A volcano is an opening in Earth’s crust through which magma rises. Magma is called lava once it reaches the Earth’s surface. Volcanoes form on land, in the ocean, and at the center of islands, but are most commonly formed in the ocean. A number of factors contribute to the formation of a volcano.
How do volcanoes form?
All volcanoes begin as magma. The magma forms when rock material melts, or becomes molten, close to where the mantle and crust meet. Molten material contains gases. Under intense pressure, the gases remain dissolved. As the molten material rises to Earth’s surface, the pressure decreases. When this happens, the gases separate from the liquid magma. This process is similar to opening a can of soda. When the cap is removed, gases escape the can of soda. Gases in the magma include water vapor, carbon dioxide, sulfur, chlorine, and fluorine. As these gases expand, they produce an explosion eruption of lava at Earth’s surface.
Volcanoes at Divergent Boundaries
Most volcanic activity occurs in the ocean along divergent plate boundaries. Magma rises from the Earth’s mantle, moving oceanic plates apart. When the magma cools, it turns into new crust. This process creates mid-ocean ridges, which are under-water mountain ranges.
Volcanoes also form along convergent boundaries where tectonic plates come together. As the plates converge, they produce a subduction zone. A subduction zone is characterized by two plates colliding, with a denser plate sinking underneath the less dense plate. The plate that sinks under melts as it enters the hot mantle and gets recycled.
Volcanoes at Hot Spots
Not all volcanoes form at plate boundaries. A few ocean ridges and volcanic island chains form by hot spots. A hot spot is a very hot region deep in the Earth that does not move. The hot spot is "so hot" that it essentially burns a hole through the moving plate, allowing magma to rise. When the lava cools, it creates an island. However, the plate continues to move over the stationary hot spot, creating more islands. This is how the Hawaiian islands were formed.
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