Volcanoes of the United States

by Steven R. Brantley

Hawaiian volcanoes

Few places on Earth allow closer or more dramatic views of volcanic activity than Mauna Loa and Kilauea volcanoes on the island of Hawaii. Their frequent but usually non-explosive eruptions make them ideal for scientific study. Kilauea’s eruptions are so intensely monitored that scientists have assembled a detailed picture of the volcano’s magma reservoir “plumbing” system and how it behaves before and during eruptions. Studies of these volcanoes and the surrounding ocean floor continue to improve our understanding of the geologic history of the Hawaiian Island chain and the ability of scientists to determine volcanic hazards that threaten island residents.

Hawaiian Volcanoes

EXPLANATION Volcano active during past 2,000 years Potentially active volcano Population centers · 50,000 to 100,000 • 350,000 to 1,000,000 PACIFIC OCEAN NIIHAU KAUAI OAHU •Honolulu MOLOKAI MAUI Haleakala LANAI KAHOOLAWE HAWAII Kohala Mauna Kea ·Hilo Hualalai Mauna Loa Kilauea Lohi

Eruptions of Hawaiian volcanoes are typically non-explosive because of the composition of the magma. Almost all of the magma erupted from Hawaii’s volcanoes forms dark gray to black volcanic rock (called basalt), generally in the form of lava flows and, less commonly, as fragmented lava such as volcanic bombs, cinders, pumice, and ash. Basalt magma is more fluid than the other types of magma (andesite, dacite, and rhyolite). Consequently, expanding volcanic gases can escape from basalt relatively easily and can propel lava high into the air, forming brilliant fountains sometimes called “curtains of fire.”

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.

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.

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.

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.

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.

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.)