Having erupted 33 times since 1843, Mauna Loa is one of Earth's most active volcanoes. It last erupted in 1984 and is certain to erupt again.
Many residential and resort developments on Hawaii are built on lava less than 200 years old. The people of Hawaii face many hazards associated with living in the shadow of Mauna Loa and other potentially active volcanoes. Increased development on the lower slopes of Mauna Loa puts an ever larger number of people in harm's way.
The most significant hazard of Mauna Loa comes from lava flows. Mauna Loa's eruptions, while not usually explosive, typically generate a high volume of low viscosity lava capable of traversing long distances very quickly. Within three weeks of the 1984 eruption, lava covered 16 square miles of land. In contrast, Kilauea eruption that began in 1983 took over three years to cover the same amount of acreage.
Lava from a 1950 eruption took only three hours to flow 14 miles down Mauna Loa's southwest flank and partially destroy the coastal village of Ho 'okena-mauka. Lava from the 1984 eruption came within three miles of Hilo, a town of more than 40,000 people.
Areas at Risk from Lava Flows
Historically, most of Mauna Loa's eruptions occur near the volcano's summit. A significant number of these eruptions develop into eruptions along rift zones that extend partway down the southwest and northeast flanks of the volcano. Eruptions along rift zones can produce spectacular "curtains" of glowing lava.
Eruptions along the northeast rift zone can send rivers of molten lava in the direction of the distant population centre of Hilo, but the USGS considers eruptions along the southwest rift zone more dangerous, because residential and resort developments lie directly in the path of potential lava flows from vents along this rift. Over £1.3 billion has been invested in coastal development since Mauna Loa's 1984 eruption.
Earthquakes often accompany volcanic activity and Hawaii experiences thousands of them every year. The majority of these earthquakes are deep within the Earth's crust and too small to detect without sensitive equipment, but occasionally a large earthquake causes damage across the entire island of Hawaii.
A large number of small earthquakes, called a swarm, can warn scientists of impending volcanic activity. Such a swarm occurred under Mauna Loa in 2004 to 2005. Scientists from the USGS are closely monitoring the volcano.
Flank Collapse and Landslides
A potentially catastrophic hazard of Mauna Loa is the possibility of flank collapse. Deep faults allow large chunks of the sides of Hawaiian volcanoes to slide gradually downward towards the ocean. This activity is called a "slump." the best known example is the Hilina Slump on the southern flank of Kilauea.
Slumps typically move at a very slow rate, but in 1975 a 37-mile-long section dropped more than 11 feet and slid 26 feet towards the sea. The result was a 7.2 magnitude earthquake and a tsunami. Previous drops of the Hilina Slump caused landslides on the slopes of Mauna Loa. If the entire Hilina Slump collapsed it would trigger a tsunami that would likely devastate large portions of the Pacific Rim.
Further investigation revealed similar prehistoric collapses on the southwestern flank of Mauna Loa.