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VolcanoWatch Weekly [9 September 2009]

Posted by feww on September 11, 2009

VOW: Toba the Sleeping Colossus

Lake Toba, Sumatra, Indonesia – Landsat photo – Source: NASA

Lake Toba is a supervolcano, 100 kilometres long and 30 kilometres wide, and 505 metres (1,666 ft) at its deepest point. Located in the middle of the northern part of the Indonesian island of Sumatra with a surface elevation of about 900 metres (2,953 ft), the lake stretches from 2.88°N 98.52°E  to 2.35°N 99.1°E.  It is the largest volcanic lake in the world. It’s also the site of a supervolcanic eruption that occurred about 74,000 years ago, a massive climate-changing event. The eruption is believed to have had a VEI intensity of 8. This eruption, believed to have been the largest anywhere on Earth in the last 25 million years, may have had catastrophic consequences globally; some anthropologists and archeologists believe that it killed most humans then alive, creating a population bottleneck in Central Eastern Africa and India that affected the genetic inheritance of all humans today. (Source: Wikipedia).

Toba Large
Lake Toba Topography.
Source: Andaman Org.

Toba catastrophe theory

The Toba catastrophe theory holds that 70,000 to 75,000 years ago, a supervolcanic event at Lake Toba, on Sumatra, plunged the Earth into a mini-ice-age lasting several thousand years, reducing the world’s human population to 10,000 or even a mere 1,000 breeding pairs, creating a bottleneck in human evolution. The theory was proposed in 1998 by Stanley H. Ambrose of the University of Illinois at Urbana-Champaign.

The Toba eruption (the Toba event) occurred at what is now Lake Toba about 67,500 to 75,500 years ago. It had an estimated Volcanic Explosivity Index of 8 (described as “mega-colossal”), making it possibly the largest explosive volcanic eruption within the last twenty-five million years. It had a volume 300 cubic km greater than the Island Park Caldera supereruption (2500 cubic km) of 2.1 million years BP.

The total amount of erupted material was estimated at about 2,800 km³ — about 2,000 km³ of ignimbrite that flowed over the ground, and some 800 km³ that fell as ash, with the wind blowing most of it to the west. The pyroclastic flows of the eruption destroyed an area of 20,000 square kilometers, with ash deposits as thick as 600 metres near the main vent [ cf, 1980 eruption of Mount St. Helens ejected about 1.2 km³;  of material, whilst the largest volcanic eruption in historic times, at Mount Tambora in 1815, emitted the equivalent of 100 km3 of dense rock.] The eruption was also about three times the size of the latest Yellowstone eruption of Lava Creek 630,000 years ago. (Source: Wikipedia).

volcanic features of toba
The eruption of 73,000 years ago left the Sibandung caldera.  Lake Toba is surrounded by two small, active volcanos as well as several updomed areas and hot springs. These features indicate that there is activity below the surface today and that pressure is rising. Samosir island, too, is evidence for upthrust from below. From the record it seems that Toba produces major eruptions every 300-400,000 years. Source: Andaman Org.

Volcanic features in and around Lake Toba:

Grey area: Present-day topographic depression
green area: Updomed areas

Area # 1.  Sibandung caldera: made 73,000 years ago by the Toba YTT event (Young Toba Ash)
Area # 2. Haranggaol caldera: made 500,000 years ago by the Toba MTT event (Middle Toba Ash)
Area # 3.  Sibandung caldera: made 800,000 years ago by the Toba OTT event (Old Toba Ash)

The MTT and OTT events were not as large as the YTT event of 73,000 years ago
but were still major eruptions of at least VEI 7.

V1 Tandukbenua (Sipisopiso) – young dacit-andesite volcano
V2 Pusubukit volcano – young dacit-andesite volcano
D1 Pardepur dacite domes
D2 Tuk-tuk rhyolite dome
HS Hot springs
Source: Andaman Org.

Recent Activity

Large earthquakes have occurred in the vicinity of the volcano more recently, notably in 1987.  Other earthquakes have occurred in the area in 1892, 1916, and 1920-1922.

Lake Toba lies near the Great Sumatran fault which runs along the centre of Sumatra called the Sumatra Fracture Zone. The volcanoes of Sumatra and Java are part of the Sunda Arc, a result of the northeasterly movement of the Indo-Australian Plate which is sliding under the eastward-moving Eurasian Plate. The subduction zone in this area is very active: the seabed near the west coast of Sumatra has had several major earthquakes since 1995, including the 9.3 2004 Indian Ocean Earthquake [followed by the deadly tsunami] and the 8.7 2005 Sumatra earthquake, the epicenters of which were around 300 km from Toba Lake. (Source: Wikipedia).

SI /USGS Weekly Volcanic Activity Report
(26 August-1 September 2009)

New activity/unrest:

Notes [Source: GVP]

RVO reported that during 28 August-3 September white and gray ash plumes from Rabaul caldera’s Tavurvur cone rose 1.5 km above the crater and produced ashfall in Rabaul town (3-5 km NW) and surrounding areas.

The Washington VAAC reported that on 6 September an explosion from San Cristóbal produced an ash plume that rose to an altitude no higher than 8.5 km (28,000 ft) a.s.l. The plume drifted 75 km W.

Ongoing Activity:

Related Links:

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FEWW Volcanic Activity Forecast


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