Volcanic Craters, Calderas, Gases, and Climate Help (page 2)

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By — McGraw-Hill Professional
Updated on Sep 4, 2011


When water seeps down to the magma, it turns to steam and can cause a phreatic eruption . When this happens, the violence of the eruption is extreme though of lower temperature. Compared to magma eruptions, there is a tremendous amount of steam, but hardly any rock fragments or ejecta .

In 1883, Krakatoa in Indonesia exploded with a phreatic eruption. It blew dust over 80km into the air with a force roughly that of 100 million tons of dynamite. The explosions were heard on Rodriguez Island, 4653km distant across the Indian Ocean, and in Australia, over 3000km away and produced orange-red sunsets for several years afterward. Ash fell on Singapore 840km to the north, Keeling Island 1155km to the southwest, and on ships that were 6076km west northwest. Ash was reported over a 750,000 square kilometer area.

The shock of the eruption caused huge waves with breakers over 40 m high, obliterating everything in their path and flinging coral blocks weighing nearly 600 tons ashore. Even though Krakatoa was not inhabited, these breakers caused over 38,000 deaths by flooding low-lying nearby coastal areas and 165 coastal villages were destroyed. When the eruption was over only 1/3 of Krakatoa, previously 45km 2 , was above sea level with newly created northern islands of scalding pumice and ash, where the sea had been 36 m in depth.

The dust from Krakatoa’s eruption also caused a decrease in temperature by Volcanoes Lava Lakes a degree centigrade worldwide. This cooling effect was reported to have lasted for ten years. When El Chichón in Mexico erupted in 1982, sulfur-containing gases were spewed into the atmosphere blocking sunlight and falling back to the earth over time as acid rain. The same thing happened with the low-ash eruption of Mount Pinatubo in the Philippines in 1991. The curiously cool Northern Hemisphere summer of 1992, following Pinatubo’s eruption and discharge of sulfuric gases, caused meteorologists to increase their look at the role of volcanic eruptions in world-wide weather patterns.

Practice problems of this concept can be found at: Volcano Practice Test

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