Uniqueness of Lake Toba

Unlike any other lake, if you think about the location — close to the equator — and look at the picture, you will realize that there is something strange about the lake. Most large lakes were formed by glaciers; large lakes close to the equator are sometimes formed by the damming of a large river. But there is no large river flowing into Lake Toba. The lake has a different and more dramatic origin.

Lake Toba marks the site of the largest volcanic eruption in the last 2 million years. It is, like Yellowstone, a “resurgent caldera”, or as geologists David Alt and Daniel Hyndman call this type of volcano, a “perfect horror”. Alt and Hyndman call resurgent calderas “perfect horrors” because they are both violent and enormous.
The size of Lake Toba may be a little difficult to grasp from the picture, so to give you some comparisons.
In Washington state, if the lake were aligned north to south in the Puget Sound area, it would stretch from Everett to Tacoma. In the Chicago area, it would stretch from Waukegan to Gary. In New York, it would stretch from West Point to the Statue of Liberty. This is one enormous volcano.

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