of the largest known volcanic eruptions took place only 74,000 yrs ago, when
over 2500 cubic kilometers of magma was ejected from Toba - a volcano-tectonic
depression that is often referred to as Earth's largest Quaternary caldera. The
caldera is 18 x 60 miles (30 by 100 km) and has a total relief of 5,100 feet
(1700 m).Over what was probably a two-week span, thousands of cubic kilometers
of debris spewed from Toba Caldera on northern Sumatra. Pyroclastic flows
(fast-moving clouds of hot gas, rock fragments, and ash) buried at least 20,000
square kilometers around the caldera. As far away as India, ash from the Toba
eruption lies in layers up to 6 meters (about 20 feet) thick; on Samosir Island,
the ash layer is more than 600 meters (more than a quarter mile) thick.
courtesy of NASA/GSFC/MITI/ERSDAC/JAROS, and the U.S./Japan ASTER Science Team.
the eruption, the ground collapsed, leaving the modern caldera, which filled
with water to make Lake Toba. Samosir Island is a resurgent volcanic dome, a
mound of rock uplifted by pressure from un-erupted magma in the chamber beneath
the volcano. The Pusukbukit Volcano on the western shore of the lake also formed
after the catastrophic eruption.
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.
NASA, The Discovery Channel, The Smithsonian Institute, USGS