Causes and consequences of pressurisation in lava dome eruptions

High total and fluid pressures develop in the interior of high-viscosity lava domes and in the uppermost parts of the feeding conduit system as a consequence of degassing. Two effects are recognised and are modelled quantitatively. First, large increases in magma viscosity result from degassing during magma ascent. Strong vertical gradients in viscosity result and large excess pressures and pressure gradients develop at the top of the conduit and in the dome. Calculations of conduit flow show that almost all the excess pressure drop from the chamber in an andesitic dome eruption occurs during the last several hundred metres of ascent. Second, microlites grow in the melt phase as a consequence of undercooling caused by gas loss. Rapid microlite growth can cause large excess fluid pressures to develop at shallow levels. Theoretically closed-system microlite crystallization can increase local pressure by a few tens of MPa, although build up of pressure will be countered by gas loss through permeable flow and expansion by viscous flow. Microlite crystallization is most effective in causing excess gas pressures at depths of a few hundred metres in the uppermost parts of the conduit and dome interior. Some of the major phenomena of lava dome eruptions can be attributed to these pressurisation effects, including spurts of growth, cycles of dome growth and subsidence, sudden onset of violent explosive activity and disintegration of lava during formation of pyroclastic flows. The characteristic shallow-level, long-period and hybrid seismicity, characteristic of dome eruptions, is attributed to the excess fluid pressures, which are maintained close to the fracture strength of the dome and wallrock, resulting in fluid movement during formation of tensile and shear fractures within the dome and upper conduit.     

Erosion by pyroclastic flows on Lascar Volcano, Chile

 Pyroclastic flows generated in the 19–20 April 1993 eruption of Lascar Volcano, Chile, produced spectacular erosion features. Scree and talus were stripped from the channels and steep slopes on the flanks of the volcano. Exposed bedrock and boulders suffered severe abrasion, producing smoothed surfaces on coarse breccias and striations and percussion marks on bedrock and large boulders. Erosional furrows developed with wavelengths of 0.5–2 m and depths of 0.1–0.3 m. Furrows commonly nucleated downstream of large boulders or blocks, which are striated on the upstream side, and thereby produced crag-and-tail structures. Erosive features were produced where flows accelerated through topographic restrictions or where they moved over steep slopes. The pyroclastic flows are inferred to have segregated during movement into lithic-rich and pumice-rich parts. Lithic-rich deposits occur on slopes up to 14°, whereas pumice-rich deposits occur only on slopes less than 4°, and mainly at the margins and distal parts of the 1993 fan. The lithic-rich deposits contain large (up to 1 m) lithic clasts eroded from the substrate and transported from the vent, whereas pumice-rich deposits contain only small (typically <2 cm) lithic clasts. These observations suggest that lithic clasts segregated to the base of the flows and were responsible for much of the erosive phenomena. The erosive features, distribution of lithic clasts and deposit morphology indicate that the 1993 flows were highly concentrated avalanches dominated by particle interactions. In some places the flows slid over the bedrock causing abrasion and long striations which imply that large blocks were locked in fixed positions for periods of about 1 s. However, shorter striae at different angles, impact marks, segregation of the deposits into pumice- and lithic-rich parts, and mixing of bedrock-derived lithic clasts throughout the deposits indicate that clasts often had some freedom of movement and that jostling of particles allowed internal mixing and density segregation to occur within the flows. Received: 15 July 1996 / Accepted: 15 January 1997     

Spatial modelling of rice yield losses in Tanzania due to bacterial leaf blight and leaf blast in a changing climate

Rice is the most rapidly growing staple food in Africa and although rice production is steadily increasing, the consumption is still out-pacing the production. In Tanzania, two important diseases in rice production are leaf blast caused by Magnaporthe oryzae and bacterial leaf blight caused by Xanthomonas oryzae pv. oryzae. The objective of this study was to quantify rice yield losses due to these two important diseases under a changing climate. We found that bacterial leaf blight is predicted to increase causing greater losses than leaf blast in the future, with losses due to leaf blast declining. The results of this study indicate that the effects of climate change on plant disease can not only be expected to be uneven across diseases but also across geographies, as in some geographic areas losses increase but decrease in others for the same disease.     

Laboratory investigations of viscous effects in replenished magma chambers

Some laboratory experiments are described which investigate the dynamical effects of replenishment of a magma chamber containing high viscosity magma by hotter, denser and much more fluid magma. In the experiments a layer of hot KNO₃ solution is emplaced beneath cold glycerine, which has a viscosity 3000 times greater. Less dense fluid is released immediately and continuously from the interface as a result of crystallization in the lower layer and rises as plumes through the overlying glycerine. Further crystallization occurs in the plumes, and the crystals fall out; but there is little mixing between the two fluids and a layer of depleted KNO₃ solution forms at the top. The experiments demonstrate that interfacial processes begin to dominate where there are large viscosity differences between adjacent fluid layers as would be the case in a rhyolitic magma chamber replenished by basaltic magma.     

Ignimbrites of the Cerro Galan caldera, NW Argentina

The 35 × 20 km Cerro Galán resurgent caldera is the largest post-Miocene caldera so far identified in the Andes. The Cerro Galán complex developed on a late pre-Cambrian to late Palaeozoic basement of gneisses, amphibolites, mica schists and deformed phyllites and quartzites. The basement was uplifted in the early Miocene along large north-south reverse faults, producing a horst-and-graben topography. Volcanism began in the area prior to 15 Ma with the formation of several andesite to dacite composite volcanoes. The Cerro Galán complex developed along two prominent north-south regional faults about 20 km apart. Dacitic to rhyodacitic magma ascended along these faults and caused at least nine ignimbrite eruptions in the period 7-4 Ma (K-Ar determinations). These ignimbrites are named the Toconquis Ignimbrite Formation. They are characterised by the presence of basal plinian deposits, many individual flow units and proximal co-ignimbrite lag breccias. The ignimbrites also have moderate to high macroscopic pumice and lithic contents and moderate to low crystal contents. Compositionally banded pumice occurs near the top of some units. Many of the Toconquis eruptions occurred from vents along a north-south line on the western rim of the young caldera. However, two of the ignimbrites erupted from vents on the eastern margin. Lava extrusions occurred contemporaneously along these north-south lines. The total D.R.E. volume of Toconquis ignimbrite exceeds 500 km³.Following a 2-Ma dormant period a single major eruption of rhyodacitic magma formed the 1000-km³ Cerro Galán ignimbrite and the caldera. The ignimbrite (age 2.1 Ma on Rb-Sr determination) forms a 30–200-m-thick outflow sheet extending up to 100 km in all directions from the caldera rim. At least 1.4 km of welded intracaldera ignimbrite also accumulated. The ignimbrite is a pumice-poor, crystal-rich deposit which contains few lithic clasts. No basal plinian deposit has been identified and proximal lag breccias are absent. The composition of pumice clasts is a very uniform rhyodacite which has a higher SiO₂ content but a lower K₂O content than the Toconquis ignimbrites. Preliminary data indicate no evidence for compositional zonation in the magma chamber. The eruption is considered to have been caused by the catastrophic foundering of a cauldron block into the magma chamber.Post-caldera extrusions occurred shortly after eruption along both the northern extension of the eastern boundary fault and the western caldera margin. Resurgence also occurred, doming up the intracaldera ignimbrite and sedimentary fill to form the central mountain range. Resurgent doming was centred along the eastern fault and resulted in radial tilting of the ignimbrite and overlying lake sediments.     

Density changes during the fractional crystallization of basaltic magmas: fluid dynamic implications

The dynamical behaviour of basaltic magma chambers is fundamentally controlled by the changes that occur in the density of magma as it crystallizes. In this paper the term fractionation density is introduced and defined as the ratio of the gram formula weight to molar volume of the chemical components in the liquid phase that are being removed by fractional crystallization. Removal of olivine and pyroxene, whose values of fractionation density are larger than the density of the magma, causes the density of residual liquid to decrease. Removal of plagioclase, with fractionation density less than the magma density, can cause the density of residual liquid to increase. During the progressive differentiation of basaltic magma, density decreases during fractionation of olivine, olivine-pyroxene, and pyroxene assemblages. When plagioclase joins these mafic phases magma density can sometimes increase leading to a density minimum. Calculations of melt density changes during fractionation show that compositional effects on density are usually greater than associated thermal effects.In the closed-system evolution of basaltic magma, several stages of distinctive fluid dynamical behaviour can be recognised that depend on the density changes which accompany crystallization, as well as on the geometry of the chamber. In an early stage of the evolution, where olivine and/or pyroxenes are the fractionating phases, compositional stratification can occur due to side-wall crystallization and replenishment by new magma, with the most differentiated magma tending to accumulate at the roof of the chamber. When plagioclase becomes a fractionating phase a zone of well-mixed magma with a composition close to the density minimum of the system can form in the chamber. The growth of a zone of constant composition destroys the stratification in the chamber. A chamber of well-mixed magma is maintained while further differentiation occurs, unless the walls of the chamber slope inwards, in which case dense boundary layer flows can lead to stable stratification of cool, differentiated magma at the floor of the chamber.In a basaltic magma chamber replenished by primitive magma, the new magma ponds at the base and evolves until it reaches the same density and composition as overlying magma. Successive cycles of replenishment of primitive magma can also form compositional zonation if successive cycles occur before internal thermal equilibrium is reached in a chamber. In a chamber containing well-mixed, plagioclase — saturated magma, the primitive magma can be either denser or lighter than the resident magma. In the first case, the new magma ponds at the base and fractionates until it reaches the same density as the evolved magma. Mixing then occurs between magmas of different temperatures and compositions. In the second case a turbulent plume is generated that causes the new magma to mix immediately with the resident magma.     

Absorption of radio waves during a solar eclipse

In this paper, the results of our observations on Al-method ionospheric absorption of radio waves on 1.8 and 2.2 MHz during the solar eclipse of 16 February 1980 are presented. The absorption decreased by about 41% and 46% of the normal value respectively at the above two frequencies at Ahmedabad following the maximum phase of the eclipse (about 77% of full disc) with a delay of 18 minutes. The quantityA _T (f) which is a measure of εN vdh is now examined for better clarity of the influence of the changes in theE-layer. The results are discussed in relation to the observations of the ionizing radiations from the sun, changes in the electron density, recombination rate and absorption in the underlyingD andE regions.     

Pumice

Cold pumice floating on water slowly absorbs water into the vesicles and eventually sinks. Experiments show that some pumice can remain afloat for over 1 1/2 years. The time taken for enough water to be adsorbed to sink depends on the pumice size, initial density, the size distribution of vesicles and the connectedness of the vesicles. Hot pumice often sinks immediately on immersion in water despite having a lower density than water. Experiments demonstrate that for any pumice there is a critical temperature above which the pumice will sink. Even pumice with a density of 0.2 g/cm³ will sink if the temperature exceeds 700 °C. The critical temperature correlates well with initial pumice density with lower-density pumice requiring higher temperatures to sink. The mechanism at low temperatures (< 150 °C) involves the absorption of water by contraction of hot air within the pumice. However, at higher temperatures conversion of absorbed water to steam in the hot pumice flushes out air, and further cooling results in condensation and absorption of water into the pumice. The experiments on hot and cold pumice suggest that all the vesicles in pumice are interconnected. This was confirmed by vacuum impregnation of pumice with resins. The behaviour of hot and cold pumice indicates that the deposits of hot and cold pyroclastic flow deposits may be distinguishable. Hot deposits will contain a significant proportion of low-density pumice, whereas cold deposits will not. Pumice falling hot onto water could also sink immediately to form subaqueous pumice-fall deposits. The physical properties of pumice were further examined by a nitrogen absorption technique and by mercury porosimetry. The former method shows that pumice has a typical surface area of 0,5 m²/g, corresponding to a sheet of material of 1 m² and 0,87µm Thick. Porosimetry shows that there are often three apparent vesicle-size populations in pumice. However, the porosimetry data gives surface areas which often greatly exceed those measured by the absorption method. The calculation of surface area by porosimetry assumes that vesicles are open cylinders. The large discrepancy with nitrogen absorption data suggests that the surface areas and proportion of small vesicles are overestimated by porosimetry and that pumice vesicles have narrow entrances. The porosimetry size distributions reflect the dimensions of pore entrances rather than the vesicles themselves. A three stage degassing history was proposed by Sparks and Brazier (1982). However, the small size population of sub-micron vesicles they identified probably represent larger ( 1µm) vesicles with narrow entrances. The experimental data indicate that pumice can degas very quickly because of the connectedness of vesicles and high internal surface areas.     

The initial giant umbrella cloud of the May 18th, 1980, explosive eruption of Mount St. Helens

The initial eruption column of May 18th, 1980 reached nearly 30 km altitude and released 10¹⁷ joules of thermal energy into the atmosphere in only a few minutes. Ascent of the cloud resulted in forced intrusion of a giant umbrella-shaped cloud between altitudes of 10 and 20 km at radial horizontal velocities initially in excess of 50 m/s. The mushroom cloud expanded 15 km upwind, forming a stagnation point where the radial expansion velocity and wind velocity were equal. The cloud was initiated when the pyroclastic blast flow became buoyant. The flow reduced its density as it moved away from the volcano by decompression, by sedimentation, and by mixing with and heating the surrounding air. Observations indicate that much of the flow, covering an area of 600 km², became buoyant within 1.5 minutes and abruptly ascended to form the giant cloud. Calculations are presented for the amount of air that must have been entrained into the flow to make it buoyant. Assuming an initial temperature of 450°C and a magmatic origin for the explosion, these calculations indicate that the flow became buoyant when its temperature was approximately 150°C and the flow consisted of a mixture of 3.25 × 10¹¹ kg of pyroclasts and 5.0 × 10¹¹ kg of air. If sedimentation is considered, these figures reduce to 1.1 × 10¹¹ kg of pyroclasts and 1.0 × 10¹¹ kg of air.