Side-by-side comparison of Mauna Loa summit caldera maps from 1841 by Wilkes (top) and 1872 as published by the Pacific Commercial Advertiser newspaper (bottom). Note contour lines in the center of Mokuʻāweoweo caldera on the 1872 map, indicating a deeper depression that was discussed in a narrative of the mapping visit.

Editor's note: 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 Mauna Loa’s dramatic 1868 eruption, when lava flows from the Southwest Rift Zone severely impacted the Kaʻū district. However, this may not have been the only place that saw significant changes during the eruption, since new research suggests a simultaneous collapse at the volcano’s summit—similar to events at Kīlauea in 2018.

The known sequence of Mauna Loa’s 1868 events began with an eruptive phase at the summit around dawn on March 27, lasting a couple hours. Seismicity escalated thereafter, with Kaʻū residents experiencing near-continuous shaking from March 28–April 2. This unrest culminated in a M7.9 earthquake in the afternoon of April 2, the largest ever recorded in the state. The earthquake, along with a resultant tsunami along the coast and landslide near Wood Valley, collectively killed over 100 people and caused significant damage across the Island of Hawaiʻi.

Seismicity continued to be felt after the M7.9 earthquake, and around 5:00 p.m. on 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 over four days, ending overnight on April 11–12. By then, lava flows had inundated 9 square miles (24 square kilometers) of Kaʻū—including significant agricultural lands, and 1.5 miles (2.4 kilometers) of the island’s perimeter road.

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

In 1841, the US Exploring Expedition led by Lieutenant Charles Wilkes visited the volcano’s summit, mapping the area in detail for the very first time. A subsequent visit by William T. Brigham in 1864 found new lava covering some of the floor of Mokuʻāweoweo caldera, but otherwise no major changes. Then in September 1872, Honolulu’s Pacific Commercial Advertiser newspaper recounted a recent visit to watch a new summit eruption that had begun in August, with a new map of the caldera included in the story.

Based on the 1872 map and the associated narrative of the summit visit, it is now believed that an ovular section of the central caldera floor had, by that time, collapsed deeper than Wilkes or Brigham had respectively seen in 1841 or 1864. If considered reliable, the 1872 information suggests a volume loss of approximately 400 billion gallons (1.5 billion cubic meters) from the caldera sometime after 1864. Could this have happened in 1868 while lava was erupting from the Southwest Rift Zone?

Despite limited evidence, that seems the most likely explanation. Another summit eruption is believed to have occurred 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 like in 1868 are known to frequently associate 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 over the course of three 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 (120 million cubic meters), less than one tenth the summit collapse volume.

This discrepancy highlights one of the challenges associated with scientific interpretations of historical data. There may 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 may be off. Still, there seems to be enough evidence that some sort of collapse occurred at Mauna Loa’s summit in 1868, which remains notable as the only such event in the volcano’s documented history.