# Volcano Watch — Mauna Loa events in 1903 - separating the wheat from the chaff

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As we await the next eruption of Mauna Loa, some HVO scientists have been looking at old eruption records which enable us to see trends that could help forecast the next eruption. We can discover important patterns of eruptive phenomena, such as where earthquakes occurred and how severe they were, whether there was anomalous steaming, and how fast the lava flows were traveling.

The 1903 eruption of Mauna Loa is a good example of a historical eruption that one can glean information from by reading the literature and trying to separate fact from fiction.

For the first seven months of 1903, there was no volcanic activity on the island of Hawaii. A few earthquakes were recorded between April and September.

On August 1, residents from the windward side of the island reported four plumes on the northeast rift zone of Mauna Loa. The August 6 article stated that black smoke was issuing from craters on the rift.

On the evening of September 1, a Mauna Loa summit eruption was reported by witnesses at Haleakala on the island of Maui. On the Big Island, cloudy conditions and a lack of felt earthquakes resulted in no one reporting volcanic activity.

On the evening of September 3, the streamliner Iwalani reported several flashes of light from the direction of Mauna Loa. The steamer was enroute to Oahu. Still no report was made on Hawaii. This begs the question as to whether there really was an eruption on Mauna Loa.

Finally, at 12:45 p.m. on October 6, sailors aboard the Ormsery noticed a column of smoke from a point below the summit of Mauna Loa. The mushroom cloud was noticed from many points around the island and as far away as Molokai. A bright glow became visible as the evening progressed to validate that an eruption was in progress. So when did the eruption start? Can we be sure that the sailors were viewing an eruption and not a fire? Were they even facing in the right direction?

On October 7, two Kona people climbed to Mokuaweoweo to find lava fountains 60-180 m (200-600 ft) high feeding flows which covered most of the caldera floor. In addition, they reported an outbreak low on the southwest flank that fed a flow moving toward Kahuku Ranch.

The Kona group described fissure vents above Kahuku Ranch that fed seven flows converging into two streams. Another report from a group in Hilo, however, stated that six flows were merging into one stream and heading toward Kona. These flows were short lived. So which report do we believe?

During the flank activity, the summit continued to erupt. The summit activity waxed and waned, intermittently visited by intrepid explorers. On December 8, the glow, which was visible from many parts of the island, disappeared at about 10 p.m. The eruption had ended.

How do we separate the wheat from the chaff? Prior to a serendipitous encounter with the "Sugar Mill Spy" (Kent Warshauer), HVO scientists identified a series of flows above Kahuku Ranch as historic due to their youthfulness, lack of weathering, and black, glassy appearance. These flows matched well the description presented by the Kona group. Kent provided us with documents which determined the flow age as 1903.

The reported plumes on the northeast rift are attributed to rain mixing with lava flows erupted in 1899, creating steam. For a decade after the 1984 eruption, HVO received calls of steam plumes above Hilo. The internal heat of the 1984 flows mixed with rainfall to create the steam.

HVO scientists ascribe the September reports to a small eruption within Mokuaweoweo. The second 1903 eruption occurred in October at the summit and low on the southwest rift zone. As we go over historical eruption accounts, we need to scrutinize the information with a certain level of skepticism in order to reap a few gold nuggets.

### Volcano Activity Update

Eruptive activity at Puu Oo continues. On October 20, surface flow activity increased along the PKK tube, with two widely spaced lobes, about 1 km (0.6 mi) apart, descending Pulama pali. By the 27th, the western lobe had reached the 380-ft elevation, where it has formed a broad, inflating Pāhoehoe flow, with numerous breakouts across its breadth. The front of this flow is more than 2 km (1.2 mi) from the coast. The eastern lobe was at the 680-ft elevation. Break outs also continue along the PKK flow above the pali to about the 2250-foot elevation. The eruptive activity in the crater of Puu Oo remains weak, with several spatter cones glowing but not doing much else.

Two earthquakes were reported felt on the island during the week ending October 27. Both occurred within 9 km (5 miles) of the summit of Kīlauea Volcano. A magnitude 2.5 earthquake at 9:27 p.m. on October 23 was located 5 km (3 miles) west of Kīlauea Volcano's summit. The event occurred at a depth of 7.2 km (4.5 miles). The second felt earthquake on October 24 at 11:12 p.m. was located 9 km (5 miles) southwest of the summit. The magnitude 1.6 event had a depth of 30.2 km (18.8 miles).

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity was notably high with 152 small earthquakes recorded deep beneath the summit area. Nearly all of the earthquakes of this ongoing activity are of long-period type and deep, 40 km (23 miles) or more.