TTO2 Natural hazards
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Plate tectonics is an important theory developed in the 1960s to explain how the continents move across the Earth's surface.
Early 20th century geologist Alfred Wegener realised that the puzzle-like fit of many the continents was more than a coincidence, but he couldn't correctly explain what powered their movement. Geologists now know that the Earth's outermost layer, the lithosphere, is divided into independently moving plates into which the continents are embedded. The plates "float" on a layer called the athenosphere. There are different types of plate boundary. Spreading centres at mid-ocean ridges are where undersea volcanoes create new plate material. Subduction zones are where one plate sinks below another, causing volcanic eruptions andearthquakes and, sometimes, building mountains. |
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Mount St. Helens is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United States. It is 96 miles (154 km) south of Seattle, Washington and 50 miles (80 km) northeast of Portland, Oregon.The volcano is located in the Cascade Range and is part of the Cascade Volcanic Arc, a segment of the Pacific Ring of Fire that includes over 160 active volcanoes. This volcano is well known for its ash explosions and pyroclastic flows.Mount St. Helens is most notorious for its catastrophic eruption on May 18, 1980, at 8:32 am PDT,[2] the deadliest and most economically destructive volcanic event in the history of the United States. Fifty-seven people were killed; 250 homes, 47 bridges, 15 miles (24 km) of railways, and 185 miles (298 km) of highway were destroyed. A massive debris avalanche triggered by an earthquake measuring 5.1 on the Richter scale, caused an eruption, reducing the elevation of the mountain's summit from 9,677 ft (2,950 m) to 8,365 ft (2,550 m) and replacing it with a 1 mile (1.6 km) wide horseshoe-shaped crater. As with most other volcanoes in the Cascade Range, Mount St. Helens is a large eruptive cone consisting of lava rock interlayered with ash, pumice, and other deposits.
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Key concepts:
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Things to know: Types of plate boundaries
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Things to know:
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