The rio caliente ignimbrite: Analysis of a compound intraplinian ignimbrite from a major late quaternary Mexican eruption

The Rio Caliente ignimbrite is a multi-flow unit orcompound ignimbrite formed during a major late Quaternary explosive rhyolitic eruption of La Primavera volcano, Mexico. The eruption sequence of the ignimbrite is complex and it occurs between lower and upper plinian air-fall deposits. It is, therefore, anintraplinian ignimbrite. Air-fall layers, pyroclastic surge, mudflow and fluviatile reworked pumice deposits also occur interbedded between ignimbrite flow units. A chaotic near-vent facies of the ignimbrite includes co-ignimbrite lag breccias segregated from proximal pumice flows. The facies locates a central vent but one which could not have been associated with a well defined edifice. Many of the lithics in the exposed lag breccias and near-vent facies of the ignimbrite appear to be fragments of welded Rio Caliente ignimbrite, and indicate considerable vent widening, or migration, during the eruption. Nearer vent the ignimbrite is thickest and composed of the largest number of flow units. Here it is welded and is a simple cooling unit. Evidence suggests that it was only the larger thicker pumice flows that escaped to the outer parts of the sheet. Detailed analysis of four flow units indicates that the pumice flows were generally poorly expanded, less mobile flows which would be produced by collapse of low eruption columns. The analogy of a compound ignimbrite with a compound lava flow is, therefore, good — a compound lava flow forms instead of a simple one when the volumetric discharge rate (or intensity) is low, and in explosive eruptions this predicts lower eruption column heights. A corollary is that the ignimbrite has a high aspect ratio. The complex eruption sequence shows the reinstatement of plinian activity several times during the eruption after column collapse occurred. This, together with erosional breaks and evidence that solidified fragments of already welded ignimbrite were re-ejected, all suggest the eruption lasted a relatively significant time period. Nearly 90 km³ of tephra were erupted. The associated plinian pumice fall is one of the largest known having a volume of 50 km³ and the ignimbrite, plus a co-ignimbrite ash-fall, have a volume of nearly 40 km³. Published welding models applied to the reejected welded blocks indicate an eruption duration of 15-20d, and a maximum average magma-discharge rate of 1.4 × 10⁴ m³/s for the ignimbrite. This is low intensity when compared with available data from other ignimbrite-forming eruptions, and concurs with all the geological evidence presented. The total eruption duration was perhaps 15-31d, which is consistent with other estimates of the duration of large magnitude explosive silicic eruptions.     

The campanian ignimbrite: a major prehistoric eruption in the Neapolitan area (Italy)

A geological, chemical and petrographical study of the Campanian ignimbrite, a pyroclastic flow deposit erupted about 30,000 years ago on the Neapolitan area (Italy), is reported. The ignimbrite covered an area of at least 7,000 km²; it consists of a single flow unit, and the lateral variations in both pumice and lithic fragments indicate that the source was located in the Phlegraean Fields area. Textural features, areal distribution and its morphological constraints suggests that the eruption was of the type of highly expanded low-temperature pyroclastic cloud. The original composition was strongly modified by post-depositional chemical changes involving most of the major and trace elements. No primary differences in the composition of the magma have been recognized. The Campanian ignimbrite is a nearly saturated potassic trachyte, similar to many other trachytes of the Quaternary volcanic province of Campania. Its chemistry indicates an affinity with the so-called «low-K association» of the Roman volcanic province.     

The dimensions and dynamics of volcanic eruption columns

Eruption columns can be divided into three regimes of physical behaviour. The basal gas thrust region is characterized by large velocities and decelerations and is dominated by momentum. This region is typically a few hundred metres in height and passes upwards into a much higher convective region where buoyancy is dominant. The top of the convective region is defined by the level of neutral density (heightH _B ) where the column has a bulk density equal to the surrounding atmosphere. Above this level the column continues to ascend to a heightH _T due to its momentum. The column spreads horizontally and radially outwards between heightH _T andH _B to form an umbrella cloud. Numerical calculations are presented on the shape of eruption columns and on the relationships between the heightH _B and the mass discharge rate of magma, magma temperature and atmospheric temperature gradients. Spreading rate of the column margins increases with height principally due to the decrease in the atmospheric pressure. The relationship between column height and mass discharge rate shows good agreement with observations. The temperature inversion above the tropopause is found to only have a small influence on column height and, eruptions with large discharge rates can inject material to substantially greater heights than the inversion level. Approximate calculations on the variation of convective velocities with height are consistent with field data and indicate that columns typically ascend at velocities from a few tens to over 200 m/s. In very large columns (greater than 30 km) the calculated convective velocities approach the speed of sound in air, suggesting that compressibility effects may become important in giant columns. Radial velocities in the umbrella region where the column is forced laterally into the atmosphere can be substantial and exceed 55 m/s in the case of the May 18th Mount St. Helens eruption. Calculations on motions in this region imply that it plays a major role in the transport of coarse pyroclastic fragments.     

Source mechanism of volcanic tremor: fluid-driven crack models and their application to the 1963 kilauea eruption

We propose a model for the mechanism of magma transport based on a fluid-filled tensile crack driven by the excess pressure of fluid. Such a transport mechanism can generate seismic waves by a succession of jerky crack extensions, if the fracture strength of rock varies in space, or if there is a difference between the dynamic and static values of the critical stress intensity factor. We also find that the opening and closing of a narrow channel connecting two fluid-filled cracks may be a source of seismic waves. Using the finite-difference method, we calculated the vibration of dry and fluid-filled cracks generated by: (1) a jerky extension at one end or at both ends and (2) a jerky opening of a narrow channel connecting two cracks. We then calculated the far-field and near-field radiation from these vibrating cracks. The spectra show peaked structures, but interestingly, most high-frequency peaks are only present in the near-field and cannot be transmitted to the far-field. The spectral features described above are often observed for volcanic tremors and in some cases for seismic signals associated with hydraulic fracturing experiments.We first consider as a model of volcanic tremor randomly occurring jerky crack extensions, and derive a formula relating the tremor amplitude to the excess pressure in the magma, the incremental area in each extension, and the frequency of extensions. These parameters are also constrained by other observations, such as the rate of magma flow.Our model was tested quantitatively against observations made in one of the best-described case histories of volcanic tremor: the October 5–6, 1963 Kilauea flank eruption. We found that a single, long crack extending from the summit to the eruptive site cannot explain the observations. The model of a steadily expanding crack ran into difficulties when quantitative comparisons were made with observations. The extension of crack area needed to explain the amplitude of volcanic tremor should accompany a large increase in tremor period which was not observed.Our second model is a chain of cracks connected by narrow channels which open and close. The length of each crack is around 1 km, the channel area connecting neighboring cracks is about 10³m², and the channel opens jerkily with the magmatic excess pressure of about 20 bars. The frequency of jerky opening of each channel is about once in 15 seconds. The channel is closed after each jerky opening, as soon as magma is moved through the channel.     

腾冲火山地热区的构造演化与火山喷发

讨论了腾冲地区在Ｄ－Ｅ２时期的板块拼合演化过程及其大地构造单元位置，用以说明腾冲火山地热区的现今区域构造条件，讨论了火山喷发时期与板块碰撞时代的关系。     

Discussion on the energy partition of volcanic eruption

Energy of volcanic eruption is divided into various ways, such as lava flows, ash production, explosion, ground vibration, air waves and so on. In order to elucidate the cruptive mechanism, energy partition of volcanic eruption in unit time is considered to be a key to solve the problem. In this short paper, partition of eruption energy was calculated for typical three basaltic volcanoes. Ratio of tremor energy to thermal energy is almost the same for both Mihara and Nyragongo in spite of their different eruptive mechanism. Tremor energy was found to be small for Kilauea Volcano compared with the above two volcanoes. This was attributed to the different source mechanism of tremor. Partition of eruption energy was calculated for Also Volcano which showed phreatic explosion in 1958. Kinetic energy is almost ten times of thermal energy. Ratio of energy converted to elastic waves to thermal energy is found to be equivalent to the ratio of energy of elastic waves to the chemical energy in case of explosion of chemical explosives.     

The interaction between volcanic gases and tephra: Fluorine adhering to tephra of the 1970 hekla eruption

The mass distribution and sorting of tephra produced in the plinian phase of the 1970 Hekla eruption was controlled by the particle size distribution, the height of the eruption column, and velocity of transport. Near the volcano the mass distribution of soluble fluorine was controlled by particle size of the deposits, but approaches the mass distribution of the tephra at longer distances. Adsorbed soluble fluorine reaches a maximum at a distance from the volcano determined by the velocity of the transporting medium.SEM studies show the soluble fluorine to be chemically adsorbed on the surface of tephra particles. The adsorption is shown by experiment to occur at temperatures below 600°C in the cooling eruption column. Evaluation of reactions in the eruption column leads to the conclusion that formation of water soluble compounds adhering to tephra is principally controlled by environmental factors and to a lesser degree by the composition of the volcanic gas phase.     

The influence of great earthquakes on volcanic eruption rate along the Chilean subduction zone

Seismic activity has been postulated as a trigger of volcanic eruption on a range of timescales, but demonstrating the occurrence of triggered eruptions on timescales beyond a few days has proven difficult using global datasets. Here, we use the historic earthquake and eruption records of Chile and the Andean southern volcanic zone to investigate eruption rates following large earthquakes. We show a significant increase in eruption rate following earthquakes of M_W > 8, notably in 1906 and 1960, with similar occurrences further back in the record. Eruption rates are enhanced above background levels for ~ 12 months following the 1906 and 1960 earthquakes, with the onset of 3–4 eruptions estimated to have been seismically influenced in each instance. Eruption locations suggest that these effects occur from the near-field to distances of ~ 500 km or more beyond the limits of the earthquake rupture zone. This suggests that both dynamic and static stresses associated with large earthquakes are important in eruption-triggering processes and have the potential to initiate volcanic eruption in arc settings over timescales of several months.     

Sensitivity of the dynamics of volcanic eruption columns to their shape

This paper presents a one-dimensional steady-state model to investigate the sensitivity of the dynamics of sustained eruption columns to radius variations with height due to thermal expansion of the entrained air, and decreases in atmospheric pressure with height. In contrast to a number of previous models using an equation known as the entrainment assumption, the new model is based on similarity arguments to derive an equation set equivalent to the model proposed by Woods [Bull Volcanol 50:169–193, 1988]. This approach allows investigation of the effect of gas compressibility on the entrainment rate of ambient air, which has been little examined for a system in which a decrease in pressure significantly affects the density stratification of a compressible fluid. The new model provides results that include two end members: one in which the volume change within the eruption columns affects only the radial expansion without changing the vertical motion, and the other is the converse. The Woods [Bull Volcanol 50:169–193, 1988] model can be regarded as being between those two end members. The range of uncertainty arises because the extremely high temperature of discharged materials from a volcanic vent, and the exceptional terminal height of the eruption columns, allow significant expansion of the gas component in the eruption columns, making them behave differently from common turbulent plumes. This study indicates that the maximum height of the eruption columns is affected considerably by this uncertainty, particularly when the eruption columns extend above a height of 10 km, at which the pressure is about one-fourth the pressure at the ground surface. Column collapse may also be suppressed in wider parameter ranges than previously estimated. However, the uncertainty can be reduced by measuring column radii through a vertical profile during actual volcanic eruptions. Accordingly, this paper suggests that appropriate observation of eruption column shapes is essential for improving our understanding of the dynamics of eruption columns.     

Relationship between earthquake and volcanic eruption inferred from historical records

Introduction From the records of Wudalianchi volcanic group eruption in 1720~1721 obtained from the Man ethnic group files of Heilongjiang General Yamen in Qing Dynasty (WU, 1998; CHEN, WU, 2003), we have discovered the eruption time, state, material and scale of Laoheishan and Hu-oshaoshan volcanoes, as well as numerous seismic records. These historical materials are discov-ered for the first time although the study on Wudalianchi volcanic group has a long history. These earthquakes co…     

Detecting volcanic eruption precursors: a new method using gravity and deformation measurements

One of the fundamental questions in modern volcanology is the manner in which a volcanic eruption is triggered; the intrusion of fresh magma into a reservoir is thought to be a key component. The amount by which previously ponded reservoir magma interacts with a newly intruded magma will determine the nature and rate of eruption as well as the chemistry of erupted lavas and shallow dykes. The physics of this interaction can be investigated through a conventional monitoring procedure that incorporates the simple and much used Mogi model relating ground deformation (most simply represented by Δh) to changes in volume of a magma reservoir. Gravity changes (Δg) combined with ground deformation provide information on magma reservoir mass changes. Our models predict how, during inflation, the observed Δg/Δh gradient will evolve as a volcano develops from a state of dormancy through unrest into a state of explosive activity. Calderas in a state of unrest and large composite volcanoes are the targets for the methods proposed here and are exemplified by Campi Flegrei, Rabaul, Krafla, and Long Valley. We show here how the simultaneous measurement of deformation and gravity at only a few key stations can identify important precursory processes within a magma reservoir prior to the onset of more conventional eruption precursors.     

Quantitative models of the fallout and dispersal of tephra from volcanic eruption columns

A theoretical model of clast fallout from convective eruption columns has been developed which quantifies how the maximum clast size dispersal is determined by column height and wind strength. An eruption column consists of a buoyant convecting region which rises to a heightH _B where the column density equals that of the atmosphere. AboveH _B the column rises further to a heightH _T due to excess momentum. BetweenH _T andH _B the column is forced laterally into the atmosphere to form an upper umbrella region. Within the eruption column, the vertical and horizontal velocity fields can be calculated from exprimental and theoretical studies and consideration of mass continuity. The centreline vertical velocity falls as a nearly linear function over most of the column's height and the velocity decreases as a gaussian function radially away from the centreline. Both column height and vertical velocity are strong functions of magma discharge rate. From calculations of the velocity field and the terminal fall velocity of clasts, a series of particle support envelopes has been constructed which represents positions where the column vertical velocity and terminal velocity are equal for a clast of specific size and density. The maximum range of a clast is determined in the absence of wind by the maximum width of the clast support envelope.The trajectories of clasts leaving their relevant support envelope at its maximum width have been modelled in columns from 6 to 43 km high with no wind and in a wind field. From these calculations the shapes and areas of maximum grain size contours of the air-fall deposit have been predicted. For the no wind case the theoretical isopleths show good agreement with the Fogo A plinian deposit in the Azores. A diagram has been constructed which plots, for a particular clast size, the maximum range normal to the dispersal axis against the downward range. From the diagram the column height (and hence magma discharge rate) and wind velocity can be determined. Historic plinian eruptions of Santa Maria (1902) and Mount St. Helens (1980) give maximum heights of 34 and 19 km respectively and maximum wind speeds at the tropopause of m/s and 30 m/s respectively. Both estimates are in good agreement with observations. The model has been applied to a number of other plinian deposits, including the ultraplinian phase of theA.D. 180 Taupo eruption in New Zealand which had an estimated column height of 51 km and wind velocity of 27 m/s.     

Rhyolitic ignimbrites in the Rogerson Graben,southern Snake River Plain volcanic province: volcanic stratigraphy,eruption history and basin evolution

The 80 km long NNE-trending Rogerson Graben on the southern margin of the central Snake River Plain, Idaho, USA, hosts a rhyolitic pyroclastic succession, 200 m thick, that records a period of successive, late-Miocene, large-volume explosive eruptions from the Yellowstone–Snake River Plain volcanic province, and contemporaneous extension. The succession, here termed the Rogerson Formation, comprises seven members (defined herein) and records at least eight large explosive eruptions with numerous repose periods. Five high-grade and extremely high-grade ignimbrites are intercalated with three non-welded ignimbrites and two volcaniclastic deposits, with numerous repose periods (palaeosols) throughout. Two of the ignimbrites are dominantly rheomorphic and lava-like but contain subordinate non-welded pyroclastic layers. The ignimbrites are typical Snake River Plain high-silica rhyolites, with anhydrous crystal assemblages and high inferred magmatic temperatures (≤ 1,025°C). We tentatively infer that the Jackpot and Rabbit Springs Members may have been emplaced from the Bruneau–Jarbidge eruptive centre on the basis of: (1) flow lineation trends, (2) crystal assemblage, and (3) radiometric age. We infer that the overlying Brown’s View, Grey’s Landing, and Sand Springs Members may have been emplaced from the Twin Falls eruptive centre on the basis of: (1) kinematic indicators (from the east), and (2) crystal assemblage. Furthermore, we have established the contemporaneous evolution of the Rogerson Graben from the emplacement of the Jackpot Member onwards, and infer that it is similar to younger half-graben along the southern margin of the Snake River Plain, formed by local reactivation of Basin and Range structures by the northeastwardly migration of the Yellowstone hot-spot. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The first tephra evidence for a Late Glacial explosive volcanic eruption in the Arxan-Chaihe volcanic field (ACVF), northeast China

A 5 mm thick tephra layer has been identified in the lacustrine sediments of Moon Lake in the Arxan-Chaihe volcanic field (ACVF) in Greater Khingan Mountains (NE China). The visible tephra layer is clearly revealed as a distinct peak in magnetic susceptibility measurements. The tephra layer consists mainly of brown vesicular glass shards and minor amounts of plagioclase, olivine and clinopyroxene. Major and minor element analysis has been carried out on the glass shards and plagioclase minerals. Glass shards show low concentrations of K₂O, similar to the eruptive products derived from post-Miocene volcanoes of the ACVF. The plagioclase phenocrysts in both lava and tephra from ACVF, and in the tephra recorded in Moon Lake are labradorites. During the Late Pleistocene to Holocene, there were also extensive explosive eruptions in the nearby Nuominhe volcanic field (NVF). Volcanic rocks from the ACVF are easily distinguished from those derived from the NVF, having distinctly different K₂O concentrations. This compositional variation is likely the result of different magmatic processes operating in the ACVF and NVF. Radiocarbon dating on organic materials from the lacustrine sediments dates the tephra layer to ca. 14,200 cal yrs BP, which implies that it was generated by a previously unknown Late Pleistocene explosive eruption in the ACVF. These results, for the first time, give a direct tephra record in this area, and suggest that identification of further tephra and/or cryptotephra in local sedimentary basins such as crater lakes of scoria cones and maars will be significant for dating the Late Pleistocene to Holocene volcanic eruptions and will help to establish a detailed record of the volcanic activity in the ACVF. The newly discovered tephra layer also provides a dated tephrochronological marker layer, which will in future studies provide a means to synchronise local sedimentary records of the climatically variable Late Glacial.     

Doppler radar sounding of volcanic eruption dynamics at Mount Etna

Besides their common use in atmospheric studies, Doppler radars are promising tools for the active remote sensing of volcanic eruptions but were little applied to this field. We present the observations made with a mid-power UHF Doppler radar (Voldorad) during a 7-h Strombolian eruption at the SE crater of Mount Etna on 11–12 October 1998. Main characteristics of radar echoes are retrieved from analysis of Doppler spectra recorded in the two range gates on either side of the jet axis. From the geometry of the sounding, the contribution of uprising and falling ejecta to each Doppler spectrum can be discriminated. The temporal evolution of total power backscattered by uprising targets is quite similar to the temporal evolution of the volcanic tremor and closely reproduces the overall evolution of the eruption before, during and after its paroxysm. Moreover, during the sharp decrease of eruptive activity following the paroxysm, detailed analysis of video (from camera recording), radar and seismic measurements reveals that radar and video signals start to decrease simultaneously, approximately 2.5 min after the tremor decline. This delay is interpreted as the ascent time through a magma conduit of large gas slugs from a shallow source roughly estimated at about 500 m beneath the SE crater. Detailed analysis of eruptive processes has been also made with Voldorad operating in a high sampling rate mode. Signature of individual outburst is clearly identified on the half part of Doppler spectra corresponding to rising ejecta: temporal variations of the backscattered power exhibit quasi periodic undulations, whereas the maximum velocity measured on each spectrum displays a sharp peak at the onset of each outburst followed by a slow decay with time. Periodicity of power variations (between 3.8 and 5.5 s) is in agreement with the occurrence of explosions visually observed at the SE vent. Maximum vertical velocities of over 160 m s^–1 were measured during the paraoxysmal stage and the renewed activity. Finally, by using a simplified model simulating the radar echoes characteristics, we show that when Voldorad is operating in high sampling rate mode, the power and maximum velocity variations are directly related to the difference in size and velocity of particles crossing the antenna beam.Editorial responsibility: A. Woods