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In volcanology, a lava dome or plug dome is a roughy circular mound-shaped protrusion resulting from the slow eruption of felsic lava (usually rhyolite and/or dacite) from a volcano. The viscosity, or stickiness, of the lava does not allow for the lava to flow very far from its vent before solidifying. Domes may reach heights of several hundred meters, and can grow slowly and steadily for months or years. The sides of these structures are composed of unstable rock debris. Due to the possibility of the building up of gas pressure, the dome can experience more explosive eruptions over time. When part of a lava dome collapses while it still contains molten rock and gases, it can produce a pyroclastic flow, one of the most lethal forms of a volcanic event. Other hazards associated with lava domes are the destruction of property, forest fires, and lahars triggered by pyroclastic flows near snow and ice. Lava domes are one of the principal structural features of many stratovolcanoes worldwide.
Some of the world's most famous active lava domes include those at Mount Merapi in central Java of Indonesia, Soufrière Hills in Montserrat, and Mount St. Helens in Washington. Lassen Peak in northern California, is one of the largest lava domes in the world and has the distinction of being the only other Cascade volcano besides Mount St. Helens to have erupted (1914 - 1921) in the 20th Century.
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