clouds and then fell through turbulent pyroclastic density currents where they accreted rims and evolved into accretionary lapilli.Editorial Responsibility: J. Stix     

Sobre un volcán de Verita recientemente descubierto en el Mioceno de Barqueros. — (Murcia)

While working to construct a geological map. A.Almela, an Engineer of Mines, discovered in Alhama de Murcia a previously unknown volcanic crater. This volcano erupted through the miocene layers and extended above them, thus establishing the date of this eruption as being post-miocene. Since there are no sedimentary rocks in this region of a more recent date it is impossible to fix the date of the eruption any more exactly. The crater is about 1Km. In the surface of the crater one finds slag, scoriaceous lava and compact rocks. The rock is composed of phlogopite, olivine and diopside in phenocrysts in a hyaloid cement with tiny grains of sanidine, olivine and diopside; and a thin layer of phlogopite in a glassy brownish black. A study was made of the chemical constituency of the rocks on the basis of three complete analyses and a comparison was made with the findings already made by Vera-Garrucha and those of Fortuna. All of the data as compiled in the five tables and four diagrams graphically represents the obtained results. The volcanic rocks have a close analogy to those ofOsann’s « Verita » type, which are shown not only by the analytical value of the magmatic parameters but also by the positions of the points shown in diagramsK-mg andLs-Fs-Ks of the three types already analized. In all the samples which have been studied from Barqueros the value of the K₂O is superior to that of Na₂O; it is also greater than that of Vera-Garrucha and less than in that of Fortuna.     

The 5,660?yBP Boquerón explosive eruption, Teide–Pico Viejo complex, Tenerife

Quantitative hazard assessments of active volcanoes require an accurate knowledge of the past eruptive activity in terms of eruption dynamics and the stratified products of eruption. Teide–Pico Viejo (TPV) is one of the largest volcanic complexes in Europe, but the associated eruptive history has only been constrained based on very general stratigraphic and geochronological data. In particular, recent studies have shown that explosive activity has been significantly more frequently common than previously thought. Our study contributes to characterization of explosive activity of TPV by describing for the first time the subplinian eruption of El Boquerón (5,660?yBP), a satellite dome located on the northern slope of the Pico Viejo stratovolcano. Stratigraphic data suggest complex shifting from effusive phases with lava flows to highly explosive phase that generated a relatively thick and widespread pumice fallout deposit. This explosive phase is classified as a subplinian eruption of VEI 3 that lasted for about 9–15?h and produced a plume with a height of up to 9?km above sea level (i.e. 7?km above the vent; MER of 6.9–8.2?×?10⁵?kg/s). The tephra deposit (minimum bulk volume of 4–6?×?10⁷?m³) was dispersed to the NE by up to 10?m/s winds. A similar eruption today would significantly impact the economy of Tenerife (e.g. tourism and aviation), with major consequences mainly for the communities around the Icod Valley, and to a minor extent, the Orotava Valley. This vulnerability shows that a better knowledge of the past explosive history of TPV and an accurate estimate of future potentials to generate violent eruptions is required in order to quantify and mitigate the associated volcanic risk.     

B.—U. N. E. S. C. O.

ABSTRACT

Sir Charles Cotton (1964) has pointed out that in an earlier paper (Carlston, 1963) which related drainage density to hydrology, there was insufficient emphasis on the role of climate in its effect on drainage density. Re-examination of the relation of drainage density to base flow in the 15 basins originally described has revealed additional evidence that base flow is affected by precipitation or recharge (a climatic variable), while varying inversely with drainage density.

Within the climatic region studied in the earlier paper (the Humid Subtropical Climate of the eastern U. S.), no evidence could be found that amount or intensity of rainfall affected the intensity of flood runoff or the scale of drainage density. In comparison with other climates, however, such as the Marine West Coast Climate, it is possible that the less intense precipitation of a marine climate may result in lower runoff intensities and lower drainage densities, however the lower mean temperatures of such climates may develop soils of generally higher infiltration capacity which would produce lower drainage densities.

A progressive increase in aridity results in a decrease in soil and vegetal cover which greatly magnifies the range of drainage densities characteristic of semi-arid regions. In such regions, where the land sur-face has a good infiltration capacity rainfall sinks readily into the dry soil (although recharge to ground water may be negligible), and runoff is virtually zero, as is drainage density, Impermeable terranes devoid of vegetal and soil cover reject the rain, runoff is briefly total and drainage density may be greatly magnified, as in the South Dakota Badlands, where drainage density runs into the hundreds. Arid or Desert Climates should produce erosional landforms with generally high drainage densities, though not reaching the magnitudes of drainage density found in the semi-arid badlands where rainfall intensities are much higher.     

A.—A. I. H. S.—I. A. S. H.

Abstract

When a river flows into a reservoir, the solid materials it carries sink to the bottom thus reducing the potential reservoir capacity. The suspended load of eight major western rivers flowing into Kainji Lake was determined as a part of the investigation of the suspended sediment of the reservoir. The sediment content in each sample was determined in parts per million.

On the whole the results of this study is in agreement with the previous conclusion that the sediment load of the Niger was low. A range of 29–221 ppm was observed within the reservoir whereas the concentration in the inflowing rivers varied from 122–953 ppm. It was however not possible to determine the total sediment load because discharge data were lacking.

The data presented herein confirm the earlier conclusion that the rate of silting is slow but indicate that locally high rate of sediment supply should give cause for anxiety regarding the possible growth of deltas as the months of these rivers.

The full potential of this study is difficult to assess at this stage but it has revealed a number of problem areas worth looking into in future years. A number of future studies believed to be capable of giving the necessary data for operation and management of the reservoir are recommended.     

Conduit-vent structures and related proximal deposits in the Las Cañadas caldera, Tenerife, Canary Islands

The Las Cañadas caldera wall and the outer slopes of the caldera provide three-dimensional exposures of numerous proximal-welded fallout deposits and have been mapped in detail. As a result, some parts of the Ucanca and Guajara Formations of the stratigraphy of Martí et al. (1994) have been divided into members that correspond to individual eruptions. Mapping has also revealed the occurrence of conduit-vent structures associated with proximal-welded fallout deposits. Conduit-vent structures consist of an upper flaring area and a lower narrow conduit. Conduit-vent geometry and dimensions include cylindrical plugs and eruptive fissures steeply dipping towards the caldera depression and elongated vents. The flaring area can be rather asymmetric and is usually filled by down-vent rheomorphic flow of the proximal fallout deposit. The lower conduits are filled by lava plug, agglutination of juveniles onto conduit walls and dyke intrusion with eventual dome extrusion. The eruption dynamics of welded fallout deposits and magma fragmentation within the conduit are consistent with an evolution from explosive to effusive. In this context conduit flow regimes evolve from turbulent to annular flow in which the conduit is progressively choked, and laminar flow leading to the final conduit closure.     

C) W.M.O.—E.C.A.F.E.

Abstract

The authors give data of water infiltrations in horizontal and vertical columns, and in a two dimensional model. The soils are two mixtures of sand and loam, and a natural sandy loam. The characteristics of these soils are given in the article.

As a conclusion, the two dimensional problem can be approached by a linear combination of data in horizontal and vertical columns.     

C.—W. M. O.—O. M. M.

Abstract

The intrusion of seawater in a tidal river is treated as a diffusion problem, characterized by a coefficient of longitudinal diffusivity.

In order to analyse the longitudinal diffusivity, a mathematical model is set up, consisting of two bodies of water, either one besides the other or one on top of the other. The two bodies are assumed to move relatively to each other, as a secondary effect of the tidal flow. It is moreover assumed that there is turbulent exchange of salt between the bodies.

It is demonstrated that the diffusion of salt into the river is greatest for an optimum value of the coefficient of exchange between the two bodies.

Exchange weaker or stronger than this optimum both diminish the salt intrusion.

The theory is applied to the Rotterdam Waterway, for which estimates of the exchange are made. Estimation of the reduction of the turbulence by stratification and hence of the vertical exchange, shows that the observed strong intrusion is explainable.

Intensified vertical mixing, for instance as provoked by compressed air, need not always result in less intrusion, and hence should be considered carefully.     

A) A.I.H.S.—I.A.S.H.

Abstract

Mathematical models are the means to characterize variables quantitatively in many groundwater problems. Recent advances in applied mathematics have perfected what is now called Adomian's decomposition method (ADM), a simple modelling procedure for practical applications. Decomposition exhibits the benefits of analytical solutions (i.e. stability, analytic derivation of heads, gradients, fluxes and simple programming). It also offers the advantages of traditional numerical methods (i.e. consideration of heterogeneity, irregular domain shapes and multiple dimensions). In addition, decomposition is one of the few systematic procedures for solving nonlinear equations. By far its greatest advantage is its simplicity of application. It may produce simple results for preliminary simulations, or in cases with scarce information. The method is described with simple applications to regional groundwater flow. Many applications in groundwater flow and contaminant transport are available in the literature.

Editor D. Koutsoyiannis; Associate editor Xi Chen

Citation Serrano, S.E., 2013. A simple approach to groundwater modelling with decomposition. Hydrological Sciences Journal, 58 (1), 1–9.     

Volcán Quizapu,Chilean Andes

Quizapu is a flank vent of the basalt-to-rhyodacite Holocene stratocone, Cerro Azul, and lies at the focus of a complex Quaternary volcanic field on the Andean volcanic front. The Quizapu vent originated in 1846 when 5 km³ of hornblende-dacite magma erupted effusively with little accompanying tephra. Between 1907 and 1932, phreatic and strombolian activity reamed out a deep crater, from which 4 km³ of dacite magma identical to that of 1846 fed the great plinian event of 10–11 April 1932. Although a total of >9 km³ of magma was thus released in 86 years, there is no discernible subsidence. As the pre-plinian crater was lined by massive lavas, 1932 enlargement was limited and the total plinian deposit contains only 0.4 wt % lithics. Areas of 5-cm and 1-cm isopachs for compacted 1932 fallout are about half of those estimated in the 1930's, yielding a revised ejecta volume of 9.5 km³. A strong inflection near the 10-cm isopach (downwind 110 km) on a plot of log Thickness vs Area^1/2 reflects slow settling of fine plinian ash — not of coignimbrite ash, as the volume of pyroclastic flows was trivial (<0.01 km³). About 17 vol.% of the fallout lies beyond the 1-cm isopach, and 82 wt% of the ejecta are finer than 1 mm. A least 18 hours of steady plinian activity produced an exceptionally uniform fall deposit. Observed column height (27–30 km) and average mass eruption rate (1.5x10⁸ kg/s) compare well with values for height and peak intensity calculated from published eruption models. The progressive aeolian fractionation of downwind ash (for which Quizapu is widely cited) is complicated by the large compositional range of 1932 juvenile pumice (52–70% SiO₂). The eruption began with andesitic scoria and ended with basaltic scoria, but >95% of the ejecta are dacitic pumice (67–68% SiO₂); minor andesitic scoria and frothier rhyodacite pumice (70% SiO₂) accompanied the dominant dacite. Phenocrysts (pl>hbopx>mt>ilmcpx) are similar in both abundance and composition in the 1846 (effusive) and 1932 (plinian) dacites. Despite the contrast in mode of eruption, bulk compositions are also indistinguishable. The only difference so far identified is a lower range of D values for 1846 hornblende, consistent with pre-eruptive degassing of the effusive batch.     

Assessing the potential for future explosive activity from Teide–Pico Viejo stratovolcanoes (Tenerife,Canary Islands)

Since the onset of their eruptive activity within the Cañadas caldera, about 180 ka ago, Teide–Pico Viejo stratovolcanoes have mainly produced lava flow eruptions of basaltic to phonoltic magmas. The products from these eruptions partially fill the caldera, and the adjacent Icod and La Orotava valleys, to the north. Although less frequent, explosive eruptions have also occurred at these composite volcanoes. In order to assess the possible evolution Teide–Pico Viejo stratovolcanoes and their potential for future explosive activity, we have analysed their recent volcanic history, assuming that similar episodes have the highest probability of occurrence in the near future. Explosive activity during the last 35000 years has been associated with the eruption of both, mafic (basalts, tephro–phonolites) and felsic (phono–tephrites and phonolites) magmas and has included strombolian, violent strombolian and sub-plinian magmatic eruptions, as well as phreatomagmatic eruptions of mafic magmas. Explosive eruptions have occurred both from central and flank vents, ranging in size from 0.001 to 0.1 km³ for the mafic eruptions and from 0.01 to < 1 km³ for the phonolitic ones. Comparison of the Teide–Pico Viejo stratovolcanoes with the previous cycles of activity from the central complex reveals that all them follow a similar pattern in the petrological evolution but that there is a significant difference in the eruptive behaviour of these different periods of central volcanism on Tenerife. Pre-Teide central activity is mostly characterised by large-volume (1–> 20 km³, DRE) eruptions of phonolitic magmas while Teide–Pico Viejo is dominated by effusive eruptions. These differences can be explained in terms of the different degree of evolution of Teide–Pico Viejo compared to the preceding cycles and, consequently, in the different pre-eruptive conditions of the corresponding phonolitic magmas. A clear interaction between the basaltic and phonolitic systems is observed from the products of phonolitic eruptions, indicating that basaltic magmatism is the driving force of the phonolitic eruptive activity. The magmatic evolution of Teide–Pico Viejo stratovolcanoes will continue in the future with a probably tendency to produce a major volume of phonolitic magmas, with an increasing explosive potential. Therefore, the explosive potential of Teide–Pico Viejo cannot be neglected and should be considered in hazard assessment on Tenerife.     

Eruptive scenarios of phonolitic volcanism at Teide–Pico Viejo volcanic complex (Tenerife,Canary Islands)

Recent studies on Teide–Pico Viejo (TPV) complex have revealed that explosive activity of phonolitic and basaltic magmas, including plinian and subplinian eruptions, and the generation of a wide range of pyroclastic density currents (PDCs) have also been significant. We perform a statistical analysis of the time series of past eruptions and the spatial extent of their erupted products, including lava flows, fallout and PDCs. We use an extreme value theory statistical method to calculate eruption recurrence. The analysis of past activity and extent of some well-identified deposits is used to calculate the eruption recurrence probabilities of various sizes and for different time periods. With this information, we compute several significant scenarios using the GIS-based VORIS 2 software (Felpeto et al., J Volcanol Geotherm Res 166:106–116, 2007) in order to evaluate the potential extent of the main eruption hazards that could be expected from TPV. The simulated hazard scenarios show that the southern flank of Tenerife is protected by Las Cañadas caldera wall against lava flows and pyroclastic density currents, but not against ash fallout. The Icod Valley, and to a minor extent also the La Orotava valley, is directly exposed to most of TPV hazards, in particular to the gravity driven flows. This study represents a step forward in the evaluation of volcanic hazard at TPV with regard to previous studies, and the results obtained should be useful for intermediate and long-term land-use and emergency planning.     

A. I. H. S. 1957–1960

Abstract

This study was made to define the relation between floods in a humid region and the causative factors that account for their variability. The study covered the basic relationships between peak discharges and hydrologic factors as well as the practical working methods for generalizing the results on a regional basis. Statistical multiple-correlation techniques were applied to hydrologic data in New England. A field and library investigation of historical flood data extended the period of flood knowledge to as much as 300 years. The relation of many topographic and climatic factors to flood peaks was tested. Flood peaks with recurrence intervals at 9 levels, from 1.2 to 300 years, were related to 6 independent variables, 3 of which are topographic, 2 climatic, and 1 orographic.