Since their first deployment in November 1978, the Total Ozone Mapping Spectrometer (TOMS) instruments have provided a robust and near-continuous record of sulphur dioxide (SO2) and ash emissions from active volcanoes worldwide. Data from the four TOMS satellites that have flown to date have been analysed with the latest SO2/ash algorithms and incorporated into a TOMS volcanic emissions database that presently covers 22 years of SO2 and ash emissions. The 1978-2001 record comprises 102 eruptions from 61 volcanoes, resulting in 784 days of volcanic cloud observations. Regular eruptions of Nyamuragira (DR Congo) since 1978, accompanied by copious SO2 production, have contributed material on approximately 30% of the days on which clouds were observed. The latest SO2 retrieval results from Earth Probe (EP) TOMS document a period (1996)(1997)(1998)(1999)(2000)(2001) lacking large explosive eruptions, and also dominated by SO2 emission from four eruptions of Nyamuragira. EP TOMS has detected the SO2 and ash produced during 23 eruptions from 15 volcanoes to date, with volcanic clouds observed on 158 days. The EP TOMS instrument began to degrade in 2001, but has now stabilized, although its planned successor (QuikTOMS) recently failed to achieve orbit. New SO2 algorithms are currently being developed for the Ozone Monitoring Instrument, which will continue the TOMS record of UV remote sensing of volcanic emissions from 2004 onwards.Volcanic eruptions vary greatly in style, duration and vigour, but all sub-aerial eruptions involve the emplacement of material, typically including water vapour and other gases, silicate ash, and aerosols, into the atmosphere above the eruption vent. The detection, analysis and tracking of the ensuing volcanic clouds and plumes is crucial for effective mitigation of volcanic hazards such as airborne ash (e.g. Casadevall 1994), understanding of magmatic degassing processes (e.g. Scaillet et al. 1998;Wallace 2001) and quantify-

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The 1994 eruption of Rabaul, in Papua New Guinea, involved a small plinian eruption at Vulcan and a vulcanian eruption on the opposite side of the caldera at Tavurvur. Vulcan's ash leachates indicate seawater interaction that is consistent with earlier observations of low sulfur dioxide emissions and the presence of ice crystals in the initial plinian eruption cloud. In contrast, Tavurvur ash leachates indicate no seawater interaction, and later sulfur dioxide emissions remained high despite low-level eruptive activity. Silicic melt inclusions indicate that the andesitic melt contained about 2 weight percent water and negligible carbon dioxide. Mafic melt inclusions in Tavurvur ash have water and carbon dioxide contents that vary systematically over the course of the eruption. The mafic melt inclusions suggest that a mafic dike intruded from below the silicic chamber and provide further evidence that mafic intrusions drive caldera unrest.

Satellite-based ultraviolet remote sensing of volcanic eruptions has produced quantitative measurements of the mass of sulfur dioxide and ash in volcanic clouds by accounting for ozone absorption and Rayleigh scattering in the atmos phere. These retrieval techniques were developed with data from the total ozone mapping spectrometer (TOMS) instruments on American, Russian, and Japanese satellites. The sulfur dioxide retrievals have been validated against groundbased Brewer and COSPEC measurements. The ash mass retrievals are in agreement with AVHRR two-band infrared ash retrievals. Daily satellite moni toring has detected, tracked, and quantified S0 2 emissions from a wide range of eruptive activity, from highly explosive to effusive types, and has produced an unprecedented 20-year record of global volcanism. Primary findings from the TOMS data are (1) observations of "excess sulfur" over that liberated during liq uid-phase degassing have indicated the existence of a volatile phase in pre empted magma; (2) indirect evidence for co-erupted H 2 S gas from apparent in crease in S0 2 mass in drifting clouds; (3) insights into the removal rates of S0 2 from the atmosphere, interactions with co-emitted ash particles, and responses to meteorological conditions; and (4) potential operational application of sulfur di oxide and ash detection for aviation hazard mitigation.

Since sulfur dioxide in volcanic eruption clouds was first measured by the total ozone mapping spectrometer (TOMS) in 1983 [Krueger, 1983], SO2 released from dozens of eruptions has been quantified. Such measurements are invaluable for addressing such problems as global climate change and dynamics, understanding the physical and chemical processes of magma chambers and volcanic eruptions, and volcanic hazards. However, validating the TOMS SO2 retrievals has been difficult due to the fleeting nature of volcanic clouds. An important goal of this validation program is to extend the range of eruptions that can be accurately measured by both ground and satellite instruments.

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Two Archean komatiitic pyroclastic rock units occuron opposite sides of the Quetico Fault in northwestern Ontario. The eastern unit, the Dismal Ashrock, is located 3 km north of Atikokan, Ontario, on the north side of the Quetico Fault within the Wabigoon Subprovince of the Superior Province. It is part of a suprascrustal sequence, the Steep Rock Group. The Grassy Portage Bay ultramafic pyroclastic rock unit (GUP) is located 100 krn to the west, on the south side of the Quetico Fault, and is part of an overturned succession comprising mafic metavolcanic rocks, GUP, and metasedimentary rocks. The Dismal Ashrock dips steeply, is little deformed, has undergone greenschist metamorphism, and is divided into komatiitic lapilli tuff, komatiitic volcanic breccia, komatiitic volcaniclastic rocks, and a mafic pillowed flow. GUP outcrops form an arcuate fold interference pattern, are strongly deformed, and have undergone amphibolite metamorphism. GUP is divided into komatiitic lapilli tuff and komatiitic volcanic breccia. Both pyroclastic units contain cored and composite lapilli, evidence for explosive volcanism. Locally, some of the lapilli fragments are highly vesicular (up to 30% by volume), greater than reported for any other komatiites. Other fragments show no vesicularity. The low vesicularity of some of the pyroclasts and, in the case of the Dismal Ashrock, their association with pillowed lava flows may indicate explosive hydrovolcanic activity. The Dismal Ashrock and GUP are high in MgO, Cr, and Ni and are unusually enriched in Fe, Ti, Zr, Mn, P, Ba, Nb, Rb, and Sr compared with other komatiites. These unique geochemical compositions are not understood at this time.Deux unites lithologiques de pyroclastites komatiitiques apparaissent de chaque cBtC de la faille Quetico, dans le nord-ouest de I'Ontario. L'unitC orientale, appelCe Dismal Ashrock, apparait B 3 km au nord d'Atikokan, Ontario, du cBtC nord de la faille Quetico, dans la sous-province de Wabigoon de la province du lac SupCrieur. Elle fait partie d'une stquence supracrustale, le Groupe de Steep Rock. L'unitC de pyroclastite ultramafique de Grassy Portage Bay (GUP) affleure B 100 km B I'ouest, du cBtC sud de la faille Quetico, et elle fait partie d'une succession inverste incluant les mCtavolcanites mafiques, GUP et les roches mCtasCdimentaires. L'unitt de Dismal Ashrock a un pendage fortement incline, est ltgttrement dCformte, mCtamorphisCe dans le facitts des schistes verts, et est subdiviske en lapilli-tuf komatiitique, breche volcanique komatiitique, volcano-clastites komatiitiques et coulCe mafique coussinke. Les affleurements de GUP prtsentent une interfkrence de plis arqds, sont intenskment dCform6s et sont m6tamorphisCs dans le faciks des amphibolites. GUP est divisCe en lapilli-tuf komatiitique et en brttche volcanique komatiitique. Les deux unites pyroclastiques contiennent des noyaux de lapilli et des lapilli composites, ce qui demontre un type de volcanisme explosif. Localement, certains des fragments de lapilli sont truffCs de vCsicules (jusqu'...