A brief history of explosive eruptions at Kilauea: 1790 and 1924 and a great 1955 video

First, kudos to the staff of Hawaii Volcano Observatory and the civil defense officials in Hawaii for the work that they are doing to keep people safe around Kilauea!

from: https://pubs.usgs.gov/fs/fs132-98/

Today (Wednesday, May 9) the USGS Hawaii Volcano Observatory put out a warning that the "steady lowering of the lava lake in "Overlook crater" within Halema'uma'u at the summit of Kilauea Volcano has raised the potential for explosive eruptions in the coming weeks." As of this announcement, the lava lake surface has dropped more than 200 m, and was subsiding at a rate of about 2 meters per hour. The water table currently lies about 520 m below the rim of Halema'uma'u. While 320 m seems like a long way to go to the water table, the USGS  says that since the early 1820's the floor of the summit crater has dropped to within 90 m of the water table at least three times.
        The fear is that the column of lava feeding that system might drop below the groundwater level and allow water into the conduit (illustrated schematically to the left). Interaction of the water with lava would result in steam-driven explosions, expelling debris around the Kilauea summit.  The notice issued by the USGS goes on to warn that in such explosions, ballistic blocks up to 2 meters across could be thrown in all directions for distances up to or exceeding 1 km. Such blocks could weigh "a few kilograms to several tons." Smaller rocks could go several miles and would be more likely to land downwind.

from U.S. National Register of Historic places

     The technical term for the process that might happen is "autocatalysis," although this term has a slightly different meaning in volcanology than in chemistry.  When magma is intruded into water, a thin cooled skin develops at the contact between the two substances. Such a skin allows magma to flow underwater without exploding because the red hot lava is prevented from contact with the water. However, if the skin is ruptured, water comes in contact with the lava and an explosion results. The explosion results in exposure of more lava to water, and so the feedback keeps the reaction going until either the water or the lava are exhausted.
     There are two known examples of such explosive eruptions at Kilauea. In1790, people were trapped  in the Ka'u Desert in what is now Hawaii Volcanoes National Park and left footprints in fresh volcanic ash that are now visited by tourists. Although legend has it that the footprints were from two parties of warriors, but research has suggested that the area was commonly used for hundreds of years (perhaps for obtaining sharp glass to use as tools) and that many of the footprints were made by women and children.
     The Hawaii Volcano Observatory was established in 1912, and in 1917, the great volcanologist, Thomas Jagger, studied Kilauea and published a major paper "Volcanologic Investigations at Kilauea" in the American Journal of Science (v. 43 (261) 161-220.) Jagger founded and remained Director of HVO until 1940. In 1924, only about 6 months after the eruption ended, he and R.H. Finch published a paper describing the second example of explosive eruptions at Kilauea, the events from February through May of that year (AJS, 8(47), 353-374, 1924). In January of that year, the lava lake in Halemaumau was fountaining and was 105 feet below the rim. In February, this subsided to 370 feet, but was relatively quiet except for numerous earthquakes on the Puna rift east of the summit, the location of current activity. The rift had vented lava in 1922 and 1923 and the possibility of further activity was openly discussed. In early April, a strong earthquake jolted Hilo, but no new cracks were observed in the ground. Earthquakes continued on the east rift zone, and after numerous earthquakes on April 21, fissures opened in the Kapoho district on April 23. Chasms developed in numerous places, some being as much as 3 feet wide. However, earthquakes diminished through the end of April.
     By May 6, the floor of Halemaumau was more than 600 feet below the rim, the actual depth not known because of avalanching material. Although the subsidence of the floor indicated lava draining from the summit, no lava flows were reported anywhere.  On May 11, for the first time since 1790, broken rocks were hurled from Halemaumau. Five explosions were reported on May 13, with one 400 pound rock being thrown 200 feet from the pit. Another explosion that day sent rocks a half mile into the air, some weightn hundreds of pounds.
    

8-ton boulder ejected May 18th, 1924, to 3500 feet from center of crater

May 18 (the date on which Mount St. Helens erupted in1980) marked the maximum of explosive activity in this sequence. One man who ventured too close to the pit died the day after his leg was crushed by ejected stones. Jagger describes "Heavy electrical storms accompanied by pisolitic mud rains followed the larger explosions. Brilliant lightnings shot through the califlower ash clouds. A marked air concussion was felt before the larger explosions." Continuous avalanching filled the air with dust and mud. Jagger took the continous explosive activity to indicate that the volcano was trying to clear a clogged throat as pressure repeatedly built up due to the debris in the conduit. He noted that the rocks ejected were not fresh lava nor were they bombs encased in lava. They were "olivine rock or gabbro...and old lava from the walls of the pit."
     The last notable explosion of the 1924 sequence was on May 24 with the ejection of rocks to 3000 feet, acconpanied by lightning and cracks of thunder, trembling ground, and a heavy deposit of ash.  Within 20 minutes, the eruption was over. The pit had enlarged to 3400 feet by 3000 feet in area, and to a depth of 1330 feet. By the end of June, seismicity had returned to normal levels.

P.S. Just discovered a nice USGS video of the 1955 east rift eruption. Could provide a good image of how the flank eruption (not the summit eruption) might proceed. And, a fun historic perspective on the technology of film making/sound track in those days!