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Unformatted text preview: U.S. Department of the Interior U.S. Geological Survey USGS Fact Sheet-002-97 Revised July 2004 What are Volcano Hazards? U.S. GEOLOGICAL SURVEYREDUCING THE RISK FROM VOL CA NO HAZ ARDS V G US S olcanoes give rise to nu mer ous geologic and hy dro log ic haz ards. U.S. Geo log i cal Sur vey (USGS) sci en tists are as sess ing haz ards at many of the almost 70 active and po ten tial ly active vol ca noes in the Unit ed States. They are close ly monitoring ac tiv i ty at the most dan ger ous of these vol ca noes and are pre pared to is sue warn ings of im pend ing erup tions or oth er haz ard ous events. Volcanic Gases Volcanoes emit gases during eruptions. Even when a volcano is not erupting, cracks in the ground allow gases to reach the sur face through small openings called fumaroles . More than 90% of all gas emit ted by vol ca noes is water va por (steam), most of which is heat ed ground wa ter (un der ground water from rain- fall and streams). Other com mon vol ca nic gas- es are car bon di ox ide, sul fur di ox ide, hy dro gen sulfi de, hy dro gen, and fl uorine. Sulfur dioxide gas can react with water drop lets in the at- mo sphere to cre ate acid rain , which caus es corrosion and harms vegetation. Carbon di- ox ide is heavi er than air and can be trapped in low ar eas in con cen tra tions that are deadly to peo ple and animals. Fluorine, which in high con cen tra tions is tox ic, can be adsorbed onto vol ca nic ash par ti cles that later fall to the ground. The fl uorine on the particles can More than 50 volcanoes in the Unit ed States have erupted one or more times in the past 200 years. The most vol ca ni cal ly active regions of the Na tion are in Alas ka, Ha waii, California, Oregon, and Washington. Vol- ca noes pro duce a wide va ri ety of hazards that can kill peo ple and de stroy prop er ty. Large ex plo sive eruptions can en dan ger people and prop er ty hundreds of miles away and even af- fect global cli mate. Some of the vol ca no haz- ards de scribed be low, such as land slides, can oc cur even when a vol ca no is not erupt ing. Eruption Columns and Clouds An explosive eruption blasts solid and mol ten rock fragments ( tephra ) and vol ca nic gas es into the air with tre men dous force. The larg est rock frag ments ( bombs ) usually fall back to the ground with in 2 miles of the vent ....
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