Volcanoes are one of nature's most destructive forces. While some of the hazards posed by volcanoes are related to eruptions, others can occur in the absence of an eruption. Some damage occurs quickly, while other damage occurs over months and years. The greatest causes of volcanic damage include lava flows, pyroclastic flows, lahars and eruptive columns.
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Lava Flow Damage
The most obvious damage created by a volcano is caused by lava flows. These streams of molten rock destroy everything in their path. Unlike damage caused by other natural disasters, which can typically be repaired or rebuilt, lava flows can permanently bury areas under hardened layers of lava. These lava flows can travel at speeds of up to 19 miles per hour, depending on the type of lava. Fires are also a great risk, as the lava flows can reach temperatures of 1,260 to 1,148 degrees Celsius. While lava flows pose a great risk to property, they rarely result in fatalities because of their slow speed.
Pyroclastic Flow Damage
Pyroclastic flows, also known as pyroclastic density currents, are mixtures of hot rock fragments and superheated gases. Like lava flows, these volcanic hazards destroy everything in their path. However, unlike lava flows, they pose a great risk to people because they travel at very high rates of speed, as high as 450 miles per hour. They also reach temperatures of up to 0-11.667 degrees C. These flows represent heavier-than-air emulsions that hug the ground like an avalanche. The 1980 eruption of Mount St. Helen's featured a lateral blast that generated a pyroclastic surge that destroyed 230 square miles. Pyroclastic flows during the 1902 eruption of Mount Pelee resulted in the death of nearly 30,000 people.
Lahar is an Indonesian term for volcanic mudflows, referring to hot or cold flows of water mixed with rock fragments. These flows are similar in nature to wet concrete that flows down the flanks of volcanoes and river valleys. As the lahars progress, they incorporate more material and can grow to 10 times their initial size. The risk of lahars is elevated with stratovolcanoes because of their explosive nature, frequent snow and ice coverage, tendency to erode and propensity for internal weakening due to hot hydrothermal fluids. They do not require an actual eruption, but can be triggered by rainfall on loose volcanic ash. Lahars and pyroclastic flows represent the most deadly volcanic hazards.
Eruptive columns can inject volcanic gases, aerosol droplets and ash into the stratosphere. These gases include sulphur dioxide, which can lead to global cooling, and carbon dioxide, which can lead to global warming. Sulphur dioxide is converted to sulphuric acid aerosols that increase the reflection of solar radiation back into space, cooling the surface. The eruption of Mount Pinatubo in 1991 released 20 million tons of sulphur dioxide over 20 miles into the atmosphere. This resulted in a drop in global temperatures for three years. The 1815 eruption of Tambora even resulted in what is known as the "year without a summer."
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