Sometimes a geologic study resembles a crime scene investigation, as seen on TV. You piece together information from eyewitnesses and compare it with your own detailed observations to arrive at an interpretation of what happened. Along the way, this may lead to dead ends, resolve many or all uncertainties, or perhaps open new possibilities unexpected when the investigation started.
We are interested in learning as much as we can about the explosive events that rocked the summit of Kilauea for 18 days in May 1924. Why? Because the past is a guide to the future, not infallible, but better than nothing. The more we know about those explosive events, the better we can estimate the causes and effects of the next explosive pulse at Kilauea.
The accompanying map shows the distribution of blocks more than 3.3 feet in average diameter, ballistically hurled out of Halemaumau in 1924, and preserved today. As physicists know, a purely ballistic trajectory is possible only in a vacuum, but we can overlook that nicety for our purposes. Such a map was not prepared in 1924; that is unfortunate, because since then, lava flows have covered parts of the block field. Nonetheless, the distribution of the large blocks observed today shows two striking patterns, one of which was noted by Hawaiian Volcano Observatory researchers in 1924.
The first is the absence of large blocks on the southwest side of Halemaumau. This was noted by Oliver Emerson and Thomas Jaggar in late May and early June 1924. The second, unnoted in HVO records of the eruption, is the concentration of large blocks on the southeastern side of Halemaumau, with one 8-ton block strewn as far as 0.6 mile from the center of the crater. The largest explosive event in 1924, on May 18, ejected blocks toward the southeast, including the 8-ton block, and killed a man named Truman Taylor.
Otherwise, the distribution of large blocks — and smaller ones down to 9.8 inches in average diameter, the smallest we’ve mapped — is reasonably uniform around the crater. In terms of sheer numbers of blocks more than 9.8 inches across, the northern flank of Halemaumau wins, though this can’t be seen on the map of only larger blocks.
These patterns may tell us something unexpected. The maximum concentration of large blocks is near where the overlook vent opened March 19, 2008. Is there a long-lasting weak spot in this area?
The abundance of large blocks along the northern and eastern rim is consistent with eyewitness reports. Emerson noted on May 28, 1924, that the northeastern part of Halemaumau was “vastly deeper” than the southwestern part, which ejected few large blocks. The day before, Ruy Finch commented that “The steam was rising from the usual vent at NE side” of Halemaumau. Perhaps there is something long-lasting about this weakness, too. Seismologists today place a magma conduit only a few hundred meters below the northeastern part of Halemaumau, and four eruptions have come from this area since 1924.
Many eyewitness accounts conflict with what we see today regarding the material that fell far beyond the ballistic limit south and west of the caldera. Almost all accounts speak only of ash, yet today we find scattered rocks called lapilli, 0.08 to 2.5 inches in diameter, in that area, capping ash that, by definition both then and now, is less than 0.08 inch in diameter.
We think this is a slip-up by the 1924 observers, a confusing shorthand for any particle smaller than a few centimeters across. Such a generalization was often made in the early 20th century, although some written accounts in 1924 do mention and even measure fragments a few centimeters in diameter, which they should have termed lapilli or gravel. Unfortunately, it is vital for estimating the energy of the eruptions to know how far these larger stones were dispersed from the vent. Our mapping of this aspect of the 1924 events is in its closing stages, and we will report on the distribution of lapilli and ash in a future Volcano Watch.
Kilauea activity update
A lava lake within the Halemaumau Overlook vent produced nighttime glow visible from the Jaggar Museum overlook and via HVO’s webcam during the past week. The lake level over the week fluctuated, because of deflation-inflation cycles, and dropped to roughly 160 feet below the floor of Halemaumau.
On Kilauea’s east rift zone, breakouts from the Peace Day tube remain active above the pali and on the coastal plain. Small ocean entries are active on both sides of the Hawaii Volcanoes National Park boundary. In addition, the Kahaualea flow, fed directly from a spatter cone on the northeastern edge of Puu Oo’s crater floor, continues to advance very slowly toward the northeast across older lava flows. As of this week, the flow front had traveled roughly three miles from Puu Oo.
Two earthquakes were reported felt recently. At 10:45 a.m. April 13, a magnitude-4.4 earthquake occurred 26 miles northeast of Ookala at a depth of 11 miles. At 12:46 a.m. Thursday, a magntitude-3.1 earthquake occurred four miles southeast of Waikoloa Village at a depth of 22 miles.
Visit hvo.wr.usgs.gov for Kilauea, Mauna Loa and Hualalai activity updates, recent volcano photos, recent earthquakes and more; call 967-8862 for a Kilauea summary; email questions to askHVO@usgs.gov.
Volcano Watch is a weekly article and activity update written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.