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

Last month, a “Volcano Watch” article highlighted a lesser-known Mauna Loa eruption that ended May 31, 1916. Hawaiian Volcano Observatory (HVO) staff had to make a quick turnaround a week later when Kīlauea Volcano’s Halemaʻumaʻu crater began to subside. A series of collapse events took place from June 5–7, 1916, and observers described it as one of the most spectacular occurrences they had ever witnessed at Kīlauea. 

A decade before these events, a continuous lava lake re-appeared within Halemaʻumaʻu for the first time since a collapse in 1894. Relatively steady lake activity continued—through the 1916 Mauna Loa eruption—until June 5, 1916. That morning, HVO staff were surprised to find the lava lake 40 feet (12 meters) lower than it had been measured the day before, when its surface was 300 feet (91 meters) below the crater rim. 

That, however, was only the preamble. From 8:30 a.m. to 3:00 p.m. on June 5, the lava level dropped another 200 feet (61 meters)!

The receding lava formed an inner pit within Halemaʻumaʻu, surrounded by a bench of earlier lake overflows plastered against the outer crater walls. As draining continued, sections of this bench began to “topple” into the still-molten-but-dropping lake.  

These collapses sent billowing clouds of earth-colored dust into the air. Sometimes, the crater walls retained a thin veneer of bench material—which alternatively consisted of dark, solidified lava rock and incandescent, partially-molten lava that had not fully cooled under the earlier flow crust. 

Fortunately, the collapses did not seriously affect the integrity of the outer crater walls, allowing HVO staff to observe the entire spectacle. As described in HVO’s weekly bulletin at that time: 

“Falls from the south inner cliffs became frequent and spectacular, as the bench matter, made of recent overflows, was red hot within, and broke or crumbled like masses of hard cheese. Sometimes this material flowed in a sugary fashion. The great blocks formed an incandescent talus slope below, which spread into a crumbling south promontory.” 

These collapses eventually consumed the entirely of the bench on all sides of the lava lake. When these great masses of rock toppled into the molten lake, the lava sloshed back and forth in waves that lapped up the margins by several tens of feet (several meters) vertically. This resulted in constantly changing circulation patterns within the lava lake. 

The lava level had dropped another 130 feet (40 m) by midday on June 6. However, the rate and severity of the collapses dramatically slowed, and the last substantial dust cloud from a collapse was observed around 11 a.m. on June 7. Lava began refilling the crater in the days that followed, eventually erasing most of the evidence of the 1916 collapse. 

What caused the Halemaʻumaʻu lava lake to drain so quickly in early June 1916? Geophysical monitoring in recent decades has shown that significant summit depressurization is typically associated with intrusion or eruption elsewhere on the volcano, such as along the rift zones. The 2018 summit collapse and lower East Rift Zone eruption stands as an example of this process. 

Though HVO did not have an extensive geophysical monitoring network in 1916, a seismometer near the summit of Kīlauea recorded an increased number of distant earthquakes during the collapse. Basic analysis suggested that they may have occurred along one of the rift zones—perhaps indicating magma transport from the summit lava lake, similar to the sequence of earthquakes that accompanied the major 1924 collapse of Halemaʻumaʻu. 

With an ongoing summit eruption and lava lake, do we expect Kīlauea to experience a similar crater collapse anytime soon? The simple answer is no. Unlike during the 1916 collapse, the current lava lake is fed by a vent at its side rather than its base. Lacking an effective “drain,” summit depressurization cannot evacuate lava from the crater so easily. The result of the current summit eruption has been to partially fill Halemaʻumaʻu crater, creating crust that is continuously uplifting and expanding to build a new crater floor. 

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Volcano Activity Updates

Kīlauea volcano is erupting. Its USGS Volcano Alert level is at WATCH (https://www.usgs.gov/natural-hazards/volcano-hazards/about-alert-levels). Kīlauea updates are issued daily.    

Over the past week, lava has continued to erupt from the western vent within Halemaʻumaʻu crater. All lava is confined within Halemaʻumaʻu crater in Hawai‘i Volcanoes National Park. Sulfur dioxide emission rates remain elevated and were last measured at approximately 1,350 tonnes per day (t/d) on June 10. Seismicity is elevated but stable, with few earthquakes and ongoing volcanic tremor. Summit tiltmeters recorded one deflation-inflation event over the past week. For more information on the current eruption of Kīlauea, see https://www.usgs.gov/volcanoes/kilauea/recent-eruption.

Mauna Loa is not erupting and remains at Volcano Alert Level ADVISORY. This alert level does not mean that an eruption is imminent or that progression to an eruption from the current level of unrest is certain. Mauna Loa updates are issued weekly.     

This past week, about 26 small-magnitude earthquakes were recorded below the summit and upper elevation flanks of Mauna Loa—the majority of these occurred at shallow depths less than 15 kilometers (9 miles) below sea level. Global Positioning System (GPS) measurements show low rates of ground deformation over the past week. Gas concentrations and fumarole temperatures at both the summit and at Sulphur Cone on the Southwest Rift Zone have remained stable over the past week. Webcams show no changes to the landscape. For more information on current monitoring of Mauna Loa, see: https://www.usgs.gov/volcanoes/mauna-loa/monitoring.  

Three earthquakes were reported felt in the Hawaiian Islands during the past week: a M2.5 earthquake 1 km (0 mi) SE of Pa‘auilo at 25 km (16 mi) depth on June 15 at 9:36 p.m. HST, a M3.3 earthquake 4 km (2 mi) SSW of Pāhala at 35 km (22 mi) depth on June 15 at 9:36 a.m. HST, and a M3.5 earthquake 2 km (1 mi) SW of Pāhala at 32 km (20 mi) depth on June 9 at 10:26 p.m. HST.

HVO continues to closely monitor Kīlauea's ongoing eruption and Mauna Loa for any signs of increased activity.    

Please visit HVO’s website for past Volcano Watch articles, Kīlauea and Mauna Loa updates, volcano photos, maps, recent earthquake info, and more. Email questions to askHVO@usgs.gov.    

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