Large volcanic eruptions are associated with rapid expansion of gases during explosive phases, and these have been known to produce some of the loudest sounds and atmospheric pressure waves known on Earth. For instance, explosions from the 1883 eruption of Krakatau were heard almost 3,000 miles (4,700 km) away, in places as diverse as the Indian Ocean islands of Diego Garcia and Rodriguez, in south India, in southeast Asia across Myan-
mar, Thailand, and Vietnam, in the Philippines, and across western and central Australia. Other volcanic eruptions that produced blasts heard for hundreds to thousands of miles include the 1835 eruption of Cosiguina in Nicaragua, the eruption of Mount Pelée in 1902, and Katmai (Alaska) in 1912. Although interesting, sounds from eruptions are relatively harmless. Explosions that produce sound waves may also be associated with much more powerful atmospheric shock waves that can be destructive.
Atmospheric shock waves are produced by the pressure changes caused by the sudden explosive release of rapidly expanding steam and gases that may sometimes exceed the speed of sound. When the gas eruptions proceed at supersonic velocities (1,000-2,700 feet [305-823 m] per second depending on the temperature and density of the gas), the shock waves may be associated with huge, expanding flashing arcs of light that pulsate out of the volcano. Firsthand accounts of these flashing arcs are rare, but observations of the 1906 eruption of Vesuvius told of flashes ranging from several times a second to every few seconds. Deafening explosive sounds followed these flashes.
Large atmospheric shock waves may be powerful enough to damage or knock down buildings and may travel completely around the world. The eruptions of Krakatau in 1883, Pelée in 1902, Asama (Japan) in 1783 and 1973, and others were recorded at atmospheric weather stations around the globe. Some of these shock waves destroyed or damaged buildings across hundreds of square miles surrounding the volcano.
Many volcanic eruptions are associated with spectacular lightning storms in the ash clouds, or in the expanding gas clouds. As this material is ejected from the volcano, many particles may rub together, creating electrical discharges seen as lightning. Intense lightning storms may typically extend for 5-10 miles (8-16 km) from the eruption, posing threats to people brave enough to remain close to the eruption, as well as to communication systems. A phenomenon known as St. Elmo's fire is sometimes associated with volcanic eruptions. It refers to a glowing blue or green electrical discharge that emanates from tall objects that are near an intense electrical charge and has been observed on ships near eruptions, including Krakatau in 1883, Vesuvius in 1906, and even during the 1980 eruption of Mount Saint Helens.
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