Toba, Indonesia
75,000 years ago
Landsat Multi-Spectral Scanner (MSS) image taken Oct 3, 1973.
Why is there a giant lake - 100km long and 30 km wide - in the middle of
the Indonesia island of Sumatra? Lakes often form in depressions caused
by glacial erosion (Great Lakes of USA) or by down-dropping of large
blocks of the crust (the lakes of East Africa), but such gentle processes
didn't cause Lake Toba in Sumatra.
In 1949 the Dutch geologist van Bemmelen reported that Lake Toba was
surrounded by a vast layer of ignimbrite rocks. Toba was apparently a
huge volcano! Later researchers found rhyolite ash similar to that in the
ignimbrite around Toba in Malaysia and even 3000 km away in India. And
oceanographers discovered a vast dusting of Toba ash on the floor of the
eastern Indian Ocean and the Bay of Bengal. These scientists quickly
realized that the Toba eruption, dated at 75,000 years ago, was the most
recent truely large eruption on Earth. Bill Rose and Craig Chesner of
Michigan Technological University combined all the information on the
extent of the Toba volcanic material to deduce that the total amount of
erupted material was about 2,800 km3. About 800 km3 was ignimbrite that
travelled swiftly over the ground away from the volcano destroying
everything in its path, and the remaining 2,000 km3 fell as ash, with the
wind blowing most of it to the west. Such a huge eruption probably lasted
nearly two weeks. Very few plants, animals or humans around this part of
Indonesia would have survived.
The eruption of such a huge amount of volcanic rock, which was previously
beneath the earth's surface, naturally caused a great collapse to occur.
The collapse formed a caldera, which filled with water creating Lake
Toba. Later, the floor of the caldera was uplifted to form Samosir, the
large island in the lake. Such uplifts are common in very large calderas,
apparently due to the upward pressure of unerupted magma. Toba is
probably the largest resurgent caldera on Earth.
There have been no historic eruptions of Toba, but large earthquakes have
occurred, the most recent in 1987 along the southern shore of the lake.
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