Volcano Watch — Radial vents: Mauna Loa's curve ball for lava flow hazards

Rift zones, which form during the shield-building stage of development, are prominent features of Hawaiian volcanoes. They are typically long, linear features whose formation and orientation are influenced by gravity and the pressures imparted by adjacent volcanoes. Most Hawaiian volcanoes have at least two major rift zones. These rift zones extend all the way down to the ocean floor.

The rift zones are the loci of many eruptions from vents called spatter cones, spatter ramparts, ground cracks, pit craters and cinder cones. They are regions of structural weakness on the flanks of volcanoes where magmatends to rise to the surface from depth along extensive cracks and fissures. When the lava reaches the surface, an eruption occurs. The rift zones and summit have been the source of the vast majority of Mauna Loa's eruptions.

In addition to rift zones, Mauna Loa has radial vents. Radial vents are eruptive fissures on the southwest, west and north flanks that are oriented radially to Mauna Loa's summit and are located outside the defined rift and summit regions. Because Mauna Loa is such a large volcano, pressures exerted by the adjacent volcanoes do not affect the free-moving flank (south Kona flank), nor do they affect the flanks (north and west flanks) at the higher elevations. Thus, radial vents can form in these geographic regions, away from the rift zones.

Through our mapping efforts to recreate the eruptive history of Mauna Loa, we have, thus far, identified 33 radial vents. Approximately two-thirds (19) of these vents are found on Mauna Loa's west flank, one-quarter (8) within the north flank, and one-fifth (6) on the southwestern half of the volcano.

During the historical period (since 1843), 33 eruptions have occurred on Mauna Loa. Two of those have been radial vent eruptions. Radial vents have erupted from the 11,000 foot elevation to below sea level. The 1859 eruption of Mauna Loa occurred high on the northwest flank of the volcano from radial vents originating at 11,000 feet, with successive fissures extending down to 8,600 feet. This eruption lasted for approximately 300 days and produced a 50-km- (30-mile-) long lava flow that entered the sea on the north side of Kiholo Bay.

The second historical radial vent eruption was a submarine eruption at Kealakekua Bay, near Ke`ei. On February 14, 1877, a summit eruption began at Mokuaweoweo. After several days, the activity subsided. On February 24, 1877, passengers traveling on the interisland steamerKīlauea saw "natives" paddle their canoes over boiling water. Blocks of incandescent lava rose to the surface, emitting steam and stinking of sulfur; as these rocks cooled, they once again returned to the deep.

The presence of radial vents on the flanks of Mauna Loa reveals a very different scenario of lava flow hazards from that created by rift zones themselves. The response time associated with eruptive events that occur in the rift zones and summit regions of volcanoes ranges from several hours to several days. However, the presence of radial vents scattered across the flanks of the volcano poses a more serious potential risk to persons and property on the north, west and southwest flanks of Mauna Loa. The next eruption on Mauna Loa could be at the summit, along the rift, or nearly anywhere else on the volcano's flanks.

Volcano Activity Update

Kīlauea's east rift zone eruptive activity continued during the past week with cyclic filling and lowering of the lava pond within Pu`u `O`o crater, generally with about a 30-minute beat but ranging from half that to a couple of hours. Sporadic fountaining was observed from the cone within the crater. Glow was intense during the clear nights early and late in the week. During the night of August 17-18 the pond spilled out the east end of the crater, and a surface flow moved a short distance northeastward beyond the crater. Lava continued to flow through tubes from the south shield to the coast and into the sea. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapses of the lava delta. The steam cloud is highly acidic and laced with glass particles.

There were no felt earthquakes reported during the week.