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Volcanic eruptions occur when magma builds up beneath the Earth's crust and forces its way to the surface. Natural vents in the crust allow magma passage to the surface, and eruptions occur when the magma that forms is l... More »

www.reference.com Science Earth Science Volcanoes

Volcanic eruptions mainly produce steam (H2 0 ), sulfur dioxide (SO 2 ) and carbon dioxide (CO 2 ). They do release other gases in lesser amounts, such as carbon monoxide (CO), helium (He), hydrogen (H 2 ), hydrogen chlo... More »

www.reference.com Science Earth Science Volcanoes

According to the United States Geological Survey, before a volcanic eruption, magma must travel upwards through rock. The travelling magma produces these precursors to an eruption: an increase in earthquakes beneath a vo... More »

www.reference.com Science Earth Science Volcanoes
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A volcano forms when magma pushes up through the Earth's crust from below, depositing lava on the surface. This lava cools, creating volcanic rock. Over time, repeated eruptions of lava build a cone-shaped mountainous st... More »

www.reference.com Science Earth Science Volcanoes

Volcanoes occur when magma and heated gases from below the Earth's crust are channeled to the surface through fissures in the crust layer. Magma is molten rock that is called lava once it reaches the surface. More »

www.reference.com Science Earth Science Volcanoes

Volcanoes occur when magma is able to reach the surface of the Earth through a gap in the crust. This typically occurs at plate boundaries, where two tectonic plates pull away or move against each other. Volcanoes can al... More »

www.reference.com Science Earth Science Volcanoes

Volcanoes can form anywhere the Earth's crust allows magma to reach the surface. Typically, this occurs around plate boundaries, either where plates are pulling apart or where one is forcing its way under another. Weak s... More »

www.reference.com Science Earth Science Volcanoes