A By volume and area, these giants are

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.

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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.

Hawaii Shield Volcanoes

Although shield volcanoes can be found all over the world, the most famous and well-studied examples are those on the Hawaiian Islands. Much of what we know about this style of volcano comes from research conducted there because the Hawaiian volcanoes are easily accessible. In some areas of the state, residents are used to seeing lava flowing across the ground and sometimes over highways and into the ocean. The most famous active shield volcanoes in Hawaii are the four on the big island: Mauna Kea, Mauna Loa, Hualalai, and Kohala.

Largest Shields in Solar System

The largest shield volcano on Earth is one most of us are unlikely ever to see because it’s on the bottom of the Pacific Ocean. The Tamu Massif‘s title as largest volcano of any kind on Earth was only confirmed by researchers studying it in 2013. It covers an area roughly the size of New Mexico.Of a similar size, but bigger still, is another shield volcano that is not even found on Earth. Olympus Mons is the largest shield volcano on Mars.

At about 375 miles in diameter and 16 miles high, it dwarfs Mount Everest, our tallest mountain above sea level.

Lesson Summary

A shield volcano is the largest kind of volcano that erupts a thin, low-viscosity, basaltic lava gently and over a large area. Basaltic lava is a type of lava that has a low viscosity, while viscosity is a measure of the ability of a fluid to flow.Slow, continuous eruptions cause hardened layers of lava to build up to create a shield shape around the caldera, the indentation at the top of the volcano. Lava can also erupt from vents, holes in the side of a volcano where it can also erupt, and lava can travel even farther from the eruption site when insulating lava tubes form. Lava tubes work like this: 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. This all begins in a magma chamber beneath the caldera.The shield volcanoes of the Hawaiian Islands are some of the most active and well-known on Earth. Although shield volcanoes can be found all over the earth, the largest shield volcano on Earth is the Tamu Massif at the bottom of the Pacific Ocean, while the largest volcano in the solar system is Olympus Mons on Mars.

Overview of Shield Volcanoes

Lava erupts from a shield volcano through the caldera and through vents
Diagram of a shield volcano
  • Shield volcanoes are the largest type of volcano
  • Layers of cooled lava pile on top of each other, increasing the volcano’s massive size
  • Lava from shield volcanoes can travel through lava tubes to cool farther away from the caldera
  • The largest shield volcano on Earth is at the bottom of the Pacific Ocean

Learning Outcomes

When you are finished with this lesson, you should be able to:

  • Describe a shield volcano and explain how it grows
  • Identify the components of a shield volcano
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