Introduction
As a recent volcanic eruption demonstrated, air travel â a convenient, speedy, and affordable means of modern transportation â was brought to a standstill by the explosive emissions from Mt. Eyjafjallajökull in Iceland from which a tephra plume (consisting of ash, dust, and solid rock) reached 5.6 mi (9 km) into the stratosphere in April 2010. At no time since the end of World War II had air traffic in the northern hemisphere experienced such a disruption. Although this particular eruption was short-lived, others, which were of a longer duration, changed the global climate.
During the last 46,000 years, major eruptions have occurred on average of one every 80 years. Some volcanoes have erupted for several consecutive years and multiple times in a single year. They discharged millions of cubic kilometers of ash into the atmosphere. As these eruptions continued, sulfur dioxide (SO2), the most active chemical agent emitted by volcanoes, combined with large quantities of emitted water vapor (H2O) and oxidized into sulfuric acid (H2SO4) aerosols. These aerosols spread around the world, reflecting the sunâs radiation and causing a decrease in global temperatures. So, an initial warming of the climate system caused by a volcanic eruption was soon followed by a cooling.
The radiative effect of volcanic aerosols generates general stratospheric (upper atmosphere) heating and tropospheric (lower atmosphere) cooling, and a tropospheric warming pattern in the winter.2 This occurs because volcanic sulfuric aerosol clouds in the stratosphere absorb heat reflected from Earth while preventing sunlight from reaching Earthâs surface. As a result, the stratosphere (upper atmosphere) heats up, while ash and pumice spread a haze across the troposphere (lower atmosphere), lowering global temperatures by a degree or more.3 Unlike the ash residue from the combustion of wood, paper, and coal, volcanic ash is composed of jagged bits of rocks, minerals, and glass, often smaller than two millimeters in size. Breathing in such a substance is lethal to humans and animals alike, and when combined with moisture, it forms wet cement-type substance capable of obliterating everything in its path.
The case studies that follow explore natural world supervolcanic catastrophes: Mt. Toba (73,000 years ago), Santorini (circa 1660â1613 BCE), Tambora (1815 CE), Krakatau (1883 CE), and the most recent large volcanic eruption, Mt. Pinatubo, in 1991.
These five accounts of highly disruptive and lethal eruptions fall into the category of supervolcanoes, a rare geological event that may not occur for hundreds of thousands of years. Such rare events pose threats to life on the planet. The eruption of Mt. Toba on the island of Sumatra around 73,000 years ago falls into this category. The second supervolcano, on the island of Thera (modern-day Santorini), circa 1660â1613 BCE, may have accelerated the collapse of Minoan civilization, sending tsunamis across the Aegean Sea. Unlike Mt. Toba, planetary life remained intact. Theraâs volcanic eruption, however, threatened the regionâs advanced commercial and economic life and its status as a thriving society. The third and fourth supervolcanoes erupted in the nineteenth century, Tambora in 1815 and Krakatau in 1883, both located in the Indonesian Volcanic Arc close to the Mt. Toba event many thousands of years before. Both disrupted atmospheric conditions in the northern hemisphere and were highly destructive to persons and property. The eruption of Mt. Pinatubo in the Philippines on June 15, 1991, destroyed property with minimal loss of life. This modern example of a supervolcano illustrates the importance of early warning systems and local people being alert.
The Mt. Toba Eruption (73,000 BP)
Supervolcanoes, those rare geological events, pose the most catastrophic threat to life on this planet. No super-eruption of Mt. Tobaâs magnitude has occurred during the Common Era (CE). It is the most studied of all known supervolcanoes in geological time, even though no precise measurements exist of its ejected material in either solid or gaseous states. Only cylinders of ice from the Greenland Sheet Project 2 (GISP2), measured for trapped gases, provide information about the magnitude of the Toba eruption.
The eruption was identified as the Younger Toba Tuff (YTT), to distinguish it from previous supervolcanoes at the same site. The Middle Toba Tuff (500,000 years Before Present; BP), the Oldest Toba Tuff (840,000 years BP), and the Haranggoal Dacite (1.2 million years BP) preceded it. The magma reservoirs from these older eruptions remained thermally stagnant for thousands of years, while the magma from the YTT began to accumulate and become more active for at least 100,000 years before its eruption.4
Its eruption expelled 6.7 mi3 (2,800 km3) of dense lava. The largest recorded eruption of Mt. Tambora in Indonesia in 1815 (whose effects historians have identified as âthe year without a summerâ) was minimal when compared to the Younger Toba Tuff. Scientists estimated its explosive capacity at 3,500 times greater than the 1815 explosion. As fractures opened in the roof of YTTâs magma chamber, massive outflows of dense magma covered 7,700 mi2 (19,930 km2) of Sumatra during a 9â14-day period. Combining the magnitude of the eruption with its intensity of around 7.8 million tons per second (7.1 billion kg/s), scientists estimated that the plume height was 20 ± 3 mi (32 ± 5 km).5
Once the plume became airborne, its gas content of H2S (hydrogen sulfide) oxidized rapidly into H2SO4 (sulfuric acid). Approximately 3.8 million tons (3.5 billion kg) of H2S would oxidize into 11 million tons (10 billion kg) of H2SO4. However, since these estimates are subject to re-evaluation with further research, they should be accepted cautiously.6 Confirmation of such high readings in the atmosphere is recorded in the Greenland Ice Sheet. With an error margin of five years, these high readings about 70,000 years ago are greater than at any time in the 110,000-year record of the GISP2.7
Additional evidence of the magnitude of this monumental event is found in the deposits of Toba tephra ejected during the volcanic explosion and spread across a wide expanse of the Indian Ocean and the South China Sea and beyond, including the surrounding areas of Borneo, Sumatra, Sri Lanka, Malaysia, Vietnam, and the Arabian Sea. Mt. Toba deposited at least 1,000 yd3 (765 m3) of ash and several gigatons of volcanic gases across this vast expanse. Additional discoveries of YTT tephra appear as deep-sea drilling continues to date ash deposits of 4 in (10 cm) or more. These covered at least 1% of Earthâs surface with 3,700 yd3 (2,829 m3) of dense rock equivalent (DRE). Researchers have uncovered as much as 35 in (90 cm) of ash in Malaysia. With each new discovery, the magnitude of this supervolcano grows and its estimated impact on the global climate system becomes clearer.
Scientists Michael R. Rampino and Stephen Self argued that the possible effects of the Toba eruption was the onset of a âvolcanic winterâ creating conditions suggested by the exchange of nuclear weapons that would create a cloud of ash caused by fireballs blocking out the energy from the sun. In the latitudes from 30° to 70°N, drops in temperatures ranging from 9â27°F (5â15°C) would have been possible, with deep freezes in the mid-latitudes. Temperatures below normal in the range of 5.4â9°F (3â5°C) may have continued for several years. More snow and sea ice would have accumulated, thereby accelerating cool conditions and leading to decades of long winters and shorter summers.8
Evidence from the Vostok ice core from Antarctica indicates that the global drop in temperatures of 7.2°F (4°C) occurred between 80,000 and 75,000 years ago.9 Although Rampino and Self acknowledged that global cooling had begun before the Toba super-eruption, they believed that the aerosol cloud accelerated the push toward full glacial conditions. Although the cloud cover would disperse in the short term, elevated levels of sulfuric acid (H2SO4) and the clouds that it created would remain in the atmosphere for at least five years after the eruption. The estimated amounts of stratospheric H2SO4 from the supervolcano range from 100 megatons (Mt) to 10 gigatons (Gt), with 6Gt of sulfuric acid remaining in the stratosphere for five years, a figure that coincides with the amounts found in the ice cores.10
The consensus among scientists suggests that the Toba supervolcano may have been a contributing factor to the instability of the climate. A decrease in ocean temperatures by 50°F (10°C) for the next millennium, explaining the expansion of ice fields in the northern hemisphere, a cooling in China and in the Pacific Ocean, all point to Toba as a causal factor.11 More speculative is the suggestion that:
climate change could have triggered the eruption via the impact of sea-level change on seismicity along the Great Sumatra Fault that runs the length of the island, accommodating oblique convergence between the Eurasian and Australasian plates. Major earthquakes have been implicated as eruption triggers and the location of Toba itself may be related to the apparent bend in the Great Sumatra Fault which could have promoted the development of large, long-lived magma reservoirs there in the first place.12
For example, one climate-change scientist has evaluated the 110,000-year record of the GISP2 and noted the correspondence of major volcanic eruptions with periods of changing climatic conditions. âThese data support the suggestion that environmental change associated with climate change has the potential to increase volcanism.â13
More recent research has suggested that even minor changes in climate can alter the position of Earthâs tectonic plates and trigger volcanic eruptions and earthquakes. The conference on Climate Forcing of Geological and Geomorphological Hazards held in London in September 2009 suggested that climate change âcould tip the planetâs delicate balance and unleash a host of geological disasters.â14 Renewed focus has been directed at rising sea levels caused by melting glaciers as the global climate warms. Added weight puts pressure on the fluids in porous rock beneath the seabed. According to researchers, this minor change in pressure may be enough to alter the frictional force that stabilizes tectonic plates and holds them in place. A small change can have a significantly larger effect.
Since most of the worldâs volcanoes are located within a few miles of the shorelines of the worldâs oceans, the melting of large ice sheets int...
