This is a map of the ocean floor.
Click on image for full size
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 located 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.
The large explosion was due to super-hot steam, created when the walls of the volcano ruptered and let ocean water into the magma chamber.
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). Tsunamis from the explosion were raised to 131 ft, and destroyed 163 villages along the coast of Java and Sumatra. Ash from the explosion rose 50 miles in altitude (higher than altitudes where airplanes fly), and it affected the weather for the next year.
Ash can have a cooling effect on weather because it remains in the sky and reduces the amount of sunlight reaching the surface.
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