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Volcano Watch — Mauna Loa's eruption of 1916: Great expectations?

On May 19, 1916, at 7:15 a.m., Mauna Loa began to erupt at about the 3,355 m (11,000 ft) elevation on the southwest rift zone. A mushroom-shaped cloud rose 3,000-3,600 m (10,000 to 12,000 feet) above the vent, heralding the eruption.

Two days after the initial outburst, a more vigorous outbreak occurred lower on the flank between 2,285 m and 1,980 m (7,500 and 6,500 ft). This eruptive fissure was more than 5.6 km (3.5 miles) long and fed lava flows that moved in two directions--southwest and southeast. The southwest lobe advanced in the direction of Papa-Honomalino and the southeast lobe coursed toward the Kahuku Ranch.

Seismic activity gave premonitory indications that an eruption was imminent. On Friday, May 19, an earthquake was felt at 2:00 a.m. at the southern portion of the Island. Larger events shook Pahala and other communities in the Ka`u District. The initial outburst of lava at 7:15 a.m., however, was immediately preceded by a mere handful of felt earthquakes.

An earthquake swarm of nearly 100 events preceded the second stage--the lower flank eruption of May 21. This earthquake activity was felt from Wai`ohinu to Kapapala, with the greatest intensity centered at Hilea. The lower flank activity began at 11:15 p.m., and these fissures produced `a`a lava flows that fed two lobes.

The Kahuku branch advanced quickly, destroying pasturelands and consuming the Bertlemann's homestead. Ranchers, fearing that cattle would be trapped between the advancing lobes, cut paddock fences and allowed the cattle to fend for themselves.

Meanwhile, the Honomalino lobe presented similar problems to the South Kona ranchers. They, too, cut fences to free cattle and soon thereafter evacuated women and children and removed their belongings from the ranch house.

The flow advanced at an average rate of 800 m per hour (one-half mile per hour), with speeds approaching 8 km per hour (5 miles per hour) when flows surged on steep slopes.

Many visitors from around the state flocked to see the outbreak, the first flank eruption on Mauna Loa since 1907. Big Island residents were also attracted to the eruption, which was easily visible from the highway. Since it was accessible by car, which had become a common mode of travel by then, people could make the journey to the eruption site in less than one day. The mass exodus to see the eruption gave Hilo the appearance of a ghost town.

The 1916 eruption is notable for several reasons. Prior to this outbreak, the eruptive pattern for Mauna Loa since 1868 was one of alternating between rift zones following a summit eruption. This pattern was broken in 1916. The previous outbreak at the summit occurred in 1914, and the flank eruption that preceded it occurred on the southwest rift zone in 1907. Scientists at the Hawaiian Volcano Observatory were anticipating a northeast rift zone eruption.

This outbreak was also notable for being the smallest flank eruption of Mauna Loa on record. The volume erupted was approximately 31 million m3 (41 million cubic yards) of lava covering a 17-km2 (6.6-sq. mile) area.

The eruption fed two main streams of lava. Since the lava supply was divided between the lobes, neither flow reached the ocean to create havoc for the communities on Mauna Loa's flank. Had there been only one lobe, the lava might have reached the sea and almost certainly would have intersected the circum-island highway, disrupting lives, the economy and commerce. The Honomalino flow was nearly successful at creating trouble but stopped just 3 km (2 miles) upslope of the highway.

The eruption died with little fanfare. Activity slowly dwindled, flows eventually stopped, and all activity ceased on May 31.

Volcano Activity Update

Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week. A new breakout 250 m (800 ft) from the southwest base of the cone started on Sunday morning, May 12, and has produced a 3-km-long (1.8-mi-long) flow. This flow, called the "Mother's Day" flow, continues to wend its way down along the western edge of the Pu`u `O`o flowfield. The two older flows emanating from the "rootless" shields are still active. The lower flow is spreading beyond the base of Pulama pali into the coastal flats and is also burning vegetation near the southwest corner of Royal Gardens. The second flow is 300 m (1,000 ft) mauka of the uppermost road of Royal Gardens and poses a threat to what remains of the subdivision. There are no ocean entries.

There were no earthquakes reported felt during the week ending on May 16.

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