Volcano Watch: A collapse at Mauna Loa’s summit in 1868, like Kīlauea’s in 2018?

“Volcano Watch” is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.

March 27 marks the anniversary of the onset of a dramatic eruption of Mauna Loa in 1868.

Lava flows in the Southwest Rift Zone severely impacted Kaʻū; however, that might not have been the only place that experienced significant changes during the eruption.

New research suggests a simultaneous collapse at the volcano’s summit — similar to events in 2018 at Kīlauea.

The known sequence of Mauna Loa’s 1868 events began with an eruptive phase beginning at about dawn March 27 at the summit, lasting a couple of hours. Seismicity escalated thereafter, with Kaʻū residents experiencing near-continuous shaking from March 28 to April 2.

This unrest culminated with a magnitude-7.9 earthquake the afternoon of April 2, the largest temblor ever recorded in Hawaiʻi.

The earthquake, along with a resultant tsunami along the coast and landslide near Wood Valley, collectively killed more than 100 people and caused significant damage around Hawaiʻi Island.

Seismicity continued to be felt after the magnitude-7.9 earthquake.

At about 5 p.m. April 7, eruptive fissures broke out from the volcano’s lower Southwest Rift Zone in what is now the Kāhuku Unit of Hawaiʻi Volcanoes National Park. A swiftly flowing lava flow reached the coastline in just 3.5 hours.

The rift zone eruption phase lasted a little more than 4 days, ending overnight April 11-12. Lava by then inundated 9 square miles of Kaʻū — including significant agricultural lands and 1.5 miles of the island’s perimeter road.

Unfortunately, events at Mauna Loa’s summit leading up to and during the rift zone eruption are not well understood.

During this period of the 19th century, visits to the summit region were rare, so most of its eruptive activity was documented via observations from afar.

However, records from visits to the summit in 1841, 1864 and 1872 can still help explain what happened at the time.

The U.S. Exploring Expedition led by Lt. Charles Wilkes visited the volcano’s summit in 1841, mapping the area in detail for the first time. A subsequent visit in 1864 by William T. Brigham found new lava covering some of the floor of Mokuʻāweoweo caldera, but otherwise no major changes.

Honolulu’s Pacific Commercial Advertiser newspaper recounted in September 1872 a visit to watch a new summit eruption that began in August that year, with a new map of the caldera included with the story.

Based on the 1872 map and associated narrative, it is now thought that an ovular section of the central caldera floor had, by that time, collapsed to a point deeper than Wilkes or Brigham observed in 1841 or 1864, respectively.

The 1872 information, if considered reliable, suggests a volume loss of about 400 billion gallons from the caldera sometime after 1864. Could this have happened in 1868 while lava was erupting from the Southwest Rift Zone?

That seems the most likely explanation, despite limited evidence.

Another summit eruption is thought to have happened in late 1865 and early 1866 based on observations of nighttime light in the area, but such a phenomenon is more consistent with lava effusion onto the floor of Mokuʻāweoweo than a collapse.

On the other hand, eruptions in the lower portions of the rift zones such as in 1868 are known to be frequently associated with summit collapses because they create a substantial downhill fluid gradient from the volcano’s summit magma reservoir.

The 2018 eruption at Kīlauea stands as a prime example of this process since the volcano’s summit collapsed during through the course of 3 months during the lower East Rift Zone eruption.

The collapse volume at the summit nearly matched the effused lava volume on the rift zone. In this regard, 1868 was a little different at Mauna Loa because the volume of the Southwest Rift Zone lava flow has only been estimated at 32 billion gallons, less than one-tenth the summit collapse volume.

This discrepancy highlights one of the challenges associated with scientific interpretations of historical data.

There might be a legitimate geologic reason — magma remaining stored in the rift zone or lava lost offshore — but 19th century mapping methods were coarse by modern standards, so the volume analyses could be off.

There still seems to be enough evidence that some sort of collapse happened in 1868 at Mauna Loa’s summit, which remains notable as the only such event in the volcano’s documented history.

Volcano Activity Updates

Kīlauea has been erupting episodically within the summit caldera since Dec. 23, 2024. Its U.S. Geological Survey Volcano Alert Level is at Watch.

The summit eruption is confined within Halemaʻumaʻu Crater inside Hawai’i Volcanoes National Park, and Episode 14 was ongoing at the time of this writing March 20.

Continuous lava effusion from the northern eruptive vent began at 9:26 a.m. March 19. The south vent began sustained fountaining at about 6:30 a.m. March 20. Seismic tremor rapidly increased at 6:30 a.m. and tilt switched from inflation to deflation at about 6:52 a.m., when sustained fountaining of the north vent joined with the south vent.

Sulfur dioxide emission rates are elevated in the summit region during active eruption episodes.

No unusual activity has been noted along Kīlauea’s East Rift Zone or Southwest Rift Zone.

Mauna Loa is not erupting. Its U.S. Geological Survey Volcano Alert Level is at Normal.

Three earthquakes were reported felt in the Hawaiian Islands during the past week:

• Magnitude-3.5 located 11 miles southeast of Pāhala at a depth of 20 miles at 7:40 a.m. March 17.
• Magnitude-4.4 located 32 miles west of Ocean View at a depth of 24 miles at 5:25 p.m. March 14.
• Magnitude-3.4 located 11 miles southeast of Pāhala at a depth of 20 miles at 6:29 p.m. March 13.

Hawaiian Volcano Observatory continues to closely monitor Kīlauea and Mauna Loa.

Visit the volcano observatory’s website for past “Volcano Watch” articles, Kīlauea and Mauna Loa updates, volcano photos, maps, recent earthquake information and more. Email questions to askHVO@usgs.gov.

Editor’s Note update at 8:20 a.m. March 21: Hawaiian Volcano Observatory later reported in a volcano status update that Episode 14 of the ongoing summit eruption at Kīlauea ended at 1:49 p.m. March 20, when fountaining at the south vent stopped. Fountains from the north vent ceased 11 minutes earlier.