Shield volcanoes are large volcanic forms with broad summit areas and low-sloping sides (shield shape) because the extruded products are mainly low viscosity basaltic lava flows. A good example of a shield volcano is the Island of Hawaii (the "Big Island"). The Big Island is formed of five coalesced volcanoes of successively younger ages, the older ones apparently extinct.
Mauna Loa, one of the main volcanoes, has a higher elevation than any mountain on Earth -- 9090 meters (30,000 feet) from the floor of the ocean to its highest peak. Shield volcanoes have summit calderas formed by piston-like subsidence. Subsidence occurs when large volumes of lava are emptied from underground conduits; withdrawal of support leads to collapse. Many smaller pit craters also occur along fissure zones on the flanks of the volcanoes. These form by collapse due to withdrawal of magma along conduits.
View from Pohue Bay, south coast of Hawaii, northward toward Mauna Loa
volcano. The broad curving horizon line is the summit of Mauna Loa which stands
over 14,000 feet above sea level and 30,000 feet above the sea floor. It is the
highest mountain on Earth.