Australia: The Land Where Time Began
Super Eruptions and Super Volcanoes (Megacalderas)
Super-eruptions have been described as the ultimate geologic hazard (Self & Blake, 2008). The term ‘super-eruption’ was first used as a volcanological term in a paper published in 1992 (Rampino & Self, 1992) to describe the Youngest Toba Tuff eruption that occurred 74 ka in northern Sumatra. The BBC was responsible for making the term popular in volcanology and with the general public by a broadcast in 2000. It is broadly understood (e.g. Sparks et al., 2005) as defining a pyroclastic eruption of magnitude (M) 8 or higher, where (Pyle, 2000):
M = log10 (mass of erupted material in kg) – 7
This threshold representing a mass of 1015 kg, and the bulk volume of a super-eruption would deposit a bulk volume of 1,000 km3, which, according to Oppenheimer & Donovan1 corresponds to a dense magma volume of approximately 450 km3. Though silicic (effusive) eruptions can be very large, none have been identified to date that approach this volume.
The term ‘super-volcano’ can be as contentious as ‘super-eruption’, though it is also commonly used in scientific literature and Oppenheimer & Donovan1 consider the definition that has been proposed (Miller & Wark, 2008) to be appropriate: a ‘super-volcano is a volcano that is associated with 1 or more super eruptions. It is important to note the considerable uncertainty involved in the reporting of the magnitude of eruption volumes and masses, notwithstanding the use of the logarithmic term used in the previous equation. Estimates of super-eruption deposits are typically based on estimating or mapping the thickness of pyroclastic (fall and current) deposits over a sufficiently representative geographic range. This requires estimates of relevant densities of the diverse materials to convert to mass (as required in the previous equation). It may be feasible to make direct measurements of mass per unit area for distal tephra fall deposits. To account for unmapped material an extrapolation or model will be required in all cases, whether it is a very thin (and potentially not preserved) ultra-distal ash fall deposits, or proximal ignimbrite that has unseen bases that rest on pre-eruption topography that is not known (e.g. Bonadonna & Costa, 2013; Burden et al., 2013). Oppenheimer & Donovan1 say many of the estimates of magnitudes of super-eruptions that have been published were based on limited deposit data and/or ‘back of the envelope’ calculations of the volumes of intra-caldera deposits, outflow sheets and tephra fallout (e.g. Rose & Chesner, 1990; Mathews et al., 2012; Gatti & Oppenheimer, 2013; costa et al., 2014 for a digest of the estimates of the Youngest Toba Tuff eruption magnitude.
Calderas are typically formed by such events as super-eruptions, given the large volumes of magma that are expelled from the crust. Therefore, the large caldera that remains following a super-eruption is the most distinctive feature (Geyer & Marti, 2008). Generally these calderas are at least 20 km in diameter, with some closer to diameters of 100 km (Lipman, 1997). In very large eruptions the timing of collapse can be critical in the dynamics of these eruptions as the ring fractures that develop above the evacuating chamber can promote rates of magma discharge that are even higher associated with the discharge of prodigious volumes of pyroclastic currents at the surface. Up to ⅓ to ½ of the ejecta can be accommodated in the depression that is formed (Mason et al., 2004). At the present many large calderas feature a ‘resurgent centre’ that typically domes the intra-caldera deposits upwards. Oppenheimer & Donovan1 say the origins of this structural uplift are not certain, though it probably reflects a combination of magma reservoir recharge and volatile exsolution (Kennedy et al., 2012; Wilcock et al., 2013).
Magma bodies that associated with supervolcanoes (megacalderas)
According to Oppenheimer & Donovan1 it is clear that in a super-eruption the magma involved was accumulated before the eruption, when the large volume of magma that is discharged by such an eruption is considered, which is exclusively silicic (dacitic through to rhyolitic) petrological affinities. The associated timescales of the assembly of very large magma chambers may take from hundreds of thousands of years to only a few thousand years, as indicated by a range of geochemical and mineralogical and geochronological studies (e.g. Wilson & Charlier, 2009; Druitt et al., 2012). As a result of the lack of mobility to erupt in the case of long-lived magmas that are crystal-rich (‘mushes’) intrusion of hotter mafic or intermediate magmas (e.g. Bachmann et al., 2002; Wotzlaw et al., 2013), may result in rejuvenation of the magma in the chamber, potentially on short timescales (Burgisser & Bergantz, 2011). The particular circumstances that ‘keep a lid on’ a reservoir of buoyant magma, which may ultimately feed a super eruption, is a related question. Though super-volcanoes consistently appear to be located on crust that is thick and of relatively low density, which promotes the deep accumulation of magmas, super-eruptions are not the only style of eruption in these volcanoes, interspersed among the massive events are smaller intrusions, explosions and extrusions (e.g. Charlier et al., 2005 on the ‘hyperactive’ Taupo volcano).
The evolutionary timescales of magma bodies have been constrained by the wide use of radioactive-decay series. The discrimination between the chemical processes that generate the silicic magma responsible for super-eruptions, and the assembly timescales, that are potentially more rapid, for the assembly and rejuvenation of a reservoir that is eruptible (Allan et al., 2013), is an important consideration in such studies. An example is observed in minerals and glasses from the Bishop Tuff; when rubidium-strontium isotope measurements were carried out on these minerals and glasses gave ages that were at least 1 million years older than the actual eruption (Halliday et al., 1989; Christensen & Halliday, 1996), which suggests a magma system that was very long-lived which was tapped intermittently during eruptions. Timescales of a few hundreds of thousands of years of crystallisation prior to the eruption is suggested by the results of uranium-lead and uranium-thorium dating on crystals of zircon and allanite that were isolated from deposits of tephra. Evidence was found (Vazquez & Reid, 2004) for part of the magma body associated with the Youngest Toba Tuff, which was of M 8.8, being in place up to 150 ka prior to eruption. A geochemical and geochronological study was carried out on the Fish Canyon Tuff, an M9 eruption that occurred 28 Ma, associated with the La Garita caldera, Colorado, which suggests a magmatic history of 440 ka, during a significant part of which reheating of the pluton took place, as up to 80 % of the magma had been crystallised (Wotzlaw et al., 2013).
Oppenheimer & Donovan1 say that evidence that is emerging concerning the thermo-mechanics of magma chambers may give some meaning to discriminating between super-eruptions and ‘ordinary’ eruptions. The thermal properties of host rocks and how they respond to repeated intrusions of magma that inflated the chamber incrementally have been modelled (Caricchi et al., 2014). According to their argument M 6 eruptions tend to be triggered by the development of overpressure in smaller chambers in the upper crust as they are repeatedly supplied with magma. The elastic behaviour of the rocks forming the wall, which are relatively cold, is a reflection of this behaviour and their propensity to fail under tensile stresses. Each event with such volcanoes only partially empties the magma chamber, though their eruptions can be relatively frequent. The chamber enlarges over time, with a net magma supply from depth that exceeds the time-averaged outputs; the chamber enlarges (e.g. Reid, 2008). The rocks comprising the walls heat up and deform in a more viscous fashion as the volume increases such that there is a reduction in the chamber overpressure. Instead of the overpressure triggering eruptions as the storage of magma increases, it is the buoyancy of the magma, with a density lower than that of the host rocks, that becomes the driving force of the eruption. The thickness of the reservoir is then the critical condition for a super-eruption, as it influences the competition between the supply of magma and cooling and crystallisation, the latter process being enhanced in reservoirs that are thinner and wider.
Oppenheimer & Donovan1 suggest the inflection that is apparent in the magnitude-frequency relationship for global (terrestrial) volcanism between events that measure M 7 and M 8 might be explained by this theoretical treatment. Sophisticated measurements of the density of silicic melts at realistic pressures and temperature of the magma provides further support for the hypothesis that it is the buoyancy of the magma that drives super-eruptions (Malfait et al., 2014).
Oppenheimer & Donovan1 address the question of whether or not super-eruptions differ qualitatively from lesser eruptions. The Mount Pinatubo eruption in 1991, at M 6.1, was the largest eruption of the 20th century. A crescendo of precursory activity occurred over the preceding moths leading to a climactic eruption on 15 June 1991. The most intense phase of the plinian eruption, which continued for about 3.5 hours (Scott et al., 1996), was accompanied by the collapse of parts of the ash and gas plume, with the result that there were substantial pyroclastic density currents and a co-ignimbrite ash cloud. The crater that remained after the eruption was 2.5 km wide.
To get close to the M 7 class in the modern period, when there some witness accounts available, there is the Tambora eruption on the island of Sumbawa, Indonesia, in 1815 of M 6.9. From contemporary reports and what can be inferred from the deposits, the style of the eruption of Pinatubo in 1991, during which increasing intensity of the eruption led to the collapse of a plinian eruption column that generated large volumes of pyroclastic density currents and the associated co-ignimbrite plumes (Sigurdsson & Corey, 1989; Oppenheimer, 2003). There are no clear indications of this paroxysmal phase, which makes it difficult to determine to what extent the greater size of Tambora relative to Pinatubo is due to the intensity or duration; Oppenheimer & Donovan1 suggesting it is probably a combination of both. A comparable M 7 eruption of the Rinjani volcano, Lombok Island, Indonesia, has been identified and dated to the mid-13th century (Lavigne et al., 2013).
These eruptions can only be inferred for eruptions of M 8 and greater, though intensities and durations of lesser events are somewhat constrained. It has been estimated that the climactic phase of the Bishop Tuff eruption of M 8.3, which occurred 0.76 Ma, which is associated with the Long Valley caldera in California may have been less than 1 week (Wilson & Hildreth, 1997). Intermittent bursts of activity spanning a few years may have been involved in other super-eruptions (Wilson, 2008), though it may have possibly been over a few centuries (e.g. Ellis et al., 2012; Svensson et al., 2013).
Wet versus dry eruptions
At M 8.1 the Oruanui eruption of Taupo volcano in New Zealand is the most recent documented eruption which erupted about 25.4 ka. Rift tectonics triggered and modulated this eruption (Allen et al., 2012), and interaction between magma and lake water were involved in episodes of the eruption (Van Eaton and Wilson, 2013). Consequently, some of the pyroclastic deposits are of particularly fine-grained material, which reflects the magma fragmentation processes that are influenced by water at the vent. The dynamics of explosive eruptions may be impacted by the phase changes associated with interaction between water and magma, the result being alternating phases of Plinian eruption and fountain collapse, and injection of ash into the atmosphere at multiple heights (Van Eaton and Wilson, 2012). Oppenheimer & Donovan1 suggest that as many supervolcanoes (megacalderas) are presently at least partially occupied by water bodies further research in this domain is warranted.
Megacalderas – location and eruption times
Eruptions of M 8 or larger over a time span of 36 Myr totalling 42 have been catalogued (Mason et al., 2004). These eruptions were found to be clustered strongly in time and space, clusters being between 36 Ma and 27 Ma, and from 13.5 Ma to the present. Megacalderas have been found to be associated with mantle plumes, rifts and subduction zones, though tectonics may play a role in triggering eruptions of these very large volcanoes. When the geographic distribution of events is considered strong biases in the dataset are readily evident, with all but 4 of them being present in the Americas, 32 in North America and 6 in the central Andes. The remaining 4 are Toba in Sumatra, Taupo in New Zealand and on the Ethiopian plateau. The ‘large magnitude explosive volcanic eruptions’ (LaMEVE) database that has been compiled more recently listed 20 M 8 and above eruptions that span the past 2.2 Myr, which Oppenheimer & Donovan1 say suggests a return period of about 100,000 years (Crosweller et al., 2012). A notable difference between the (Mason et al., 2004) and LaMEVE databases is that in the latter identifies 6 megacalderas from the Quaternary in Japan. It is likely, however, that there are many other super-eruption deposits still to be discovered or recognised (Donovan & Oppenheimer, 2014).
A case has recently been made for the occurrence of super eruptions on Mars (Michalski & Bleacher, 2013), though the geomorphological evidence presented may not be sufficiently compelling for some volcanologists to accept the proposal.
It is apparent that the deposits of super-eruptions can cover a wide area, though they can also be concentrated locally at the site of the associated caldera.
The areal extent of tephra fall is one of the astonishing aspects of super-eruptions, sourced largely – and possibly exclusively – from the co-ignimbrite plumes (e.g. Matthews et al., 2012). For example, at several terrestrial sites in India deposits of ash of the Youngest Toba Tuff has been identified; and in many deep-sea sediment cores from as far away as the Arabian Sea, 4,000 km from Toba; and in cores from the bed of Lake Malawi (Lane et al., 2103). The potential to investigate the regional environmental impacts of the eruption has been provided by studying the overlying and underlying deposits at such sites. The minimum estimate of the mass of the tephra fallout is 2 x 1015 kg.
In some cases, such as past eruptions of the Yellowstone caldera (mostly in Wyoming, USA), very wide expanses of continental areas were probably covered with ash; though the vast majority of the dust from super-eruptions would have settled in the oceans in many other cases (Izett & Wilcox, 1982). Invaluable chronological markers can be represented by such widespread layers in archaeological and palaeoenvironmental contexts (e.g. Lane et al., 2014).
Some of the most dramatic landscapes around megacalderas (supervolcanoes) are formed by pyroclastic density currents (pyroclastic flows). They are composed predominantly of ash and pumice and are typically referred to as ignimbrite. High temperature, > 550oC, characterises the parent flows, and is often evident in the deformation and sintering of clasts that form a welded ignimbrite or sillar. Outside the caldera ignimbrites can reach thicknesses of hundreds of metres, but inside the crater they can reach as much as a kilometre or more. Plateaux are often formed by them that become incised deeply, generating complex, spectacular topography with canyons that are deep and are steep-sided.
In the case of super-eruptions, the quantities of tephra that are deposited rapidly on the landscape are clearly very substantial. Entire valleys may become choked in areas that have been inundated by pyroclastic currents, with the result that drainage systems are disrupted substantially. Vast areas can be covered by distal deposits of tephra fallout, which can also have substantial impacts that leave clear signals in the sedimentological record (Gatti et al., 2013; Williams et al., 2009).
Super-eruptions – impacts
There has been much debate of the global impacts of super-eruptions on climate, especially in the context of the eruption of the Youngest Toba Tuff (reflecting in particular its consequences for human populations of the present). Making progress on unravelling the story of the Toba eruption has, however, proven to be very challenging because of the timing of the eruption that occurred during a period of very significant oscillations of the climate, the Dansgaard-Oeschger cycles, and the uncertainty in regard to the quantity of sulphur that was co-emitted that reached the stratosphere, a critical factor when evaluating the degree of climate forcing that is attributable to the Toba super-eruption (e.g. Oppenheimer et al., 2011). It is possible to compare the various climate models with regard to extremely large releases of sulphur to the atmosphere, though it has remained problematic to match the results of modelling to the relevant palaeoclimate data (e.g. Robock et al., 2009; Timmreck et al., 2012; Segschneider et al., 2013). The significance of the dynamical integration of the components of models of Earth systems, of the state and sensitivity of the climate that prevailed at the time of the eruption, and of adequately representing the microphysical processes that were occurring in clouds of volcanic aerosols, is highlighted by these studies. Some of the models (e.g. Robock et al., 2009) have assumed, in the case of the Youngest Toba Tuff eruption, there was a very large release of sulphur, 200 times greater than that of Mount Pinatubo in 1991. In the case of the Youngest Toba Tuff, however, petrological considerations point towards the magma having relatively low sulphur content (Scailett et al., 1998; Chesner & Luhr, 2010).
Future super-eruptions – scenario
The timing and location of the next super-eruption can only be speculated about. Included among possible candidates are Toba, Yellowstone and Taupo, which raises the possibility of future super-eruptions occurring in the tropics or mid-latitudes of the Northern and Southern hemispheres. At Yellowstone it has been revealed by seismological investigation that there is a shallow magma reservoir that holds a total volume of more than 4,300 km3, ⅓ of which is molten (Chu et al., 2010). There is also the possibility that a volcano that has not previously experienced a super-eruption could potentially experience one in the future.
The question is what would be the effect of a future super-eruption on the present inhabitants of the Earth, and Oppenheimer & Donovan1 suggest that much would depend on the state of preparedness of the human population at the time. Attempts to model risk scenarios have yet to be made of such an event, though a number of summaries of generic consequences have been made (e.g. Self, 2006; Oppenheimer, 2011; Donovan & Oppenheimer, 2014), which were based on inferences from studies of the Youngest Toba Tuff and other eruptions, and the results of climate and Earth system models. Pyroclastic currents (pyroclastic flows) for an event of M 8 or M 9, e.g., could be expected to extend for up to 100 km radially from the volcano. An area of a few tens of thousands of km2 would be engulfed by incandescent pumice to a depth of up to 200 m. That the chance of surviving such an event would be vanishingly small is illustrated by the eruption in 1902 of Mt Pelée which killed all but 1 of the inhabitants of the town of St Pierre. The sole survivor only survived because he was in a prison cell at the time the pyroclastic current covered the town. Oppenheimer & Donovan1 suggest there may be some chance of surviving initially, beyond the fringes of the ignimbrite deposits of the super eruption, though subsequently many would eventually die from exposure, burns and other injuries.
Oppenheimer & Donovan1 have pointed out some possible results of a future super-eruption:
Thick tephra fallout would affect a much wider zone.
Substantial building damage could be expected, that would probably claim many more victims, where more the 0.5 m of ash accumulates.
The electromagnet effects of airborne ash would generally compromise telecommunications and power lines would be brought down (e.g. Wilson et al., 2012).
For a long time after, as well as during a future super-eruption, air quality and visibility would be poor across a vast area, as a result of the windblown ash.
The many challenges of mounting search-and-rescue operations would be compounded by this.
Ash fallout would make many roads and railways impassable, and aviation would be hazardous, and mud flows would inundate valleys.
In places where ash accumulates to depths greater than a few centimetres farming and agriculture would be affected severely.
Much of North America was blanketed in tephra from eruptions of Yellowstone in the past. Safe food and water resources would become increasingly scarce, and there would be a high potential for severe social unrest.
Power cuts and shortages of medicines would cripple hospitals.
Water could become scarce in arid regions.
Fluoride and other chemicals could be leached from the ash and contaminate surface water.
Power shortages would cause pumped water supplies to dwindle.
The effort required responding to such a disaster and mitigating loss of life in most regions that are badly affected, especially to prevent or control the outbreak of infectious diseases, would be huge.
If a super-eruption occurred outside the tropics it is likely the impacts on humans would be influenced by the season in which the eruption occurred. A scenario for a winter eruption might prove to be even deadlier than an eruption in the tropics as the problems of exposure of millions of people would be compounded by the freezing conditions. If an eruption occurred in the summer the result could be a deeper hemispheric climate response (Timmreck and Graf, 2006) with an immediate impact on crops and livestock, which would be a greater threat to food security. In the zone affected by ash fallout agriculture would be affected for years, and potentially decades, and result in rainfall deficits that can be expected following a super-eruption. The regional climate would be perturbed for much longer than a few years of residence of aerosols of sulphuric acid in the stratosphere that is typical for lesser eruptions. As silicic ash is comparable in brightness to snow it therefore would reflect sunlight that would otherwise be absorbed by vegetation and soil. A surface cooling of about 5oC throughout the year in North America has been predicted by one climate model following an eruption from Yellowstone (Jones et al., 2007).
The most extreme of the scenarios for super-eruptions include the demise of technological civilisation, which has led Mike Rampino to suggest that the number of extra-terrestrial civilisations could be constrained by volcanism (Rampino, 2002). According to his argument the impacts on global climates would severely reduce the yield of global agriculture which could potentially lead to ‘widespread starvation, famine, disease, social unrest, financial collapse, and severe damage to the underpinnings of civilisation’ (Rampino, 2002).
Oppenheimer & Donovan1 suggest the 2007-2008 and 2010-2011 global food crises (e.g. Rosset, 2008), which has been implicated by some in the increase of civil unrest such as that manifested in the Arab Spring (e,g. Lybbert & Morgan, 2013). There are many studies that link climate stress to conflict (e.g. Hsiang et al., 2013), though the implied environmental determinism is often rejected (e.g. Raleigh et al., 2014). With the present state of knowledge it is hard to rule out the hypothesis that an extremely severe international crisis will result from a future super-eruption.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|