A volcano is an opening in the earth's surface through where lava, hot gases, and rock fragments erupt. Such an opening occurs when melted rock from deep within the earth blasts through the surface. Most volcanoes are atop mountains, particularly cone-shaped mountains. The mountains themselves are also called volcanoes.Eruptions of volcanic mountains are spectacular sights. In some eruptions, huge fiery clouds rise over the mountain, and glowing rivers of lava flow down its sides. In other eruptions, red-hot ash and cinders shoot out the mountaintop, and large chunks of hot rock are blast high into the air. A few eruptions are so violent that they blow the mountain apart.Some eruptions occur on volcanic islands. Such islands are the tops of volcanic mountains that have been built up from the ocean floor by repeated eruptions. Other eruptions occur along narrow cracks in the ocean floor.
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