William I. Rose

Retrieval of sizes and total masses of particles in volcanic clouds using AVHRR bands 4 and 5

The advanced very high resolution radiometer (AVHRR) sensor on polar orbiting NOAA satellites can discriminate between volcanic clouds and meteorological clouds using two-band data in the thermal infrared. This paper is aimed at developing a retrieval of the particle sizes, optical depth, and total masses of particles from AVHRR two-band data of volcanic clouds. Radiative transfer calculations are used with a semi-transparent cloud model that is based on assumptions of spherical particle shape, a homogeneous underlying surface, and a simple thin cloud parallel to the surface. The model is applied to observed AVHRR data from a 13-hour old drifting cloud from the August 19, 1992, eruption of Crater Peak/Spurr Volcano, Alaska. The AVHRR data fit in the range of results calculated by the model, which supports its credibility. According to the model results, the average of effective particle radius in the test frame of this cloud is in the range of 2 to 2.5 μm, the optical depth at 12 μm is about 0.60–0.65. The total estimated mass of ash in the air amounts to 0.24–0.31×106 tons, which is about 0.7–0.9% of the mass measured in the ashfall blanket. Sensitivity tests show that the mass estimate is more sensitive to the assumed ash size distribution than it is to the ash composition.

Volatilization, transport and sublimation of metallic and non-metallic elements in high temperature gases at Merapi Volcano, Indonesia

Abstract Condensates, silica tube sublimates and incrustations were sampled from 500–800°C fumaroles and lava samples were collected at Merapi Volcano, Indonesia in Jan.–Feb., 1984. With respect to the magma, Merapi gases are enriched by factors greater than 105 in Se, Re, Bi and Cd; 104–105 in Au, Br, In, Pb and W; 103–104 in Mo, Cl, Cs, S, Sn and Ag; 102–103 in As, Zn, F and Rb; and 1–102 in Cu, K, Na, Sb, Ni, Ga, V, Fe, Mn and Li. The fumaroles are transporting more than 106 grams/day ( g d ) of S, Cl and F; 104–106 g/d of Al, Br, Zn, Fe, K and Mg; 103–104 g d of Pb, As, Mo, Mn, V, W and Sr; and less than 103 g d of Ni, Cu, Cr, Ga, Sb, Bi, Cd, Li, Co and U. With decreasing temperature (800-500°C) there were five sublimate zones found in silica tubes: 1) cristobalite and magnetite (first deposition of Si, Fe and Al); 2) K-Ca sulfate, acmite, halite, sylvite and pyrite (maximum deposition of Cl, Na, K, Si, S, Fe, Mo, Br, Al, Rb, Cs, Mn, W, P, Ca, Re, Ag, Au and Co); 3) aphthitalite (K-Na sulfate), sphalerite, galena and Cs-K. sulfate (maximum deposition of Zn, Bi, Cd, Se and In; higher deposition of Pb and Sn); 4) Pb-K chloride and Na-K-Fe sulfate (maximum deposition of Pb, Sn and Cu); and 5) Zn, Cu and K-Pb sulfates (maximum deposition of Pb, Sn, Ti, As and Sb). The incrustations surrounding the fumaroles are also chemically zoned. Bi, Cd, Pb, W, Mo, Zn, Cu, K, Na, V, Fe and Mn are concentrated most in or very close to the vent as expected with cooling, atmospheric contamination and dispersion. The highly volatile elements Br, Cl, As and Sb are transported primarily away from high temperature vents. Ba, Si, P, Al, Ca and Cr are derived from wall rock reactions. Incomplete degassing of shallow magma at 915°C is the origin of most of the elements in the Merapi volcanic gas, although it is partly contaminated by particles or wall rock reactions. The metals are transported predominantly as chloride species. As the gas cools in the fumarolic environment, it becomes saturated with sublimate phases that fractionate from the gas in the order of their equilibrium saturation temperatures. Devolatilization of a cooling batholith could transport enough acids and metals to a hydrothermal system to play a significant role in forming an ore deposit. However, sublimation from a high temperature, high velocity carrier gas is not efficient enough to form a large ore deposit. Re, Se, Cd and Bi could be used as supporting evidence for magmatic fluid transport in an ore deposit.

Eruptive history of Earth's largest Quaternary caldera (Toba, Indonesia) clarified

Single-grain laser-fusion {sup 40}Ar/{sup 39}Ar analyses of individual sanidine phenocrysts from the two youngest Toba (Indonesia) tuffs yield mean ages of 73{plus minus}4 and 501{plus minus}5 ka. In addition, glass shards from Toba ash deposited in Malaysia were dated at 68{plus minus}7 ka by the isothermal plateau fission-track technique. These new determinations, in conjunction with previous ages for the two oldest tuffs at Toba, establish the chronology of four eruptive events from the Toba caldera complex over the past 1.2 m.y. Ash-flow tuffs were erupted from the complex every 0.34 to 0.43 m.y., culminating with the enormous (2500-3000 km{sup 3}) Youngest Toba tuff eruption, caldera formation, and subsequent resurgence of Samosir Island. Timing of this last eruption at Toba is coincident with the early Wisconsin glacial advance. The high-precision {sup 40}Ar/{sup 39}Ar age eruption of such magnitude may provide an important marker horizon useful as a baseline for research and modeling of the worldwide climatic impact of exceptionally large explosive eruptions.

Dispersal of ash in the great Toba eruption, 75 ka

One of Earth9s largest known eruptions, the Toba eruption of 75 ka, erupted a minimum of 2800 km 3 of magma, of which at least 800 km 3 was deposited as ash fall. This ash may be entirely of coignimbrite origin and dispersed widely because of high drag coefficients on the predominantly bubble-wall shards. Shards of this shape are broken from the walls of spherical vesicles, which formed in high abundance in isotropic strain shadows near phenocrysts in this crystal-rich magma.

Contribution of C1- and F-bearing gases to the atmosphere by volcanoes

As halogen gases catalyse the destruction of ozone in the stratosphere1–3, it is important to quantify the natural emissions of halogens from active volcanoes. Here we use equilibrium thermodynamics to predict the speciation of Cl and F in volcanic gases and provide new estimates of the global emission rates to the atmosphere. Our calculations show that HCl and HF are the dominant species of Cl and F in volcanic gases, at least several orders of magnitude more abundant than all other species. We estimate the annual global volcanic fluxes of HCl and HF to be 0.4–11 Tg (1012 g) and 0.06–6 Tg, respectively. On average, <10% of these emissions come in large explosive eruptions that transmit them efficiently to the stratosphere. Although they are infrequent, large volcanic eruptions may inject significant amounts of HCl and HF into the stratosphere. Passively degassing volcanoes are also a major source of tropospheric HF, although sea salt is the main source of tropospheric HC1.

Scavenging of volcanic aerosol by ash: Atmospheric and volcanologic implications

The initial concentrations of S (1,600 ppm) and Cl (1,100 ppm) in the high-Al2O3 basaltic magma from the 1974 eruptions of Fuego Volcano, Guatemala, were inferred from trapped glass inclusions in phenocrysts. During the explosive eruptions, as much as 33% of the S and 17% of the Cl fell quickly back to Earth as acid aerosol particles absorbed on the ash. An additional 5% of the S and 20% of the Cl was trapped in the silicate ash. The remaining S and Cl was released to the atmosphere. By estimating the volume of ash and applying the above values for S and Cl, 2.2 × 1011 g of S and 1.6 × 1011 g of Cl were calculated to be the atmospheric contribution of the 1974 Fuego eruption. These figures are minimum values because an undetermined amount of intrusive magma may have contributed volatiles to the eruption. Airborne in-the-plume measurements, together with the existing approaches, are the best way to eliminate this uncertainty. The absolute concentrations of scavenged elements on ash are seen to be a function of plume flux and particle trajectory, both of which vary greatly during an eruption. Intense pyroclastic activity produces higher S/Cl ratios in the coating acquired by the ash particles; this implies that there are higher S/Cl ratios in volcanic gas during more explosive phases of an eruption.

Origin, speciation, and fluxes of trace-element gases at Augustine volcano, Alaska: Insights into magma degassing and fumarolic processes

Thermochemical modeling predicts that trace elements in the Augustine gas are transported from near-surface magma as simple chloride (NaCl, KCl, FeCl2, ZnCl2, PbCl2, CuCl, SbCl3, LiCl, MnCl2, NiCl2, BiCl, SrCl2), oxychloride (MoO2Cl2), sulfide (AsS), and elemental (Cd) gas species. However, Si, Ca, Al, Mg, Ti, V, and Cr are actually more concentrated in solids, beta-quartz (SiO2), wollastonite (CaSiO3), anorthite (CaAl2Si2O8), diopside (CaMgSi2O6), sphene (CaTiSiO5), V2O3(c), and Cr2O3(c), respectively, than in their most abundant gaseous species, SiF4, CaCl2, AlF2O, MgCl2 TiCl4, VOCl3, and CrO2Cl2. These computed solids are not degassing products, but reflect contaminants in our gas condensates or possible problems with our modeling due to “missing” gas species in the thermochemical data base. Using the calculated distribution of gas species and the COSPEC SO2 fluxes, we have estimated the emission rates for many species (e.g., COS, NaCl, KCl, HBr, AsS, CuCl). Such forecasts could be useful to evaluate the effects of these trace species on atmospheric chemistry. Because of the high volatility of metal chlorides (e.g., FeCl2, NaCl, KCl, MnCl2, CuCl), the extremely HCl-rich Augustine volcanic gases are favorable for transporting metals from magma. Thermochemical modeling shows that equilibrium degassing of magma near 870°C can account for the concentrations of Fe, Na, K, Mn, Cu, Ni and part of the Mg in the gases escaping from the dome fumaroles on the 1986 lava dome. These calculations also explain why gases escaping from the lower temperature but highly oxidized moat vents on the 1976 lava dome should transport less Fe, Na, K, Mn and Ni, but more Cu; oxidation may also account for the larger concentrations of Zn and Mo in the moat gases. Nonvolatile elements (e.g., Al, Ca, Ti, Si) in the gas condensates came from eroded rock particles that dissolved in our samples or, for Si, from contamination from the silica sampling tube. Only a very small amount of rock contamination occurred (water/rock ratios between 104 and 106). Erosion is more prevalent in the pyroclastic flow fumaroles than in the summit vents, reflecting physical differences in the fumarole walls: ash vs. lava. Trace element contents of volcanic gases show enormous variability because of differences in the intensive parameters of degassing magma and variable amounts of wall rock erosion in volcanic fumaroles.

Fumarole incrustations at active central american volcanoes

In 11 yr of sampling at 14 volcanoes in Guatemala, El Salvador, Nicaragua and Costa Rica, we have identified 47 minerals in incrustations depositing at approximately 100 different high temperature fumaroles. Most of these minerals are sulfates. The most abundant and most frequently found minerals are: sulfur, hematite, halite, sylvite, gypsum, ralstonite, anhydrite, thenardite and langbeinite. Incrustation suites deposit around fumaroles to produce a zonal pattern which is a response to the rapidly changing temperature and oxygen pressure at the mouth of the vent. The observed zoning pattern can be explained by the reaction of a volcanic gas composed of H2O, SO2, CO2, HC1 and HF, along with trace amounts of volatile cations, which interacts with the atmosphere and the fumarole wallrock. This interaction is aided at lower temperatures by the formation of sulfuric acid. The mineralogies and descriptions of incrustations at fumaroles at a large number of other volcanoes from every part of the world are similar to what we have found in Central America. Thus we believe our conclusions have general applicability.

Integrating retrievals of volcanic cloud characteristics from satellite remote sensors: a summary

Volcanic eruptions are events that rapidly and suddenly disperse gases and fine particles into the atmosphere, a process most conveniently studied from the synoptic satellite perspective, where remote sensing offers a practical tool for spatial and temporal measurements. Meteorological satellites offer approximately 20 years of archived data, which can be analysed for measurements of masses of SO2 and fine volcanic ash in spatial two–dimensional arrays and integrated with other meteorological data. The satellite data offer a tool to study volcano–atmosphere interactions in a quantitative way. They provide information of unique value for understanding the fate and transport of fine silicates with significant health hazards and for addressing the problem of volcanic cloud hazards to jet aircraft. Studies of satellite data have demonstrated the following. (1) Volcanic clouds from convergent plate boundary volcanoes contain large and variable excesses of SO2. (2) The second day of atmospheric residence for volcanic clouds has significantly higher SO2 than the first, suggesting that early volcanic H2S may be converting to SO2. (3) Complete conversion of SO2 to sulphate in the stratosphere occurs at an efolding rate of approximately 120 days. SO2 loss from stratospheric volcanic clouds occurs at an e–folding rate of approximately 35 days, and the SO2 loss rate for volcanic clouds which only barely reach the stratosphere is rapid (efolding only a few days). The latter limits the stratospheric aerosol build–up from smaller eruptions. (4) Fine volcanic ash (with diameters of less than ca.25μm) in drifting volcanic clouds retrieved after 10 h or more appear to represent a small fraction (less than 2% of the total mass) of the total mass of magma erupted, and also a small fraction (less than 20%) of the total mass of fine ash erupted. This is probably explained by the fact that the total mass is greatly reduced by aggregation processes within the volcanic cloud. (5) The amounts of fine ash decrease faster in volcanic clouds of larger eruptions, supporting the self–removal processes suggested by Pinto et al. in 1989. (6) Evidence for a strong role of ice in the fallout and aggregation of volcanic cloud ash is considerable. (7) In many cases, volcanic clouds separate into higher SO2–rich portions and lower ash–rich portions. The two portions follow different trajectories and the lower, ash–rich portions are affected by interactions with moist tropospheric air.

Evaluation of gases, condensates, and SO2 emissions from Augustine volcano, Alaska: the degassing of a Cl-rich volcanic system

After the March–April 1986 explosive eruption a comprehensive gas study at Augustine was undertaken in the summers of 1986 and 1987. Airborne COSPEC measurements indicate that passive SO2 emission rates declined exponentially during this period from 380±45 metric tons/day (T/D) on 7/24/86 to 27±6 T/D on 8/24/87. These data are consistent with the hypothesis that the Augustine magma reservoir has become more degassed as volcanic activity decreased after the spring 1986 eruption. Gas samples collected in 1987 from an 870°C fumarole on the andesitic lava dome show various degrees of disequilibrium due to oxidation of reduced gas species and condensation (and loss) of H2O in the intake tube of the sampling apparatus. Thermochemical restoration of the data permits removal of these effects to infer an equilibrium composition of the gases. Although not conclusive, this restoration is consistent with the idea that the gases were in equilibrium at 870°C with an oxygen fugacity near the Ni−NiO buffer. These restored gas compositions show that, relative to other convergent plate volcanoes, the Augustine gases are very HCl rich (5.3–6.0 mol% HCl), S rich (7.1 mol% total S), and H2O poor (83.9–84.8 mol% H2O). Values of δD and δ18O suggest that the H2O in the dome gases is a mixture of primary magmatic water (PMW) and local seawater. Part of the Cl in the Augustine volcanic gases probably comes from this shallow seawater source. Additional Cl may come from subducted oceanic crust because data by Johnston (1978) show that Cl-rich glass inclusions in olivine crystals contain hornblende, which is evidence for a deep source (>25km) for part of the Cl. Gas samples collected in 1986 from 390°–642°C fumaroles on a ramp surrounding the inner summit crater have been oxidized so severely that restoration to an equilibrium composition is not possible. H and O isotope data suggest that these gases are variable mixtures of seawater, FMW, and meteoric steam. These samples are much more H2O-rich (92%–97% H2O) than the dome gases, possibly due to a larger meteoric steam component. The 1986 samples also have higher Cl/S, S/C, and F/Cl ratios, which imply that the magmatic component in these gases is from the more degassed 1976 magma. Thus, the 1987 samples from the lava dome are better indicators than the 1986 samples of degassing within the Augustine magma reservoir, even though they were collected a year later and contain a significant seawater component. Future gas studies at Augustine should emphasize fumaroles on active lava domes. Condensates collected from the same lava-dome fumarole have enrichments ot 107–102 in Cl, Br, F, B, Cd, As, S, Bi, Pb, Sb, Mo, Zn, Cu, K, Li, Na, Si, and Ni. Lower-temperature (200°–650°C) fumaroles around the volcano are generally less enriched in highly volatile elements. However, these lower-termperature fumaroles have higher concentration of rock-forming elements, probably derived from the wall rock.