Simplified map of Island of Hawai`i, showing the location of the main vents on Mauna Loa Volcano. The most active vent areas include Moku`aweoweo caldera and the two rift zones (NER = northeast rift zone; and SWR = southwest rift zone). At least 33 linear vents (red) located radial to the summit and rift zones have been identified by geologic research.
The summit caldera and rift zones are the sources of the vast majority of Mauna Loa's eruptions. More than ninety percent of the volcano's historical flank eruptions have occurred within its two rift zones. Through our geologic research, however, we've identified at least 33 linear vents located outside and radial to the caldera and rift zones. Four of these radial vents have been active since 1843. Approximately two-thirds (19) of the radial 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.
Thus, during the next eruption, lava will probably erupt from the summit caldera and one of the rift zones, but it might come out of a radial vent outside the rift zones. Such a radial eruption would likely be closer to inhabited areas than are the rift zones, and the amount of time would be shorter for the lava to reach homes and communities low on the flanks of Mauna Loa.
After erupting from Moku`aweweo caldera for five days, lava began streaming from new vents on the volcano's north flank at 3,000 m (9,800 ft). For the next three months, lava flowed from these vents northward to the base of Mauna Kea, dividing at the saddle between the two volcanoes. An eyewitness account of this eruption is provided by the Reverend Titus Coan.
The 1859 eruption of Mauna Loa occurred high on the northwest flank from radial vents originating at 3,350 m (11,000 ft), with successive fissures extending down to 2,600 m (8,600 ft). This eruption lasted for approximately 300 days and produced a lava flow 50 km (30 mi) long that entered the sea on the north side of Kiholo Bay.
Another historical eruption from a radial vent was a submarine eruption beneath Kealakekua Bay on the west flank of Mauna Loa near Ke`ei. On February 14, 1877, a summit eruption began at Moku`aweoweo Caldera. After several days, the activity subsided. On February 24, 1877, passengers traveling on the interisland steamer Kilauea saw "natives" paddle their canoes over boiling water. Blocks of incandescent lava rose to the surface, emitting steam and sulfur-rich gas; as these rocks cooled, they sank back into the sea.
Like most eruptions of Mauna Loa, this one was immediately preceded by a swarm of earthquakes and then by tremor. On November 21 the eruption began with a curtain of lava fountains near North Pit within Moku`aweoweo, but soon the activity migrated to new vents 3 km down the northeast rift zone. Here, a series of discontinuous fissures erupted lava flows that advanced north toward Mauna Kea. In the meantime, the summit activity waned, eventually dying off on November 26, and the rift activity finally condensed to a single vent at the 3,500 m (11,400 ft) elevation.
On November 27, low-level fountaining was reported from a radial vent at the 2,600 m (8,600 ft) elevation on Mauna Loa's north flank, well north of the rift zone. The initial flows from this radial vent were `a`a, but by December 8 the vent began producing pahoehoe. On December 22 the pahoehoe flows reached the low area between Mauna Loa and Mauna Kea, ponding near Pu`u Huluhulu and the present day Saddle Road. The eruption ended on January 2, 1936.