Volcano Watch — Study of Kīlauea's 1924 explosive eruption resembles a CSI

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Sometimes a geologic study resembles a crime scene investigation (CSI), 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.

Study of Kīlauea's 1924 explosive eruption resembles a CSI ...

Map showing ballistic blocks of the 1924 eruption: red, more than 1.5 m in average diameter; yellow, 1-1.5 m. Shading shows lava flows younger than 1924 outside Halema‘uma‘u, and dashed line, the limit of ballistic blocks erupted on March 19, 2008. Crater in 1912 widened to nearly its present diameter during the 1924 eruption.

(Public domain.)

We are interested in learning as much as we can about the explosive events that rocked the summit of Kīlauea for 18 days in May 1924. Why? Because the past is a guide to the future, not infallible, but far 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 Kīlauea.

The accompanying map shows the distribution of blocks more than 1 m (3.3 ft) in average diameter, hurled out of Halema‘uma‘u ballistically 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 HVO researchers in 1924.

The first is the absence of large blocks on the southwest side of Halema‘uma‘u. 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 Halema‘uma‘u, with one eight-ton block strewn as far as 1 km (0.6 mi) from the center of the crater. The largest explosive event in 1924, on May 18, ejected blocks toward the southeast, including the eight-ton block, and killed Truman Taylor.

Otherwise, the distribution of large blocks—and even of smaller ones down to 25 cm (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 25 cm across, the northern flank of Halema‘uma‘u 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 on 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 Halema‘uma‘u 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 Halema‘uma‘u. 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 Halema‘uma‘u, 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 the accounts speak only of ash, yet today we find scattered rocks called lapilli (2–64 mm [0.08–2.5 inches] in diameter) in that area, capping ash that, by definition both then and now, is less than 2 mm (0.08 in) in diameter.

We think that 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 actually 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.

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Volcano Activity Update

A lava lake within the Halema‘uma‘u Overlook vent produced nighttime glow that was visible from the Jaggar Museum overlook and via HVO's Webcam during the past week. The lake level over the past week fluctuated, due to deflation-inflation cycles, and dropped to roughly 60 m (160 ft) below the floor of Halema‘uma‘u.

On Kīlauea'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 Hawai‘i Volcanoes National Park boundary. In addition, the Kahauale‘a flow, fed directly from a spatter cone on the northeastern edge of Pu‘u ‘Ō‘ō'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 5 km (3 mi) from Pu‘u ‘Ō‘ō.

There were two earthquakes reported felt in the past week. On April 13, at 10:45 a.m. local time, a magnitude-4.4 earthquake occurred 42 km (26 mi) northeast of ‘O‘ōkala at a depth of 18 km (11 mi). On April 18, at 12:46 a.m., a magntitude-3.1 earthquake occurred 6 km (4 mi) southeast of Waikōloa Village at a depth of 35 km (22 mi).