Mauna Loa is known for its effusive eruptions that produce spectacular lava flows. However, some of the volcanic products found on Mauna Loa are pyroclastic or explosive in character.
Evidence of a more violent style of eruption has been identified in the form of debris fans. These deposits are largely made up of solidified blocks, lapilli (pieces 2.5 inches), and ash.
The age and general location of the deposits were discussed in an earlier “Volcano Watch” article published in 2018. Here, we describe the rock types and sizes, which are used to help constrain the vent locations and size of these unusual explosive eruptions.
Generally, the ejected rocks are roughly equivalent in size and abundance along the central axis of each fan, and they decrease in size with increasing distance from the caldera rim.
We documented over 1,000 clasts from the debris fan on the northwest rim. The clasts were dominated by ponded lava ejecta, both unaltered and altered, totaling 78% percent. Pahoehoe clasts, both unaltered and altered, are the second most abundant, at 20 percent, while gabbroic xenoliths and bread-crust or crackle-surface rocks each comprise 1 percent of the deposit. Clasts in the northwest fan range in size from 1 to 87 inches in diameter, with an average size of 10 inches.
The east, or Mauna Loa Cabin apron, is also dominated by ponded lava ejecta, both unaltered and altered. We documented over 1,700 clasts, and ponded lava comprises 98% of the lithics, while pahoehoe clasts, both unaltered and altered, comprise only about 1% of the deposit. Bread-crust or crackle-surfaced rocks also comprise 1% of this deposit, along with a single gabbroic lapillus that was noted.
In the east fan, most larger-sized clasts (greater than 16 inches) are found in the northern section of the fan. Lithic ejecta in the north ranges from 0.8 to 60 inches in length, with an average of 8 inches. To the south, the clasts are much smaller and more abundant. The ash fraction is over 70% of the deposit, burying almost all of the underlying pahoehoe flows. The clasts range in size from 0.4 to 26 inches, with an average size of 5 inches.
West fan clasts, types, and sizes are randomly scattered at varying distances from the caldera rim, with no clear dispersal pattern. We cataloged over 340 clasts; pahoehoe, both unaltered and altered, is the most abundant at 44%, followed by ponded lava, both unaltered and altered, at 24% . Clasts range from 0.80 to 35 inches, with an average of 8 inches.
Surprisingly, the two most uncommon rocks in the other explosive fans are quite abundant in the west fan. Gabbros comprise 21% of the deposit, while bread-crust or crackled-surface rocks comprise 11% of the deposit.
The flow samples indicate the explosions occurred at depth, and had to break through overlying lava flows. Many of the pahoehoe blocks are rounded, suggesting that they were tumbled in the vent or recycled a few times before ejection. Gabbros represent crystalized lava lake(s).
The most unusual blocks have “bread-crust” textures or have water-quenched-type outer surfaces (crackled). We interpret the bread-crust-textured clasts as the ejected clots of residual melts of crystallizing shallow magma bodies, similar to those that produced the gabbroic xenoliths. This residual melt was still molten or plastic when the explosive activity occurred. In contrast, the crackle-surfaced types are the result of hot rocks being rapidly quenched.
So why is this information so important? Based on simple geometry of the arcuate shaped deposit fans, clast type, and size distribution, we can determine that the sources of these explosive eruptions were from vents on the caldera floor adjacent to the deposits found on the rim.
We know that explosive eruptions have occurred on Mauna Loa before, and that this type of activity is likely to happen again. Although no one lives at the summit of Mauna Loa, explosive eruptions have the potential to pose a threat to hikers, aircraft, and communities downwind that could experience ashfall. This study adds another important dimension in reconstructing the long-term eruptive history of Mauna Loa.
Volcano Activity Updates
Kilauea volcano is erupting. its USGS Volcano Alert level is at WATCH. Kilauea updates are issued daily
Lava returned to the western vent within Halema‘uma‘u crater at about 10:45 a.m. on Jan. 18, following a pause lasting about three days. All lava is confined within Halema‘uma‘u crater in Hawaii Volcanoes National Park. Sulfur dioxide emission rates remain elevated and were measured at approximately 2,100 tonnes per day on Jan. 19. Seismicity is elevated but stable, with few earthquakes and ongoing volcanic tremor. Summit tiltmeters show variable deflation and inflation trends over the past week. For more information on the current eruption of Kilauea, 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 60 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 6 miles. GPS measurements show no major deformation over the past week. Gas concentrations and fumarole temperatures at both the summit and at Sulphur Cone on the Southwest Rift Zone remain stable. Webcams show no changes to the landscape.
One earthquake was reported felt in the Hawaiian Islands during the past week: a magnitude-3.5 earthquake 8 miles SSE of Fern Forest at 4 miles depth on Jan. 16 at 12:04 p.m.
Visit https://www.usgs.gov/observatories/hawaiian-volcano-observatory for past Volcano Watch articles, updates, photos, maps, recent earthquake info, and more. Email questions to askHVO@usgs.gov. Volcano Watch is a weekly article and activity update written by USGS Hawaiian Volcano Observatory scientists and affiliates.
