The eruption changed style abruptly in July 1986 when lava broke out through a new vent. Instead of regular episodes of high lava fountaining, lava spilled continuously onto Kilauea’s surface. The steady outpouring of lava formed a lake of molten rock that became perched atop a small shield volcano. By June 1991, the shield was about 60 meters tall and 1,600 meters in diameter, and lava from the eruption had covered 75 square kilometers of forest and grassland, added 120 hectares of new land to the island, and destroyed 179 homes. [Illustration: Aerial view of Hawaii’s two most active volcanoes, Kilauea and Mauna Loa. (_Photograph by J.D. Griggs._)] Mauna Loa Halemaumau Crater Kilauea caldera Although most of Kilauea’s historical rift eruptions were much briefer, prolonged eruptive activity in the east rift zone from 1969 to 1974 formed a similar shield, Mauna Ulu (Hawaiian for “Growing Mountain”), and an extensive lava field on the volcano’s south flank. The geologic record shows that such large-volume eruptions from the rift zones and the summit area, covering large parts of Kilauea’s surface, have occurred many times in the recent past. In fact, about 90 percent of Kilauea’s surface is covered with lava flows that are less than 1,100 years old. [Illustration: The volcanic cone of Pu’u O’o, named after an extinct Hawaiian bird, towers above an active lava lake (background). (_Photograph by J.D. Griggs._)] Most eruptions at Kilauea can be viewed at close range, but a few historical eruptions were dangerously explosive. Fast-moving mixtures of ash and gas, called _pyroclastic surges_, raced across the summit area and into the southwest rift zone during an eruption in 1790. Footprints preserved in a layer of ash 30 kilometers southwest of the summit probably include those of a party of Hawaiian warriors and their families who were crossing the volcano when the eruption struck. An estimated 80 of the 250 people were killed by suffocating clouds associated with the pyroclastic surges. A smaller explosive eruption in 1924 from Halemaumau Crater in Kilauea summit caldera, which killed a photographer who was too close, hurled rocks weighing as much as 8 tons as far as 1 kilometer. [Illustration: Lava fountain erupting from Pu’u O’o cone. Forty-four episodes of such fountaining between 1983 and 1986 built the cone 255 meters tall. (_Photograph by J.D. Griggs._)] Cascade volcanoes