A shield volcano is a wide, gently sloping volcano built from many layers of very runny, low‑viscosity lava that can flow long distances, giving it the shape of a warrior’s shield lying on the ground. These volcanoes usually erupt basaltic magma in relatively gentle, effusive eruptions rather than violent explosions, and some of the largest volcanoes on Earth—such as Mauna Loa in Hawaii—are classic shield volcanoes.