Knowledge of the frequencies of highly explosive, moderately explosive, and nonexplosive eruptions would be useful in a variety of volcano studies. Historical records are generally incomplete, however, and contain very little quantitative data from which explosive magnitude can be estimated. Only the largest eruptions have a complete record back to the early 19th Century; other important explosive events went unrecorded prior to about 1960. Only a handful of the very biggest eruptions are represented in the geologic record, so it will be impossible to augment historical records post facto. A composite estimate of the magnitude of past explosive eruptions, termed the Volcanic Explosivity Index (VEI), is proposed as a semiquantitative compromise between poor data and the need in various disciplines to evaluate the record of past volcanism. The VEI has been assigned to over 8000 historic and prehistoric eruptions, and a complete list is available in a companion document.

[1] The 1783-1784 Laki flood lava eruption in Iceland emitted $122 megatons (Mt) SO 2 into the atmosphere and maintained a sulfuric aerosol veil that hung over the Northern Hemisphere for >5 months. The eruption columns extended to 9-13 km and released $95 Mt SO 2 into the upper troposphere/lower stratosphere (i.e., the polar jet stream), enforcing a net eastward dispersion of the plumes which reacted with atmospheric moisture to produce $200 Mt of H 2 SO 4 aerosols. Away from source, the Laki aerosols were delivered to the surface by subsiding air masses within anticyclones. We show that $175 Mt of H 2 SO 4 aerosols were removed as acid precipitation and caused the extreme volcanic pollution (i.e., dry fog) that effected Europe and other regions in 1783. The remaining $25 Mt stayed aloft at tropopause level for >1 year. The summer of 1783 was characterized by extreme and unusual weather, including an unusually hot July in western Europe, most likely caused by perseverance of southerly air currents. The following winter was one of the most severe winters on record in Europe and North America. In these regions, the annual mean surface cooling that followed the Laki eruption was about À1.3°C and lasted for 2-3 years. We propose that the upper troposphere/lower stratosphere aerosols from Laki disrupted the thermal balance of the Arctic regions for two summers and were the main mechanism for the associated climate perturbations. Eruptions of Laki magnitude have occurred in the recent past in Iceland and will occur again. If such an eruption were to occur today, one of the most likely immediate consequences would be disruption to air traffic over large portions of the Northern Hemisphere.

Knowledge of the frequencies of highly explosive, moderately explosive, and nonexplosive eruptions would be useful in a variety of volcano studies. Historical records are generally incomplete, however, and contain very little quantitative data from which explosive magnitude can be estimated. Only the largest eruptions have a complete record back to the early 19th Century; other important explosive events went unrecorded prior to about 1960. Only a handful of the very biggest eruptions are represented in the geologic record, so it will be impossible to augment historical records post facto. A composite estimate of the magnitude of past explosive eruptions, termed the Volcanic Explosivity Index (VEI), is proposed as a semiquantitative compromise between poor data and the need in various disciplines to evaluate the record of past volcanism. The VEI has been assigned to over 8000 historic and prehistoric eruptions, and a complete list is available in a companion document]. The problem also arises in studies of freqency of various kinds of volcanic events, for use both in understanding fundamental controls of volcanism and in quantifying volcanic hazards. To be useful, studies of historic volcanism need: (1) up-to-date, readily manageable historical information about eruptions, including dates and the nature of activity, and (2) a basis for comparing the scale or magnitude of each type of activity. We describe here a simple scheme for estimating explosive magnitude, with notes on its use and limitations. PREVIOUS WORK Compilations of Volcanological Information Information about historic eruptions is scattered widelythroughout the historical and geological literature. The Catalogue of Active Volcanoes and Solfatara Areas of the World (IAVCEI, Rome, Italy, 1950-1975) is an exceptionally valuable compilation of such information in a text and table format. ) provide valuable supplements to the Catalogue also in a text and table format. The most recent addition to these sources, and one which includes information from the sources mentioned above plus a comprehensive study of other literature, is another Smithsonian project, Volcanoes of the World, a regional directory, gazetteer, and chronology of volcanism in the last 10,000 years [Simkin et al., 1981]. This last source contains a more complete chronology than any previous compilation and is also in a flexible, computer-based format which allows for easy data retrieval. Compilations of Volcanic Eruption Records for the Purpose of Comparison With Climate RecordsLamb [1970, 1977] used atmospheric opacity, temperature records, and volcanological information to estimate the amount of dust introduced in to the upper atmosphere by each of approximately 250 eruptions. When determined from atmospheric opacity, his ratio of dust veil index/maximum extent (dvi/Emax) is a realistically direct measure of the dust injection. Unfortunately, dvi can only be evaluated in this way for the relatively small number of eruptions identifiable in atmospheric records. Early eruptions, small eru...

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