Volcanoes
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A volcano is an area of Earth's surface through which magma and volcanic gases pass. There are two types of volcanic eruptions, nonexplosive and explosive. Nonexplosive eruptions are the most common types of eruptions and they can release huge amounts of lava. Explosive eruptions are much rarer than nonexplosive eruptions, however the effects of explosive eruptions can be very destructive. The key to whether an eruption will be explosive lies in the silica, water, and gas content of the magma. Magma erupts as either lava or pyroclastic material, which forms when magma is blasted into the air and hardens. Explosive eruptions produce mostly pyroclastic material. Over many years a volcano's eruptions may alternate between lava and pyroclastic eruptions. The Ring of Fire contains nearly 75% of the world's active volcanoes. (Holt and Science Technology, page 250-253, 261)
Image: (disasters-grade7.wikispaces.com)
Image: (disasters-grade7.wikispaces.com)
Ring of Fire
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