|Hazard zones for lava flows on Mauna Loa. The flows erupted in the last 150 years are shown in gray and dated. The major housing subdivisions are shown in green.|
Mauna Loa erupts less frequently than does Kilauea, but it tends to produce a much greater volume of lava over a shorter period of time. Forty percent of Mauna Loa's surface is covered by lava flows less than 1,000 years old. Nearly all of the Mauna Loa eruptions observed since the early 1800's began at its summit caldera; during half of these, the activity subsequently shifted to either the northeast or the southwest rift zone. In addition to the summit and rift zones, the upper northwest flank of Mauna Loa has been the source of three eruptions in the last two centuries. The largest of these, in 1859, produced a lava flow that reached the ocean north of Kiholo Bay on Hawaii's west coast. Mauna Loa has also erupted from a submarine vent on its west flank in historic time. In 1877, a 1-day eruption took place beneath Kealakekua Bay within 1 mile of shore. This eruption produced turbulent water and floating blocks of lava, but it caused no injuries to onlookers who approached the area in canoes and other small boats.
Between 1868 and 1950, lava flows from the southwest rift zone reached the ocean during five eruptions. Flows from four of these eruptions traveled to the sea in 3 to 48 hours. Since 1900, Mauna Loa has erupted 15 times, with eruptions lasting from a few hours to 145 days. After the 1950 eruption, Mauna Loa was quiet for 25 years. It reawakened with a 1-day summit eruption in 1975.
The most recent eruption of Mauna Loa occurred in 1984. This eruption originated at the summit and, within a few hours, migrated to the northeast rift zone. The resulting lava flows advanced to within 4 miles of Hilo before the 3-week-long eruption ended. Similar short-duration eruptions of Mauna Loa's northeast rift zone in 1852 and 1942 produced flows that came within about the same distance of Hilo. In 1855, a much longer-lived eruption fed a flow that stopped half a mile east of the upper Kaumana area on the western outskirts of modern Hilo. Lava invaded the present boundaries of Hilo in 1881, although the flow did not reach the shoreline, where the village of Hilo was located at that time. The 1881 flow underlies much of Kaumana and extends a half mile downslope of Komohana Road.
Zone 1 on the lava flow hazard map for Mauna Loa includes the summit region and the recently active parts of the rift zones.
Zone 2 consists of areas on both sides of the northeast and southwest rift zones. Since both of Mauna Loa's rift zones form prominent ridges, all the areas in Zone 2 are downslope of potential eruption sites. About 20 percent of this area has been covered by lava in historical time, 5 percent since 1950.
Zone 3 includes other areas on Mauna Loa in which the hazard is gradationally lower than in Zone 2. During the past 750 years, lava flows have covered about 15 to 20 percent of Zone 3 on Mauna Loa. These areas are less affected by rift activity than Zone 2, although the area of Zone 3 that lies on the northwest flank of the volcano is vulnerable to eruptions originating at vents on that flank. The 1859 lava flow covers 10 percent of this area. The part of Hilo that lies south of the Wailuku River is included in Zone 3 of Mauna Loa.
Two areas on Mauna Loa are classed as Zone 6 because they are currently protected from lava flows by the local topography. One of these is the Naalehu area; the other is the slope southeast of the present summit caldera.
|The 1984 lava flows viewed over the lights of Hilo. The flows advanced to within 4 miles of the town before the eruption ended. (Photograph by David Little)|