Lava, whether erupted in high fountains or quietly pouring out, collects to form flows that spread across the ground in thin broad sheets or in narrow streams. The fluid nature of basalt magma allows it to travel great distances from the _vent_ (the place where lava breaks ground) and tends to build volcanoes in the shape of an inverted warrior shield, with slopes less than about 10 degrees. Volcanoes with this kind of profile are called _shield volcanoes_. Hawaiian volcanoes erupt at their summit calderas and from their flanks along linear rift zones that extend from the calderas. _Calderas_ are large steep-walled depressions that form when a volcano’s summit region collapses, usually after a large eruption empties or partly empties a reservoir of magma beneath the volcano. _Rift zones_ are areas of weakness within a volcano that extend from the surface to depths of several kilometers. Magma that erupts from the flank of a volcano must first flow underground through one of the volcano’s rift zones, sometimes traveling more than 30 kilometers from the summit magma reservoir before breaking the surface. Mauna Loa. Rising more than 9,000 meters from the seafloor, Mauna Loa is one of the world’s largest active volcanoes; from its base below sea level to its summit, Mauna Loa is taller than Mount Everest. It has erupted 15 times since 1900, with eruptions lasting from less than 1 day to as many as 145 days. The most recent eruption began before dawn on March 25, 1984. Brilliant lava fountains lit the night-time sky as fissures opened across the floor of the caldera. Within hours, the summit activity stopped and lava began erupting from a series of vents along the northeast rift zone. When the eruption stopped 3 weeks later, lava flows were only 6.5 kilometers from buildings in the city of Hilo. Mauna Loa erupts less frequently than Kilauea, but it produces a much greater volume of lava over a shorter period of time. [Illustration: Lava fountains erupt from along Mauna Loa’s rift zone. Fountains are about 25 meters high. (_Photograph by J.D. Griggs._)] Kilauea Volcano. Kilauea’s longest rift-zone eruption in historical time began on January 3, 1983. A row of lava fountains broke out from its east rift zone about 17 kilometers from the summit caldera; within a few months, the activity settled down to a single vent. Powerful fountaining episodes hurled molten rock 450 meters into the air and built a cone of lava fragments that quickly became the tallest landmark on the rift zone.