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Parts of the Shield Volcano

Named for their resemblance to warrior shields, shield volcanoes are build from slow-moving lava flows that cool in thin sheets. These sheets build up over hundreds of thousands of years, creating the wide, gentle sloops that shield volcanoes are known for. Shield volcanoes are usually a few miles in height with diameters sometimes exceeding 100 miles, covering hundreds of square miles. They are some of the largest volcanoes in the world. Their lava flows are slow moving and thus the easiest for scientists to study.

Summit Caldera
- A volcano's flat peak is created by the summit caldera.
  
  The summit caldera, named because of its cauldron shape, is a large depression located at the top of the volcano. Before a shield volcano forms, its caldera is just a crack in the earth's surface with a large deposit of magma, or molten rock, beneath it. When the magma quickly moves to the surface, it leaves an empty space under the ground. Having nothing to support it, the ground collapses, creating the caldera, which could have a diameter as great as three miles. Lava continues to flow out of the caldera, hardening in sheets that build up the long slopes that make up the volcano's sides.
Magma and Magma Chamber
- When lava cools, it returns to its rock state.
  
  Also called a magma reservoir, the magma chamber is located at the bottom of the volcano, near the center. This chamber holds the magma that eventually erupts from the caldera or the fissures. The magma in shield volcanoes is usually melted basaltic rock and comes from the upper mantle, or the mantle itself, inside the earth. The magma moves into the magma chamber through separations in the tectonic plates. When magma reaches the surface, it becomes lava.
Central Vent and Fissures
- Shield volcanoes aren't just found on earth. Mars also has a shield volcano, Olympus Mons.
  
  The central vent connects the magma chamber and the summit caldera, allowing the movement of the magma between the two. The central vent is a pipe-like structure. Magma may also flow from fissures, or rift zones, on the sides of the volcano. Some shield volcanoes actually have eruptions more often from the rifts than from the central vent. Multiple lava flows from fissures often create lava plateaus, long flat sections found on the sides of the volcano mountain.

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