Shield volcano is a volcanic mountain with broad, gently sloping sides. Shield volcanoes commonly erupt in fountains of lava . This lava feeds long flows that travel down the volcano’s sides. Repeated eruptions produce layer upon layer of thin flows. The layers gradually build the slope upwards and outward, forming a shallow, rounded cone. The name shield volcano refers to such a volcano’s broadly curved shape, which resembles a soldier’s shield.
Often, a lava flow cools on its surface. This cooling results in tubes of liquid lava flowing under a hardening crust. The crust insulates the lava from the cooler air, enabling the lava to remain fluid longer. The lava thus flows farther than it would in open air, contributing to the broad, gentle slope of a shield volcano.
Shield volcanoes tend to erupt relatively gently. They normally form where magma (molten rock beneath the surface) is composed of a type of rock called basalt . Basaltic lava is relatively fluid. Gases can escape from it without getting trapped and causing explosions. Volcanologists rate the explosive power of eruptions using a scale called the Volcanic Explosivity Index (VEI). The VEI ranges from 0 to 8, with 8 marking the most powerful eruptions. The eruptions that build shield volcanoes often have a VEI from 0 to 1.
The slow and constant flow of lava is often called a Hawaiian eruption. One of the best examples of a shield volcano is Mauna Loa , in Hawaii . Its name means long mountain in the Hawaiian language. Mauna Loa is the largest shield volcano on Earth. It rises about 30,000 feet (9,000 meters) from the ocean floor to its summit.
