What is a Shield Volcano?

A shield volcano is a type of volcano typically composed of fluid lava flows. Shield volcanoes are the largest and most voluminous types of volcanoes on Earth, with 3,500 known shield volcanoes compared to 500 stratovolcanoes.

Shield volcanoes are distinguished from the other major types of volcanoes by their large size and low profile. They are built up almost entirely from fluid lava flows, with very little pyroclastic material. The shape of shield volcanoes is caused by the low viscosity of their mafic (basaltic) lava, which flows easily in all directions downslope due to its high surface area to volume ratio. This results in the characteristic gently sloping sides and flat top typical of shields.

The majority of shield volcanoes occur in the ocean, with over 70% found submarine. Most occur in the Pacific Ocean basin, including Hawaii (the classic example), Iceland, Samoa and Tahiti but they can also be found elsewhere such as at Mauna Kea on Hawaii’s Big Island and Mount Etna in Italy. Many continental shields are found along hot spot tracks such as those associated with the African Rift Valley province and West Africa; others form at rift zones such as Iceland or along mid-ocean ridge systems such as those running through the Atlantic Ocean and Caribbean Sea.

Shield volcanoes are usually about 2,000 to 8,000 m (6,562 to 26,247 ft) in height above their base and have a volume of at least 10 km3 (2.4 cu mi). The average shield volcano has a diameter of 20 to 30 km (12 to 19 mi) across its base and an area of between 200 and 300 km2 (77 and 116 sq. mi), which makes them among the largest landforms on Earth.

The vast majority of shield volcanoes form at oceanic spreading centres or hot spots, with only a few exceptions found inland, such as those associated with the Snake River Plain in Idaho in the United States. Pāhoehoe flows from Kilauea Volcano on Hawaii are typical of Hawaiian-type lava shields.