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.
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.
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.
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