What is a Shield Volcano? - Definition, Facts & Examples

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  • 0:02 Shield Volcano
  • 0:45 Formation
  • 2:37 Shield Volcano Components
  • 3:10 Hawaii Shield Volcanoes
  • 3:43 Largest Shields in…
  • 4:21 Lesson Summary
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Lesson Transcript
Instructor: Mary Ellen Ellis
A shield volcano is an impressive thing. By volume and area, these giants are the biggest volcanoes on earth - but the largest one known is actually on another planet. Learn all about them in this lesson.

Definition of Shield Volcano

Of the three types of volcano, the shield is the largest in terms of area. Cinder cone volcanoes are impressive for their explosive eruptions and classic looks. Stratovolcanoes are like cinder cones on steroids, with big eruptions and a large size. Shield volcanoes, although quiet, are immense.

Shield volcanoes are the largest volcanoes in terms of volume, and in diameter, can be tens to hundreds of kilometers across, or more. A shield volcano gets its name from its appearance. It is large in terms of area, but much flatter than the other two types of volcanoes. Viewed from above, it looks like a warrior's shield: slightly raised in the center with long, gently-sloping sides.

Formation of Shield Volcanoes

What makes a shield volcano special and gives it a different shape compared to other types of volcanoes is its lava. The lava that erupts from a shield volcano is typically basaltic, a type that has a low viscosity. Viscosity is a measure of the ability of a fluid to flow. Think of pouring molasses out of a jar and compare it to how water flows. In this example, water has a low viscosity in comparison to molasses with its relatively high viscosity.

The basaltic lava of a shield volcano has a low viscosity compared to the lava, ash, and rock that erupt from other types of volcanoes. Its low viscosity means it can flow quickly and far, covering a large area. The lava from each eruption eventually hardens, and the next eruption flows over and hardens on top of the previous one. Over time, the multiple eruptions build up, one on top of the other, to give us a shield volcano.

Shield volcanoes are able to grow as large in area as they do because they erupt regularly for long periods of time. Unlike a cinder cone that may erupt violently and unexpectedly and then remain quiet for hundreds or thousands of years, shield volcanoes let out a near-continuous and gentle flow of lava. This results in the gradual building-up of layer upon layer to create a large shield.

Another phenomenon that allows shields to grow so large is called a lava tube. As lava flows away from the site of an eruption, it cools. The lava exposed to the air cools and hardens more quickly than the lava underneath it. Depending on the conditions, that outer lava can harden into a shell that insulates the deeper lava. The insulation keeps it fluid longer and allows it to travel far from the eruption site. You can see, and even walk in, lava tubes at Lava Beds National Monument in Northern California.

Shield Volcano Components

The bulk of a shield volcano is the shield itself, but there are other important parts as well. The top of the volcano is an indentation called the caldera. Lava erupts from this site, but it is not the only place where lava emerges. Vents on the sides of the shield are openings through which lava can also erupt.

Under the caldera is a magma chamber deep below the crust. Plumes of magma from the chamber work upward through cracks in the crust. When it emerges from the crust in an eruption its name changes from magma to lava.

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