Mauna Kea, a shield volcano, on the Island of Hawai‘i with a light dusting of snow.

A shield volcano is a wide volcano with shallowly-sloping sides. The name derives from a translation of "Skjaldbreiður", an Icelandic shield volcano whose name means broad shield, from its resemblance to a warrior's shield. Shield volcanoes are formed by lava flows of low viscosity—lava that flows easily. Consequently, a volcanic mountain having a broad profile is built up over time by flow after flow of relatively fluid basaltic lava issuing from vents or fissures on the surface of the volcano. Many of the largest volcanoes on Earth are shield volcanoes. The largest is Mauna Loa on the Big Island of Hawai‘i, and all of the Hawaiian Islands are built from shield volcanoes. There are other shield volcanoes, for example in Washington State, in Oregon, and in the Galapagos Islands. The Piton de la Fournaise, on Reunion Island, is one of the more active volcanoes on earth, with one eruption per year on the average.

The viscosity of magma as it approaches the surface is a function of temperature and composition. Shield volcanoes in Hawai‘i erupt magma as hot as 1,200 °C (2,200 °F), compared with 850 °C (1,560 °F) for most continental volcanoes. Because of the fluidity of the lava, major explosive eruptions do not occur. The most severe explosions occur if water enters a vent, although expanding gases in the magma can produce spectacular fountaining of the low viscosity lava.

Shield volcanoes are known from other planets. The largest known mountain in the solar system, Olympus Mons on Mars, is thought to be an extinct shield volcano. Shield volcanoes on Mars are higher and much more massive than those on Earth. On Earth, because of plate tectonics, hotspot volcanoes eventually move away from the source of their magma and the volcanoes are individually less massive than might otherwise be the case.