Richard B. Stothers

The Great Tambora Eruption in 1815 and Its Aftermath

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 worlds 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.

Flood Basalt Volcanism During the Past 250 Million Years

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.

Major optical depth perturbations to the stratosphere from volcanic eruptions: Pyrheliometric period, 1881–1960

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), Soufriere and Pelee (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, Bishops 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.

The great dry fog of 1783

A persistent dry haze hung over Europe during the second half of 1783. Spawned by the Laki basalt fissure eruption in southern Iceland, this fog evoked much contemporary written commentary, from which the course of events is here reconstructed in a quantitative way. It was the densest European dry fog since the late Middle Ages, and it lay primarily in the troposphere. Spreading broadly toward the south and east, it nevertheless remained mostly confined to the North Atlantic, western Eurasia, and the Arctic. Previously it was believed by many to have risen to the middle stratosphere and to have blanketed much of North America. Composed of sulfuric-acid aerosols, its total mass reached about 200 megatons, as determined from its observed optical thickness. Several authors have pointed out that it may have been responsible for the cold winter of 1783–84, which caused much economic and social distress in many parts of the Northern Hemisphere. As the earliest dry fog to be studied scientifically, it remains the paradigm even today and poses an interesting challenge to climate modelers.

Geological Rhythms and Cometary Impacts

Time-series analysis reveals two dominant, stable long-term periodicities approximately equal to 33 � 3 and 260 � 25 million years in the known series of geological and biological upheavals during the Phanerozoic Eon. Because the cycles of these episodes agree in period and phase with the cycles of impact cratering on Earth, these results suggest that periodic comet impacts strongly influence global tectonism and biological evolution. These two periodicities could arise from interactions of the solar system with interstellar clouds as the solar system moves cyclically through the Galaxy.

Climatic and Demographic Consequences of the Massive Volcanic Eruption of 1258

Somewhere in the tropics, a volcanoexploded violently during the year 1258, producing amassive stratospheric aerosol veil that eventuallyblanketed the globe. Arctic and Antarctic ice coressuggest that this was the worlds largest volcaniceruption of the past millennium. According tocontemporary chronicles, the stratospheric dry fogpossibly manifested itself in Europe as a persistentlycloudy aspect of the sky and also through anapparently total darkening of the eclipsed Moon. Basedon a sudden temperature drop for several months inEngland, the eruptions initiation date can beinferred to have been probably January 1258. Thefrequent cold and rain that year led to severe cropdamage and famine throughout much of Europe.Pestilence repeatedly broke out in 1258 and 1259; itoccurred also in the Middle East, reportedly there asplague. Another very cold winter followed in1260–1261. The troubled periods wars, famines,pestilences, and earthquakes appear to havecontributed in part to the rise of the Europeanflagellant movement of 1260, one of the most bizarresocial phenomena of the Middle Ages. Analogies can bedrawn with the climatic aftereffects and Europeansocial unrest following another great tropicaleruption, Tambora in 1815. Some generalizations aboutthe climatic impacts of tropical eruptions are madefrom these and other data.

VOLCANIC DRY FOGS, CLIMATE COOLING, AND PLAGUE PANDEMICS IN EUROPE AND THE MIDDLE EAST

Dry fogs spawned by large volcanic eruptions cool the climate by partially blocking incident sunlight and perturbing atmospheric circulation patterns. The climatic and epidemiological consequences of seven intense volcanic dry fogs of the past 21 centuries, detected in Europe and the Middle East, are investigated by using historical reports, supplemented by tree-ring data and polar-ice acidity measurements. The signal-to-noise ratio in the historical data is very high. In four cases, the first winter following the eruption was exceptionally cold. The eruptions preceding these frigid first winters are known, or strongly suspected, to have occurred at high northern latitudes. Two of the other dry fogs are linked unambiguously to tropical eruptions, after each of which the first winter was comparatively mild. The following few years tended to be cooler on the average in all six of the instances that can be checked. Famine and disease pandemics ensued, with the epidemics in all cases reaching the Mediterranean area within 1 to 5 years after the eruptions. In at least five cases, the contagion responsible for the mass mortality was probably plague.

Flood basalts and extinction events

The largest known effusive eruptions during the Cenozoic and Mesozoic Eras, the voluminous flood basalts, have long been suspected as being associated with major extinctions of biotic species. Despite the possible errors attached to the dates in both time series of events, the significance level of the suspected correlation is found here, by an objective, direct method, to be 1% to 4%. Statistically, extinctions lag eruptions by a mean time interval that is indistinguishable from zero, being much less than the average residual derived from the correlation analysis. Oceanic flood basalts, however, must have had a different biological impact, which is still uncertain owing to the small number of known examples and differing physical factors. Although not all continental flood basalts can have produced major extinction events, the non-correlating eruptions (including most or all of the oceanic flood basalts) may have led to smaller marine extinciton events that terminated at least some of the less catastrophically ending geologic stages. Consequently, the 26 Myr quasiperiodicity seen in major marine extinctions may be only a sampling effect, rather than a manifestation of underlying periodicity.

A New Explanation of the Blazhko Effect in RR Lyrae Stars

An interpretation of the modulating Blazhko effect in RR Lyrae stars is presented here, in which turbulent convection inside the hydrogen and helium ionization zones becomes cyclically weakened and strengthened owing to the presence of a transient magnetic field that is generated in situ by either a turbulent or a rotational dynamo mechanism. Successful predictions, both qualitative and quantitative, are made for the small changes of the primary pulsation period, the fluctuations of light and velocity amplitudes, the slow periodicity and irregularity of the Blazhko cycles, the restricted ranges of effective temperature for the RRab and RRc Blazhko variables, and the complex correlations between the primary period change, amplitude change, and mean effective temperature. Characteristic features of the predicted light and velocity curves at high and low amplitudes, even though they are based on radiative stellar models, agree well with the observed features, for the most part. The present theory of the Blazhko effect is simple enough that it does not require any basic change in our current understanding of RRab and RRc stars as being purely fundamental-mode and first-overtone radial pulsators. It also accounts naturally for the observed fact that hotter and cooler classes of periodic variable stars do not exhibit the Blazhko effect.

Dynamical instability as the cause of the massive outbursts in Eta Carinae and other luminous blue variables

A new type of stellar envelope structure has been computationally discovered at very high stellar masses. The outer part of the envelope resembles a nearly detached, diffusely filled shell overlying an ultrahot surface of small radius. This structural anomaly is caused by a large iron bump occurring in the new opacities of Iglesias et al. (1992). The new stellar models with normal metallicity encounter a strong ionization-induced dynamical instability in the outer envelope as they rapidly transit the H-R diagram after the end of central hydrogen burning. Preliminary evolutionary and hydrodynamical calculations successfully mimic the most basic observed properties of Eta Carinae and other very luminous blue variables. The Humphreys-Davidson sloped line in the H-R diagram, however, seems to be unrelated to these variables, and is instead the observed terminus of the main-sequence phase of evolution if convective core overshooting is insignificant.