A supervolcano’s plumbing system

What was the most devastating natural disaster ever to face humans? It would be tempting to suggest the Indian Ocean tsunami of 26 December 2004, but that is because most people remember it with horror. In fact the worst the Earth ever flung at us was much further back in our history and left a huge spike of sulfates in the Greenland icecap at around 73 thousand years ago. This relic of volcanic aerosols that had blasted into the stratosphere was tracked back to a 100 by 30 km caldera in Sumatra now occupied by a lake (Lake Toba) that is 500 m deep in places and almost filled by a slightly off-centre island. The eruption explosively ejected 2800 cubic kilometres of magma, of which an estimated 800 km3 fell as ash across a wide swath of the tropics westwards of Sumatra at least as far as Arabia and East Africa; the line of march taken by anatomically modern humans migrating from Africa. In India and Malaysia the Toba ash layer reaches 5-10 m thickness and probably occurs undetected as a thin layer across the entire tropics. Around 1010 tonnes of sulfuric acid belched out, some to enter and linger in the stratosphere, which is estimated to have caused an average decrease in average global temperatures of 3.0 to 3.5 °C for several years. Studies of human mtDNA hint at a genetic bottleneck around the time of Toba’s eruption and a large decrease, perhaps as much as 60%, in the global population of Homo sapiens. But humans survived or quickly filled devastated land in India, where stone tools are found both below and just above the Toba ash layer.

Landsat image of Lake Toba, the largest volcan...
Landsat image (120 km across) of Lake Toba, the largest volcanic crater lake in the world. (credit: Wikipedia)

The largest volcanic eruption in the last 26 Ma, there can be little doubt that no other natural catastrophe had as large an influence on humanity as did Toba. Of course, slower processes such as climate change and ups and downs of sea level lay behind the repeated spread of humans out of Africa and probably their evolution as a whole. The drama of the Toba event has drawn attention to the massive risk posed by supervolcanoes in general, such as that centred on Yellowstone in the NW US, which show signs of activity 640 ka after its last major explosive event. Toba certainly is not dead, for its peculiar island of Samosir has been uplifted steadily since the eruption by about 450 m, probably due to influx of magma deep beneath the surface, and experiences shallow earthquakes. What lies in the guts of supervolcanoes is literally a hot topic and a new 3-D imaging method has been applied to Toba.

English: Batak village on Samosir island, Lake...
Traditional village on Samosir island, Lake Toba. (credit: Wikipedia)

Seismic tomography that uses background or ambient seismic noise has become a powerful technique for studying the crust and lithosphere when small-amplitude short-wavelength Rayleigh and Love surface waves are monitored to pick up subsurface reflecting bodies and measure variation in wave speed with depth. The greater the density of seismometers deployed, the finer the resolution of deep crustal features and 40 such detectors are in place around Lake Toba. A team of Russian, French and German geophysicists have reported new results bearing on how magma may be accumulating beneath the vast caldera (Jaxibulatov, K. et al. 2014. A large magmatic sill complex beneath the Toba caldera. Science, v. 346, p. 617-619). Down to about 7 km the tomography has picked up a structurally homogeneous low-speed zone directly beneath Samosir Island that the authors attribute to the 73 ka explosive eruption. Beneath that several magma sills appear to dominate the sub-caldera crust, possibly responsible for the post eruption uplift within the caldera: the precursor to a layered intrusive body and each an increment towards a further huge eruption.

Interpretation of seismic tomography cross section of Toba. Greens to reds increasingly negative shear speed anomaly. Showing magma sills in lower crust and 74 ka damage zone above 7 km. (credit: Jaxibulatov et al. 2014
Interpretation of seismic tomography cross section of Toba. Greens to reds increasingly negative shear speed anomaly. Showing magma sills in lower crust and 74 ka damage zone above 7 km. (credit: Jaxibulatov et al. 2014

Petrologists probe Minoan collapse

Partial panorama of Santorini and Thera caldera
Modern Santorini and the drowned Thera caldera. Image via Wikipedia

A burning topic for Bronze Age archaeologists, such as the delightful  Bettany Hughes – biographer of Helen of Troy, is the explosive collapse of the volcano Thera (modern Santorini) whose distant effects (ash and tsunamis)wiped out the Minoan civilisation of Crete around 1600 BCE, giving rise to Plato’s legend of Atlantis. It was a big one alright, hurling of the order of 60 km3 of pulverised magma skywards, though not the largest historic eruption: that involved 160 km3 from the Tambora volcano on Indonesia’s island of Sumbawa in 1815. The inhabitants of Santorini simply disappeared, after evacuating their homes during precursor earthquakes and small eruptions, which were then buried beneath many metres of tephra when Thera literally ‘blew its top’. Little ash fell on Crete, yet its northern coast shows clear signs of a major tsunami. The reason for such an engulfing wave is revealed by the nature of Thera’s eruption: after evacuating magma, the edifice collapsed to form a caldera clearly revealed by the elliptical bay around which the remnants stand as the various islands of Santorini.  Caldera formation would have displaced vast amounts of sea water.

Santorini has been well studied by volcanologists, still being an astonishingly awesome spectacle as well as preserving the full record of the eruption and the archaeology that it buried (http://santorini-eruption.org.uk/). Empirical research reveals four distinct eruptive phases probably over a period of a few months. The explosive force of the final catastrophe probably resulted from seawater reaching the sub-volcanic magma chamber: not a difficult feat of imagination. What has not been known is how the magma evolved over times leading up to the cataclysm, and that is a knotty issue for all volcanoes that pose a major threat because of evidence for repeated and perhaps cyclic activity. A new technique is now capable of lifting the veil on such purely magmatic evolution, and is based on the changes that took place in minerals that crystallised over lengthy periods while the magma cooled slowly at depth but was periodically added to (Druitt, T.H et al. 2012. Decadal to monthly timescale of magma transfer and reservoir growth at a caldera volcano. Nature, v. 482, p. 77-80).

Such phenocrysts are commonly found in fragments of pumice that make up Theran tephra, and they are commonly zoned in a concentric fashion, especially those of the mineral feldspar, each zone marking a phase of growth that occasionally traps samples of magma in the form of now glassy inclusions. The zones mark chemical changes in the magma as new pulses are added in the sub-volcanic chamber, and sometimes temperature changes and loss of gas. Although the zone boundaries a are expected to be sharp in terms of chemical differences, in practice they are blurred as a result of element diffusion at high temperatures. Diffusion is a predictable process and so the degree of blurring indicates the time at which a new zone formed relative to that of eruption and cooling, when diffusion would have stopped abruptly. Rates of high-temperature diffusion depend on the element concerned. So using a suite of trace elements in feldspar zones gives a variety of chronometers. A fast-diffusing element such as Mg can chart changes of the order of decades to months, while a more sluggish trace element – for instance titanium – can examine changes on longer timescales.

The results obtained by the authors present a surprise: although Thera had last erupted catastrophically 18 ka previously, additional magma recharged the volcano only in the last few decades before it extinguished life on Santorini and set the Minoan civilisation on a downward spiral. Indeed, magma continued to be added even in the last few months. Calderas, such as that at Yellowstone in the western US, to which are linked ancient ash layers covering areas hundreds and thousands of kilometres away, pose threats as large and even bigger than Thera. If Thera is anything to go by, they lie in repose long after an eruption and signs of recharge may herald eruption in the near future. The Yellowstone caldera, that has lain dormant for 640 ka is indeed showing signs of magmatic ‘stoking’, as the Earth’s surface there is slowly bulging. It produced ‘supereruptions’ that dwarfed Thera at 2.1 Ma (2500 km3), 1.3 Ma (280 km3) and 0.6 ka (1000 km3). For each of these and several other calderas there are abundant tuffs that carry phenocrysts, whose zonation is yet to be checked for signs of past behaviour by their local magma chambers.