A two-part Volcano Watch in late January presented evidence for large explosions from Kilauea. Ash rose into, or even above, the jet stream and was carried east of the summit by prevailing westerly winds. Perhaps six such explosions took place in the past 1,000-1,200 years. What about Kilauea’s smaller explosions? They are far more frequent than the powerful ones. In our ongoing study of explosive eruptions, we have not forgotten these relatively common events. And now a student intern has shed more light on them.
A fortuitous circumstance during the past month resulted in important discoveries about one or two of the smaller explosions. Cornell University (Ithaca, NY) conducts a “Field Program in Earth and Environmental Systems” on the island every year. Students in the program live in Waimea for three months, studying the ocean, atmosphere, biosphere, and solid Earth, as well as Hawaiian history and culture. At the end of the program, each student undertakes a 3-week internship somewhere on the island.
HVO was fortunate to have two student interns of the Cornell program from April 24 to May 11. Sarah Menassian did computer coding with the deformation group, and Ben Haravitch conducted field work with the geologist studying Kilauea’s explosions. Both interns gave to HVO at least as much as they received. Mahalo nui loa, Sarah and Ben!
Ben, who is just completing his junior year at Cornell, conducted a field investigation of the distribution of ballistic projectiles from one or two of the smaller explosions to rock Kilauea’s summit in the late 1700s or early 1800s. He measured the length, width, and height of some 364 large rocks in the southern part of Kilauea’s caldera.
These rocks are strewn across the ground surface west and southwest of Keanakako`i Crater.
Many of the rocks are large enough to have formed impact pits when they landed. Former HVO geologist Chester Wentworth first described these pits in 1957. They are still visible today, though rather muted, because they are partly filled with younger debris erupted by lava fountains in 1959, 1971, and 1974.
Ben generally measured only the largest rock every few tens of meters. Probably 100-1000 times as many smaller rocks litter the ground surface, but the larger sizes provide the most information regarding the power of the explosion.
The distributions of the rocks themselves, and of their sizes, indicate an important feature: the deposit produced by each explosion forms a narrow lobe reaching outward from the caldera, rather than a continuous blanket around the caldera. One lobe produced the impact pits that Wentworth described. Another is somewhat farther west and left few, if any, preserved pits.
Such a pattern of overlapping fallout lobes is exactly what the 1924 explosions produced, too. One explosion dropped debris in one place, the next in another, and so on until a complete quilt was produced with enough overlap to make a continuous cover.
Ben made the important observation that one particular type of rock was confined to a third lobe spreading outward from the caldera. This rock is very rich in the mineral olivine, is typically oxidized to a reddish hue, and generally is quite dense. All rocks of this type are confined to a narrow belt about 450 m (450 yards) wide and oriented southeast from the direction of Halemaumau. One of these rocks, 2.6 km (1.6 miles) from the center of Halemaumau, weighs almost 4 tons; another, 3.1 km (1.9 miles) from the center, weights nearly 1 ton.
A curious fact about the distribution of the distinctive rocks is that they are, on average, larger the farther they are from their vent. Normally, larger rocks should not travel as far as smaller ones. If, however, the ejection velocity from the vent is the same for a large and a small rock, the larger will travel farther ballistically, owing to momentum. The far-flung, large, olivine-rich rocks may be an example of this uncommon process–high ejection velocities allowed large rocks to be thrown farther than their smaller cousins.
Ben’s work is but a stitch in a large tapestry that will eventually tell the story of Kilauea’s explosions, but that’s the way tapestries are created. Well done, Ben!
Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater. Lava continues to flow through the PKK lava tube from its source on the flank of Pu`u `O`o to the ocean, with frequent surface flows breaking out of the tube near the 2,300-ft elevation, and a persistent flow, known as the “March 1 breakout,” active on the coastal plain. The March 1 breakout is waning, however, and active lava was limited to a small area 1.5 km (0.9 mile) from the coast.
Lava continues to enter the ocean at East Lae`apuki, in Hawai`i Volcanoes National Park. The active lava bench continues to grow following the major collapse of November 28 and is now approximately 1,000 m (3,300 ft) long by 315 m (1,000 ft) wide, with a total surface area of 18 ha (44 acres). A small collapse on the western side of the bench overnight on May 6-7 removed about 1 hectare (2.5 acres).
Access to the ocean entries and the surrounding area remains closed, due to significant hazards. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.
There were three earthquakes reported felt on Hawai`i Island in the past week. A magnitude-2.8 event at a depth of 20 km (12.5 miles) beneath Kilauea’s summit was felt by residents of the Volcano Golf Course Subdivision at 5:47 a.m. H.s.t. on May 12. Also felt in the Volcano area was a magnitude-3.3 earthquake located 5 km (3 miles) south of Kilauea’s summit at a depth of 14 km (8.5 miles) at 3:10 p.m. H s.t. on May 13. On May 18 at 9:00 a.m. H.s.t., a magnitude-2.7 earthquake at a depth of 0.7 km (0.4 miles) occurred 4 km (2.5 miles) east of Pu`ulena Crater and was felt by residents of Kapoho and Leilani Estates.
Mauna Loa is not erupting. During the past week, seismic activity remained low beneath the volcano’s summit, with 1 earthquake detected. Extension of line lengths between locations spanning the summit, indicating inflation, continues at low rates.
This article was written by scientists at the U.S. Geological Survey’s Hawaii Volcano Observatory and is republished by HawaiiNews.com with permission.