Quantitative analytical methods are used to reconstruct the course of events during and after the cataclysmic eruption of Mount Tambora, Indonesia, on 10 and 11 April 1815. This was the world's greatest ash eruption (so far as is definitely known) since the end of the last Ice Age. This synthesis is based on data and methods from the fields of volcanology, oceanography, glaciology, meteorology, climatology, astronomy, and history.

A detailed chronology of major stratospheric dust veils from 1881 to 1960 has been constructed by searching the primary literature for relevant observational data of various kinds, especially pyrheliometry. Data from 23 observing sites in both hemispheres have been reduced in a rigorous fashion to yield a table of pyrheliometric optical depth perturbations as a function of year, month, and latitude band. To convert to visual reference wavelengths, the tabular entries should be multiplied by a time‐dependent factor that is somewhat greater than unity. Ten measurable volcanic dust veils have been established in this manner and have been analyzed as to their generation, transport, decay, and total mass. These clouds arose from eruptions of Krakatau (1883), an unidentified volcano (1890), Soufrière and Pelée (1902), Santa Maria (1902), Ksudach (1907), Katmai (1912), Puyehue (1921), Paluweh (1928), Komagatake (1929), and Quizapu (1932). Other turbidity indicators have also been used in the analysis, including starlight extinction, purple twilight glows, color of Sun and Moon, polarization of blue sky light, Bishop's ring around the Sun, and dark lunar eclipses. Global stratospheric aerosol loadings have been computed from the peak turbidities. Agreement with the aerosol masses derived from polar ice cores is good in the important cases of Krakatau, Santa Maria, and (after a major correction of the ice core value) Katmai. During the long period 1881–1992, about 80% of all stratospheric aerosols generated by the largest sulfur‐producing eruptions were emplaced during the two short time intervals 1883–1902 and 1982–1991. The long‐term (1881–1992) average annual production rate of stratospheric SO2 from the largest eruptions was 0.8 Tg yr−1, about half the average rate since 1981. Implications of the principal results are discussed.

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A chronology of the initiation dates of major continental flood basalt volcanism is established from published potassium-argon (K-Ar) and argon-argon (Ar-Ar) ages of basaltic rocks and related basic intrusions. The dating is therefore independent of the biostratigraphic and paleomagnetic time scales. Estimated errors of the initation dates of the volcanic episodes determined from the distributions of the radiometric ages are, approximately, plus or minus 4 percent. There were 11 distinct episodes during the past 250 million years. Sometimes appearing in pairs, the episodes have occurred quasi-periodically with a mean cycle time of 32 +/- 1 (estimated, error of the mean) million years. The initiation dates of the episodes are close to the estimated dates of mass extinctions of marine organisms. Showers of impacting comets may be the cause.