Lake Toba

Im April 2003 reiste ich mit Bernd Biedermann nach Sumatra und verbrachte 3 Tage am Lake Toba (TukTuk). Eine sehr angenehme, ruhige Insel, außerdem extrem günstig. Gute Unterkünfte gab es bereis für 3 Euro pro Nacht im touristisch erschlossenen aber wenig besuchten TukTuk auf der Insel Samosir. Die große Caldera zeigt heute außer wenigen Fumarolen keine vulkanische Aktivität mehr.

Anreise von nahezu allen Richtungen aus mit öffentlichen Verkehrsmittel kein Problem. Wir kamen wegen akuter Faulheit mit Privatfahrer aus Berestagi und reisten in Richtung Medan weiter. Kostete je Strecke ca. 25 Euro für Fahrzeug mit Fahrer.

Allgemeine Informationen zum Toba (Sumatra, Indonesia)

Satelittenaufnahme des Lake Toba	Country:	Indonesia
	Subregion Name:	Indonesia - Sumatra
	Volcano Type:	Caldera
	Volcano Status:	Holocene
	Last Eruption:	Unknown
	Summit Elevation:	2157 m
	Latitude:	2.58°N
	Longitude:	98.83°E
The 35 x 100 km Toba caldera, the Earth's largest Quaternary caldera, was formed during four major Pleistocene ignimbrite-producing eruptions beginning at 1.2 million years ago. The latest of these produced the Young Toba Tuff (YTT) about 74,000 years ago. The YTT represents the world's largest known Quaternary eruption, ejecting about 2500-3000 cu km (dense rock equivalent) of ignimbrite and airfall ash from vents at the NW and SE ends of present-day Lake Toba. Resurgent doming forming the massive Samosir Island and Uluan Peninsula structural blocks postdated eruption of the YTT. Additional post-YTT eruptions include emplacement of a series of lava domes, growth of the solfatarically active Pusukbukit volcano on the south margin of the caldera, and formation of Tandukbenua volcano at the NW-most rim of the caldera. Lack of vegetation suggests that this volcano may be only a few hundred years old (Chesner and Rose, 1991).

Toba caldera produced the largest eruption in the last 2 million years. The caldera is 18 x 60 miles (30 by 100 km) and has a total relief of 5,100 feet (1700 m). The caldera probably formed in stages. Large eruptions occurred 840,000, about 700,000, and 75,000 years ago. The eruption 75,000 years ago produced the Young Toba Tuff. The Young Toba Tuff was erupted from ring fractures that surround most or all of the present-day lake.

Map of the Toba caldera from Knight and others (1986). Samosir Island and the Uluan Peninsula are parts of one or two resurgent domes. Lake sediments on Samosir indicate at least 1,350 feet (450 m) of uplift. Pusukbukit, a small stratovolcano along the west margin of the caldera, formed after the eruption 75,000 years ago. There are active solfataras on the north side of the volcano.

Comparison of volumes produced by some of the greatest volcanic eruptions. The Young Toba Tuff has an estimated volume of 2,800 cubic kilometers (km) and was erupted about 74,000 years ago. The Huckleberry Ridge Tuff, erupted at Yellowstone 2.2 million years ago, has a volume of 2,500 cubic km. The Lava Creek Tuff, erupted at Yellowstone 600,000 years ago, has a volume of 1,000 cubic km. The May 1980 eruption of Mount St. Helens produced 1 cubic km of ash. Not shown is the Fish Canyon Tuff of the San Juan Mountains of Colorado. The Fish Canyon Tuff was erupted 27.8 million years ago and has an estimated volume of 3,000 cubic km.

The volume of the youngest eruption is estimated at 2,800 cubic km, making the eruption the largest in the Quaternary. Pyroclastic flows covered an area of at least 20,000 square km. Up to 1200 feet (400 m) of Young Toba Tuff is exposed in the walls of the caldera. On Samosir Island the tuff is more than 1800 feet (600 m) thick. Ash fall from the eruption covers an area of at least 4 million square km (about half the size on the continental United States). Ash from the eruption has been recovered from deep-sea cores taken in the Bay of Bengal and in India, roughly 300 miles (500 km) inland (1,900 miles, 3100 km from Toba). Rose and Chesner suggested the ash may have reached central Asia and the Middle East. Ninkovich and others (1978) estimated of the height of the eruption column to be 30 to 50 miles (50 to 80 km) for the Young Toba Tuff. Rose and Chesner, after a study of the shapes of the ash shards, concluded this estimate was too high by a factor of 5 or more.

The pumice erupted 75,000 years ago is calc-alkalic quartz-latite to rhyolite in composition (68%-76% silica).

There have been no eruptions at Toba in historical time. The area is seismically active with major earthquakes in 1892, 1916, 1920-1922, and 1987.

Toba is located near the Sumatra Fracture Zone (SFZ). Stratovolcanoes in Sumatra are part of the Sunda arc. Volcanism is the result of the subduction of the Indian Ocean plate under the Eurasian 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 Eurasian plate in this case). The rate of subduction is 6.7 cm per year. From Knight and others (1986).