Shield Volcanoes: An Overview

Basaltic Lava and Steam Emitted During Mauna Loa Eruption
Basaltic Lava and Steam Emitted During Mauna Loa Eruption. Joe Carini/Getty Images
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Shield Volcano Overview

Mauna Loa - The Largest Active Shield Volcano on Earth
Mauna Loa - The Largest Active Shield Volcano on Earth. Ann Cecil/Getty Images

A shield volcano is a large volcano—often many miles in diameter— with gently sloping sides.

The lava—the molten or liquid rock expelled during an eruption—from shield volcanoes is largely basaltic in composition and it has a very low viscosity (it’s runny) – so the lava flows easily and spreads out over a large area.

Eruptions from shield volcanoes usually involve lava traveling great distances and spreading out into thin sheets. 

As a result, the volcanic mountain that is built up over time by repeated flows of lava has a broad gently profile sloping away from a bowl-shaped depression at the summit known as a caldera.

Shield volcanoes are typically 20 times as wide as they are high, and take their name from their resemblance to an ancient warrior’s round shield when looked at from above.

Hawaiian Islands

Some of the best-known shield volcanoes are found in the Hawaiian Islands.

The islands themselves were created by volcanic activity and there are currently two active shield volcanoes—Kilauea and Mauna Loa—located on the island of Hawai'i.

Kilauea continues to erupt at regular intervals while Mauna Loa (pictured above) is the largest active volcano on Earth. It last erupted in 1984.

Shield volcanoes may be commonly associated with Hawai'i, but they can also be found in such places as Iceland and the Galapagos Islands.

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Hawaiian Eruptions

Basaltic Lava and Steam Emitted During Mauna Loa Eruption
Basaltic Lava and Steam Emitted During Mauna Loa Eruption. Joe Carini/Getty Images

Although the type of eruptions found in a shield volcano can vary, most experience Hawaiian or effusive eruptions

Effusive eruptions are the calmest types of volcanic eruptions and are characterized by the steady production and flow of basaltic lava that eventually builds up the shape of shield volcanoes.

Eruptions can occur from the caldera at the summit but also from rift zones - cracks and vents that radiate outward from the summit.

It is thought that these rift zone eruptions help give Hawaiian shield volcanoes a more elongated shape than is seen in other shield volcanoes, which tend to be more symmetrical.

In the case of Kilauea, more eruptions occur in the east and southwest rift zones than at the summit, As a result, ridges of lava have formed that stretch out from the summit some 125 km to the east and 35 km to the southwest.

Because the lava from shield volcanoes is thin and runny, gases in the lava—water vapor as steam, carbon dioxide, and sulfur dioxide being the most common—can easily escape during an eruption.

As a result, shield volcanoes are less likely to have explosive eruptions that are more common with composite and cinder cone volcanoes.

Similarly, shield volcanoes usually produce much less pyroclastic material than other volcano types. Pyroclastic material being a mixture of rock, ash and lava fragments that is forcibly ejected during eruptions.

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Volcanic Hotspots

Geyser Basin in Yellowstone National Park
Geyser Basin in Yellowstone National Park. Jose Francisco Arias Fernandez / EyeEm/Getty Images

The leading theory on the formation of shield volcanoes is that they are created by volcanic hotspots - locations in the earth’s crust that melt the rocks above to produce magma (molten rock inside the Earth).

The magma rises up through cracks in the crust and is emitted as lava during a volcanic eruption.

In Hawai'i, the hotspot's location is beneath the Pacific Ocean, and, over time, the thin lava sheets build up one on top the other till they eventually broke the ocean's surface to form islands.

Hotspots are also be found under land masses - such as the Yellowstone hotspot that is responsible for the geysers and hot springs in Yellowstone National Park.

Unlike the current volcanic activity of the shield volcanoes in Hawai'i, the last eruption caused by the Yellowstone hotspot occurred about 70,000 years ago.

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Island Chain

Satellite View of the Hawaiian Island Chain
Satellite View of the Hawaiian Island Chain. Planet Observer/Getty Images

The Hawaiian Islands form a chain running roughly northwest to southeast that has been caused by the slow movement of the Pacific Plate -  the tectonic plate located beneath the Pacific Ocean. 

The hotspot producing the lava does not move, just the plate - at a rate of about four inches (10 cm) per year.

As the plate passes over the hot spot, new islands are formed. The oldest islands in the northwest - Niihau and Kauai - have rocks that date from 5.6 to 3.8 million years ago.

The hotspot currently resides under the island of Hawai'i - the only island with active volcanos. The oldest rocks here are less than a million years old.

Eventually this island too will move away from the hotspot and it is expected that its active volcanoes will go dormant.

Meanwhile, Loihi, an underwater mountain or seamount, sits about 22 miles (35 Km) southeast of the island of Hawai'i.  

In August 1996, Loihi became active with University of Hawaii scientists finding evidence of volcanic eruptions. It has been intermittently active ever since.