TOBA BOTTLENCK. Three years ago it was my privilege to study and map a region of Snowdonia, rightly known for its rugged grandeur and scenic beauty. Formed from heat and pressure and sculpted by ice and water, it really is a land of majesty, achingly beautiful and distinctly memorable. I have spent fifteen years walking its mountains and valleys, it never gets old, it never disappoints.There is a great synform that runs through the Snowdon edifice, trending NE- SW. It continues down through the Moel Hebog massif until it curves gradually to the east where it joins a discontinuous series of further synformal structures, turning north and finally west again to meet with the Snowdon/ Cwm Idwal Syncline. Clearly then, there is a sub-circular feature delineating a large ring like geological structure. My mapping area of 25 km3 covered a portion of the SW quadrant of this great oval. It is not a topographical feature, indeed, it has been buried, baked and distorted by later events including the closing of the ancient Iapetus Ocean and the subsequent Caledonian Orogeny. The effects of the Quaternary glaciations obscured or removed remaining evidence of the palimpsest or ghost crater. Too large to be described simply as a crater, it was in fact, a monstrous Caldera. Only intense geological scrutiny can reveal its existence, but like its younger Brother to the north of the Iapetus Suture, the Glen Coe Caldera, it is most definitely there. 448 million years ago, this region was on the edge of a basin, back arc volcanics thrust once deep marine strata to the surface. Bimodal magmas produced almost every variety of volcaniclastic deposition and emplacement. As the large chamber depleted, the caldera sank, inrushing waters produced cataclysmic phreatic eruptions until new injections from the upper mantle pushed the region up again, and so a cycle of sub aqueous and subaerial eruptions continued until the final collapse of the great volcanic pile. Eruptions on a reduced scale continued periodically for thousands of years, but the faulted and sunken structure finally became a quiescent marine environment once more until deposition and the orogeny to the North created greenschist rocks of slates, phylites, chloritised basalts and altered acid tuffs. Had there been any life on the proximal shores adjacent to the Snowdon Caldera 448 million years ago, it would have been snuffed out by the ignimbrite surges, bombs, lavas and tephras. Distal life too, would have suffered. Ashfall and volcanic winters take their toll. But not until the Devonian would there have been tentative amphibious excursions onshore, so only the graptolites, brachiopods and trilobites were caught up in the tectonic turmoil as old oceanic crust descended beneath the Avalonian Terrane. But what if there had been a diversity of terrestrial biota? What would become of them in such catastrophic circumstances? This region of the Avalon microcontinent, located barely 20 degrees above the South pole, was not an equable place to be from an anthropological perspective. From the perspective of any life form, it was challenging. Gondwana was still shifting to the south Pole producing global cooling and exposing continental shelves, while heightened volcanic activity exacerbated the fragility of floral and faunal existence. Geological history is littered with such environmental challenges, peaking in mass extinction as ecosystems fail. But this is millions of years ago. When the ancient Earth is observed from this temporal distance, its seems we are remote from such catastrophes. After all this time, surely the rough edges of our world are smoothed off and apart from a few burps like Mt St Helens or Eyjafjallajökull, Earth is a fairly benign place for the continued evolution of life. Its worth considering then, the eruption of Toba, a great Caldera in Indonesia. This event dwarfs the ancient glen Coe and Snowdon events and they were huge. It was only about 70,000 years ago, a mere trice in Geological terms. As with all large calderas, it was rarely just one vent, but many usually at half graben structures on the periphery or Ring. So it was with Toba, many vents concurrently erupting with Plinian columns miles high and great super heated clouds tens of miles wide and further as the Hadley Cells and trade winds carried the ash around the globe. The ashfall was in the hundreds of metres locally, at least 600 metres adjacent to the largest vent, and in far off India there was a continuous cover of at least 150mm over most of the sub continent. (Lake Malawi in East Africa has deposits with the identical geochemistry of Toba). This was enough to kill off vegetation over tens of thousand of square kilometres. The food chain was broken, the animal populations would have died off in vast numbers. Including humans. The human race was young, mobility was at the walking pace of the frailest and mass migration at a speed quick enough to relocate to safe sustainable environments was impossible. It is estimated that the ashfall and the Volcanic Winter that followed this VEI 8 event, reduced the Human Population to barely more than 12,000 and as little as 2,000 individuals across the Planet. Genetic studies of the people of India indicates that 600 or less survived there. The Genetic Bottleneck studied by many disciplines point to the Toba Event as a very probable cause. There is debate still but it seems clear, almost inevitable, that a global catastrophe occurred 70,000 years ago. Greenland Ice cores, stratigraphic field studies and palaeobotanists concur. The Earth was not a hospitable place then. Mankind barely survived. The famous quotation by Dr Ian Malcolm of Jurassic Park fame; ...Life Finds A Way..” but only just. So perhaps we should remember that the Toba Catastrophe was really just yesterday cosmologically and the Sunda Arc is still very active. We are living through a hiatus with Vesuvius, Mt St Helens and the tragic Boxing Day Tsunami - reminders of what this still very fretful Planet is capable of. If we hadnt such short memories we would consider our petty anthropological preoccupations with race, religion and the perpetuation of the Haves before the Have Nots, as so much irrelevant buffoonery that could be swept away in a few short weeks. Images; 1, The Toba Caldera Lake - NASA - USGS 2, Geologists studying the Youngest Toba Tuff strata. Fox News. 3, Graphic comparison of Eruption Volumes. Wiki.
Posted on: Sat, 23 Aug 2014 21:54:07 +0000
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