olelog

The theme of the ninth Accretionary Wedge hosted by Julian at Harmonic Tremors is significant geological events. I suppose that an event that nearly wiped out all human beings could be called significant.

In 1998 Stanley Ambrose proposed that the eruption of the supervulcano Toba reduced the world's human population to 10,000 or even a mere 1,000 breeding pairs. The findings were supported by DNA evidence. The last glacial period was preceded by one thousand years of the coldest temperatures of the Later Pleistocene, 71 to 70 thousand years ago. The intriguing but as yet unproven speculation is that it was caused by the eruption of Toba, Sumatra, leading to a “volcanic winter”. Toba was the largest known explosive eruption in the last two million years.

The length or severity of global cooling caused by the Toba eruption is debatable. The Indian subcontinent contains extensive deposits from the eruption. In a series of stone artefacts unearthed in southern India (Jwalapuram) suggest that local human populations remained in the region after the Toba eruption. The prehistoric tools were found in more than 7.5 m of sedimentary layers sandwiching a layer of ash produced by the Toba eruption, and the tools above were essentially at the same level of evolution as those below. The findings published in Science of 6 July 2007 seem to indicate that the impact of the Toba eruption was not as significant as earlier thought.

The Toba supervolcano-eruption produced the largest known volcanic eruption on earth during the past 2 million years. About 71,500 years ago, give or take a rough 4,000 years, an estimated minimum of 2800 km3 of magma were erupted, of which at least ca. 800 km3 was transported in atmospheric ash plumes that blanketed an area from the South China Sea to the Arabian Sea. This total volume is even more than the Yellowstone super-volcano eruption, much debated a couple of years ago, with a volume of an estimated 2500 km3 . The eruption led to the final formation of one of Earths largest calderas, the 35x100 km wide Toba caldera (See landsat image).

There have been no eruptions at Toba in historical time. Toba is located near the Sumatra Fracture Zone. I have marked this fracture zone as “Fault Zone” on the landsat image, where it is clearly visible as a light line. On the map it is marked by _._._. The movement directions are shown by arrows. Stratovolcanoes in Sumatra are part of the Sunda arc. Volcanism is the result of the subduction of the Indian Ocean plate under the Sunda plate. The subduction zone is marked by the Java Trench. The geologic symbol for a subduction zone is a line with "teeth" (black triangles). The teeth are on the over-riding plate. The rate of subduction is 6.7 cm per year.