Lava Bodies Research Articles

Effusive eruption of viscous silicic magma triggered and driven by recharge: a case study of the Cerro Chascon-Runtu Jarita Dome Complex in Southwest Bolivia

The Cerro Chascon-Runtu Jarita Complex is a group of ten Late Pleistocene (∼85 ka) lava domes located in the Andean Central Volcanic Zone of Bolivia. These domes display considerable macroscopic and microscopic evidence of magma mixing. Two groups of domes are defined chemically and geographically. A northern group, the Chascon, consists of four lava bodies of dominantly rhyodacite composition. These bodies contain 43–48% phenocrysts of plagioclase, quartz, sanidine, biotite, and amphibole in a microlite-poor, rhyolitic glass. Rare mafic enclaves and selvages are present. Mineral equilibria yield temperatures from 640 to 750 °C and log ƒO2 of –16. Geochemical data indicate that the pre-eruption magma chamber was zoned from a dominant volume of 68% to minor amounts of 76% SiO2. This zonation is best explained by fractional crystallization and some mixing between rhyodacite and more evolved compositions. The mafic enclaves represent magma that intruded but did not chemically interact much with the evolved magmas. A southern group, the Runtu Jarita, is a linear chain of six small domes (<1 km3 total volume) that probably is the surface expression of a dike. The five most northerly domes are composites of dacitic and rhyolitic compositions. The southernmost dome is dominantly rhyolite with rare mafic enclaves. The composite domes have lower flanks of porphyritic dacite with ∼35 vol.% phenocrysts of plagioclase, orthopyroxene, and hornblende in a microlite-rich, rhyodacitic glass. Sieve-textured plagioclase, mixed populations of disequilibrium plagioclase compositions, xenocrystic quartz, and sanidine with ternary composition reaction rims indicate that the dacite is a hybrid. The central cores of the composite domes are rhyolitic and contain up to 48 vol.% phenocrysts of plagioclase, quartz, sanidine, biotite, and amphibole. This is separated from the dacitic flanks by a banded zone of mingled lava. Macroscopic, microscopic, and petrologic evidence suggest scavenging of phenocrysts from the silicic lava. Mineral equilibria yield temperatures of 625–727 °C and log ƒO2 of –16 for the rhyolite and 926–1000 °C and log ƒO2 of –9.5 for the dacite. The rhyolite is zoned from 73 to 76% SiO2, and fractionation within the rhyolite composition produced this variation. Most of the 63–73% SiO2 compositional range of the lava in this group is the result of mixing between the hybrid dacite and the rhyolite. Eruption of both groups of lavas apparently was triggered by mafic recharge. A paucity of explosive activity suggests that volatile and thermal exchanges between reservoir and recharge magmas were less important than volume increase and the lubricating effects of recharge by mafic magmas. For the Runtu Jarita group, the eruption is best explained by intrusion of a dike of dacite into a chamber of crystal-rich rhyolite close to its solidus. The rhyolite was encapsulated and transported to the surface by the less-viscous dacite magma, which also acted as a lubricant. Simultaneous effusion of the lavas produced the composite domes, and their zonation reflects the subsurface zonation. The role of recharge by hotter, more fluid mafic magma appears to be critical to the eruption of some highly viscous silicic magmas.

The evolution of lava flows from ephemeral vents at Mount Etna: Insights from vesicle distribution and morphological studies

Lava flow units from ephemeral vents at Mount Etna are characterized by a cross-flow subdivision into zones with different surface morphology, symmetrically distributed with respect to the centerline of the flow. These zones are: (1) a central zone ( cp zone) with a relatively smooth surface near the vent; (2) a lateral zone ( lp zone) covered by a carpet of clinkers overlying the massive lava body; and (3) a lateral levée zone ( ll zone). In many cases the cp and lp zones are separated by an inward-dipping groove, and the cp zone is from some decimeters up to more than 1 m thicker than the adjacent lp zone. In sections perpendicular to the flow direction, the vesicles corresponding with the cp zone are systematically distributed in a ring-like region and are characterized by a sub-elliptical shape with their minor axes along a direction radial to the center of such region. Vesicles corresponding with the lp zone are still elliptical and their major axes tend to be parallel to the nearest cooling surface. Vesicularity is the lowest at the center of the ring-like region where vesicles approach a spherical shape. In the upper part of the ring-like region, vesicle accumulation and coalescence below the crust produces one or more gas-rich layers which act as levels of preferential detachment for the formation of lava tubes. The observed morphological characteristics and reconstructed vesicle distribution patterns are consistent with a radial distribution of velocity within the lava flow unit and suggest the existence of an inner plug flow region where the velocity gradient is zero. A simple evolution scheme is proposed in which a lava flow unit from ephemeral vent invariably evolves to a lava tube through the downward migration of the plug flow region. The progressive reduction of the cross-flow section due to cooling produces a pressure increase within the flowing lava body and the thickening of the active central portion of the lava flow unit in order to satisfy mass conservation.

A model for the opening of ephemeral vents in a stationary lava flow

A model is proposed for the opening of ephemeral vents in the solid front of a stationary lava flow. The front is considered to be a viscoelastic shell which deforms due to the pressure exerted on it by the inner fluid lava. The vent opens when the normal stress in the front overcomes the tensile strength of solid lava. Cooling of the front is considered and the increase in crust thickness, as the isothermal surface at the solidus temperature deepens into the lava body, is calculated. The thermal effect is negligible, if the timescale for the opening of ephemeral vents is on the order of days, as is often observed. Considering stationary flow lengths of a few hundred meters, this timescale corresponds to an average viscosity of the crust of 10 12 Pa s, assuming reasonable values of the crust strength and initial thickness. Observation of ephemeral vents on Mount Etna is found to be consistent with the model predictions.

Structure of lava flows of the Nimbin Rhyolite, northeast New South Wales

The Tertiary Nimbin Rhyolite in northeast New South Wales comprises tiers of lava extending laterally up to 12 km. The geographic distribution of crystal‐poor (nearly aphyric) and crystal‐rich lava types indicates that these tiers comprise coalesced lava domes and limited flows rather than extensive flows. Bodies of crystal‐poor lava exhibit flow foliation and flow folds which aid in locating vents, whereas the weak foliation in bodies of crystal‐rich lava makes their internal structure difficult to resolve. The spacing of vents throughout the lava complex also suggests multiple lava bodies, ranging in diameter from 2 to 4 km.

Effusive silicic volcanism in the Central Andes: The Chao dacite and other young lavas of the Altiplano‐Puna Volcanic Complex

The largest known Quaternary silicic lava body in the world is Cerro Chao in north Chile, a 14‐km‐long coulée with a volume of at least 26 km3. It is the largest of a group of several closely similar dacitic lavas erupted during a recent (&lt; 100,000 year old) magmatic episode in the Altiplano‐Puna Volcanic Complex (APVC; 21‐24°S) of the Central Andean Volcanic Zone. The eruption of Chao proceeded in three phases. Phase 1 was explosive and produced ∼1 km3 of coarse, nonwelded dacitic pumice deposits and later block and ash flows that form an apron in front of the main lava body. Phase 2 was dominantly effusive and erupted ∼22.5 km3 of magma in the form of a composite couleé covering ∼53 km2 with a 400‐m‐high flow front and a small cone of poorly expanded pumice around the vent. The lava is homogeneous with rare flow banding and vesicular tops and selvages. Ogives (flow ridges) reaching heights of 30 m form prominent features on its surface. Phase 3 produced a 6‐km‐long, 3‐km‐wide flow that emanated from a collapsed dome. Ogives are subdued, and the lava is glassier than that produced in previous phases. All the Chao products are crystal‐rich high‐K dacites and rhyodacites with phenocrysts of plagioclase, quartz, hornblende, biotite, sphene, rare sanidine, and oxides. Phenocryst contents reach 40–60 vol % (vesicle free) in the main phase 2 lavas but are lower in the phase 1 (20–25%) and phase 3 (∼40%) lavas. Ovoid andesitic inclusions with vesicular interiors and chilled margins up to 10 cm are found in the later stages of phase 2 and compose up to 5% of the phase 3 lava. There is little evidence for preemptive zonation of the magma body in composition, temperature (∼840°C), ƒO2 (10−11), or water content, so we propose that eruption of the Chao complex was driven by intrusion of fresh, hot andesitic magma into a crystallizing and largely homogeneous body of dacitic magma. Morphological measurments suggest that the Chao lavas had internal plastic viscosities of 1010 to 1012Pa s, apparent viscosities of 109 Pa s, surface viscosities of 1015 to 1024 Pa s, and a yield strength of 8×105 Pa. These estimates indicate that Chao would have exhibited largely similar rheological properties to other silicic lava extrusions, notwithstanding its high phenocryst content. We suggest that Chao's anomalous size is a function of both the relatively steep local slope (20° to 3°) and the available volume of magma. The eruption duration for Chao's emplacement is thought to have been about 100 to 150 years, with maximum effusion rates of about 25 m3 s−1 for short periods. Four other lavas in the vicinity with volumes of ∼5 km3 closely resemble Chao and are probably comagmatic. The suite as a whole shares a petrologic and chemical similarity with the voluminous regional Tertiary to Pleistocene ignimbrites of the APVC and may be derived from a zone of silicic magmatism that is thought to have been active since the late Tertiary. Chao and the other young lavas may represent either the waning of this system or a new episode fueled by intrusions of mafic magma.

Thermal distributions of hot volcanic surfaces constrained using three infrared bands of remote sensing data

Infrared remote sensing provides a means for radiometry of hot volcanic features. Previously, two spectral band measurements have been combined to solve simple 2‐component models of lava surface temperatures. However, pixel dimensions often exceed the widths of lava bodies. In such cases, 3‐component models are more realistic. Here, a procedure for solving such a model from three infrared observations is presented. It is then demonstrated using Thematic Mapper Simulator data recorded over Mount Etna, Italy, in 1991. By constraining temperatures and pixel‐filling fractions of the three different thermal components in this way, improved estimates of radiant emittance can be obtained. The techniques described could be applied to satellite data of volcanic, and other, hot spots (including fires).

Thermal distributions at fumarole fields: implications for infrared remote sensing of active volcanoes

Multispectral satellite data recorded in the short-wavelength infrared (SWIR) region have been used to estimate temperatures of hot volcanic features such as fumaroles and lava bodies. This has required the assumption that thermal distributions across the surfaces in question can be adequately represented by a very few (usually two) temperature components. We investigate here the applicability of such simple descriptions to fumarole fields, an important class of thermal phenomena. Our surface temperature surveys, conducted at Momotombo (Nicaragua) and Vulcano (Italy), suggest that SWIR emission from the interiors of high-temperature fumarole vents far exceeds that from their surroundings. In contrast, cooling crust on active lava bodies can contribute more SWIR radiation than the small areas of exposed incandescent material that are often present. Such a distinction should be discernible in the relative response of the two Landsat Thematic Mapper (TM) SWIR bands, providing a basis for identification of the volcanic nature of thermal anomalies in Landsat TM or similar data, even in the absence of field observations. However, estimates of subpixel temperatures and heat flux, made from such satellite data, must be considered unreliable.

Infrared monitoring of volcanoes by satellite

Sensors on future satellite platforms will be used routinely for volcano monitoring, with infrared channels offering the potential to detect and measure temperatures of features such as lava bodies and fumarole fields. This is possible even though the surface temperature distributions associated with such phenomena are typically inhomogeneous at the pixel scale, so long as simplifying assumptions can be made. To test the applicability of the technique, we have carried out field studies of a range of volcanic thermal manifestations using portable infrared thermometers. Rock surface temperatures around fumarolic vents exhibit greater stability with time than those associated with actively degassing lava bodies, but in both cases volcanogenic fumes can significantly attenuate the emitted infrared radiation. The scale of the temporal variability, at a given wavelength, dictates the quantitative value of isolated satellite 'snapshots'. If more spectral channels in the infrared were available on remote sensing platforms then more components of the surface temperature distribution could be extracted from the data. Imaging spectrometry offers the greatest flexibility, although data processing, including correction for atmospheric effects, would present difficulties.

Tectonic and volcanic evolution of Panarea (Aeolian Islands, Italy)

Based on new geophysical, volcanological, structural and radiometric data, a morphological-structural reconstruction of the submarine part of the Panarea volcanic edifice is provided and a model for the evolution of the entire complex is proposed. Panarea is a small part of a much larger submarine edifice whose overall shape is one of a truncated cone with an eastern protrusion. The base of the cone is about 1500 m below sea level; it has a diameter of about 23 km and an area of 460 km 2. Several lobes are present on the steep eastern slopes of the cone and the wide submarine top is of a morphology which is more complex than previously thought. The evolution of the Panarea volcanic complex occurred in three stages: 1. (1) Development of a central volcano in the western sector of the complex. The island of Panarea represents the top of this volcano. 2. (2) Eastward shifting of volcanic activity, producing development of the lobed eastern sector of the complex. 3. (3) Extensive caldera collapse of the central part of the complex and emplacement, within the collapsed area, of domes and lava bodies of various affinities, some of which are basaltic. NE-SW oriented structures seem to have had a fundamental role during the development of the Panarea volcanic complex. These structures have been active until recent times and are of regional significance. Indeed, the evolution of nearby Stromboli has been controlled by the NE-SW trending structures. Panarea and Stromboli also display similar magnetic anomaly trends, geochemical affinities, and temporal relationships in volcanism.

A comparison of Miocene and Archean rhyolite hyaloclastites: Evidence for a hot and fluid rhyolite lava

In this paper, we compare the geology and petrography of Miocene and Archean submarine rhyolite hyaloclastites. The hyaloclastites are sparsely (10% or less) plagioclase- (± quartz and pyroxene-) phyric. The hyaloclastites consist of a feeder dyke from which branch lava lobes and irregularly shaped lava pods. The lava bodies consist of a holocrystalline core with microlitic texture, grading outward into a flow-layered rim zone and, finally, into obsidian. The proportion of plagioclase and pyroxene microlites decreases outward. Some layers of the rim zone may be pumiceous (vesicularity up to 50%, vesicle size 1 mm or less), but most of the lava has less than 5% vesicles one or a few cm long. The obsidian shows perlitic fracture patterns. The lava bodies grade through an in-situ breccia into a hyaloclastite composed of angular obsidian granules and, in many cases, of fragments of lava lobes. Evidence for alteration at high temperature is as follows: in the Archean rhyolite hyaloclastites, plagioclase microlites are overgrown by quartz-albite spherulites. Furthermore, parts of the Miocene and Archean hyaloclastite have been cemented and granules have been marginally replaced by quartz and albite. Hyaloclastite cemented at high temperature locally shows columnar joints. At low temperatures, obsidian has been hydrated and/or has been replaced by clay minerals, zeolites, chlorite or prehnite. “Chess-board” albite and fibroradial prehnite in Archean hyaloclastite is possibly a pseudomorph after zeolites. The sparsely porphyritic nature of the lava and the absence of microlites from the quenched glass suggests that the thyolite hyaloclastites extruded at high (near liquidus) temperature. Furthermore pumice is present only locally, in the flow-layered rim zone and in fragments derived from that zone. These features suggest that vesiculation was inhibited by the weight of the water column. High temperature and possibly the volatile (H 2O) content explain the relatively low viscosity and shear strength of the lava, and resulted in the flow morphology particular to this type of hyaloclastic rhyolite flows.

Contents volume 22, 1984

Multispectral satellite data recorded in the short-wavelength infrared (SWIR) region have been used to estimate temperatures of hot volcanic features such as fumaroles and lava bodies. This has required the assumption that thermal distributions across the surfaces in question can be adequately represented by a very few (usually two) temperature components. We investigate here the applicability of such simple descriptions to fumarole fields, an important class of thermal phenomena. Our surface temperature surveys, conducted at Momotombo (Nicaragua) and Vulcano (Italy), suggest that SWIR emission from the interiors of high-temperature fumarole vents far exceeds that from their surroundings. In contrast, cooling crust on active lava bodies can contribute more SWIR radiation than the small areas of exposed incandescent material that are often present. Such a distinction should be discernible in the relative response of the two Landsat Thematic Mapper (TM) SWIR bands, providing a basis for identification of the volcanic nature of thermal anomalies in Landsat TM or similar data, even in the absence of field observations. However, estimates of subpixel temperatures and heat flux, made from such satellite data, must be considered unreliable.

Pyroclastic geology of the rhyolitic volcano of La Primavera, Mexico

Explosive eruptions of this large Quaternary rhyolitic volcano have produced voluminous pumice fall deposits and ignimbrites. A stratigraphie study reveals more than 30 fall deposits, totalling more than 90 km3, which have come from many different vents. Some are of plinian type, and one is among the largest known in the world. The main ignimbrite (Rio Caliente) is of intra-plinian type. In all, 140 km3 of rhyolitic material were erupted during the past 105 years, of which 50 km5 are dispersed well outside the volcano. The dense rock equivalent volume is 60 km3, of which fall deposits, ignimbrites and lava bodies comprise 45 %, 25 % and 30 % respectively. The average output rate of 0.06 km3 per century is nearly an order of magnitude less than for the most productive rhyolitic volcanoes known. A circular area 10 km across in the centre of La Primavera contains updomed lacustrine ashes and associated sediments, including a remarkable giant pumice bed, which probably accumulated in a caldera lake. La Primavera has no record of historical eruptions, and hot springs are the only signs of present activity, but the updoming is thought to be due to the uprise of a new acid pluton beneath the volcano and future eruptions are probable.

Numerical analysis of lava lake cooling models; Part I, Description of the method

A computer method for numerical solution of problems in transient heat flow has been developed and applied to cooling models of Hawaiian lava lakes. This report and part II by Peck, Hamilton, and Shaw apply the method to Alae lava lake as the first detailed test against a systematic program of temperature measurements in lava lakes carried out by the Hawaii Volcano Observatory. The method is neither a finite element nor finite difference method, as the terminology of numerical analysis is currently employed, though there are similarities to both. It is an adaptation of a computer method that we call the method of explicit cell balances. Input-output balances are computed between cells in a grid system simulating the lava lake geometry. Computation explicitly performs the cell balances sequentially in the same way the "cooling wave" moves into the lava body without recourse to numerical solutions of sets of simultaneous equations. The procedure clearly identifies the physical processes governing thermal effects: the effects of the latent heat of crystallization, effects of rainfall permeating the lava surface, and effects of large variations in density and thermal conductivity. The accuracy of numerical approximations was tested against exact solutions for an extrusive sheet without sources. Deviations were less than 1 degrees C in all cases. Agreement with data for Alae lava lake given in part II indicates that the method can be used with good confidence for refinement of thermal properties and identification of heat transfer mechanisms in other lava lakes as well as for predictive tests of cooling histories for hypothetical systems involving complicated heat transfer mechanisms.

Beach/Shallow Marine Hyaloclastite Deposits and Their Geological Significance: An Example from Gran Canaria

Basanite lava streams which flowed over dry land, a beach, littoral pools, and into a shallow sea produced a complex hyaloclastite/massive basalt passage zone. Small localized breccias-basal breccias-formed when the lava flowed over littoral pools or alternatively was met by surges. Massive lava bodies entirely overlain by hyaloclastite, and with upwards convex, irregularly brecciated top surfaces, resulted when rising tide continuously overstepped lava flows that were advancing in the opposite direction to the incoming tide. In this way hyaloclastite, encrusting massive lava bodies, will continuously build up to the high tide sea level, and then become covered with massive, subaerial lavas. Massive lava units with a downward convex shape, deeply cutting into the hyaloclastites and partly unconsolidated basement rocks, formed at a later stage, but at ebb tide. The shapes and distribution of the latter massive lava bodies are entirely controlled by the morphology of the hyaloclastite that formed during rising tide at an earlier stage. The general applications of tide-formed structures in beach/shallow marine environment are discussed.

Petroleum Fields with Reservoirs of Volcanic Rocks, Japan: ABSTRACT

As 5 of 11 major oil and gas fields found during the last 15 years in Japan have volcanic-rock reservoirs, they have become an important objective in exploration for petroleum in Japan. Japanese oil and gas fields have been found mainly in Neogene sedimentary basins developed on the Japan Sea coast along the northern half of Honshu. The basin, associated with volcanic activities, began its depression in the early Miocene, but it continued to subside through the Neogene and Quaternary. Volcanic-rock reservoirs are present in the formations deposited during middle Miocene and early Pliocene times. They are composed of liparitic, dacitic and/or andesitic lava, agglomerate, and tuff breccia. Intergranular pores are the main cause for the porosity, but many fractures and vugs which may provide additional porosity are known. Volcanic-rock reservoirs have a rough resemblance to carbonate-rock reservoirs in that fractures and vugs are predominant, formation resistivity is higher than surrounding formations, and the shapes of volcanic-rock masses commonly show reeflike forms. However, the decisive difference between them is that whereas carbonate rocks may be source rocks as well as reservoir rocks, the volcanic rocks are not source rocks. Therefore it is important that, in searching for petroleum in volcanic-rock reservoirs, source rocks must be confirmed close by. Each volcanic-rock reservoir has been found to have its own pore continuity. Some reservoirs have good pore continuity but others do not. For development of these fields, the difficulty is in determining the location and magnitude of the lava bodies which have good porosity. End_of_Article - Last_Page 1444------------