This is a map of the ocean floor.
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Image from: NOAA/NESDIS/National Geophysical Data Center, Boulder, CO
One of the most powerful volcanic explosions in the history of the world occured at Krakatoa in the last century.
Krakatoa was formerly a volcanic island found in the strait between Java and Sumatra. It was located near a region where the Indo-Australian plate subducts under the Eurasian plate.
In May, 1883, a series of eruptions commenced which continued until August 27, 1883, when a cataclismic explosion blew the island apart.
It is believed that the large explosion was due to super-hot steam, which was created when the walls of the volcano ruptered and allowed ocean water into the magma chamber. Explosions of steam, of which Krakatoa is the ultimate example, are known as "Phreatic" eruptions.
The island exploded with the force of 100 megatons (the Hiroshima bomb was about 20 kilotons). The explosion was heard as far away as Madagascar (2,200 miles). Ash from the explosion rose 50 miles in altitude, into the stratosphere, where it affected weather patterns for the next year. Large amounts of ash which reach the stratosphere can have a cooling effect on weather because the ash remains in the sky and reduces the amount of sunlight reaching the surface. Tsunamis from the explosion were raised to 131 ft, destroying 163 villages along the coast of Java and Sumatra, and drowning 36,000 people.
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