# Volcano Watch — Not all eruptions as passive as this

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Kīlauea is famous for eruptions of the type we are now experiencing - quiet effusion of lava that makes the eruption approachable. However, not all of Kīlauea's eruptions have been so passive and approachable.

Kīlauea is famous for eruptions of the type we are now experiencing - quiet effusion of lava that makes the eruption approachable. However, not all of Kīlauea's eruptions have been so passive and approachable. Sixty-eight years ago today, on May 10, 1924, a violent phreatic (steam) eruption began in Halemaumau that sent repeated columns of ash high into the sky. The explosions continued for 18 days, with the largest explosions occurring on May 18. The steam explosions hurled blocks of rock as large as eight tons up to 0.6 miles from the crater; these blocks can still be seen surrounding Halemaumau. One of these blocks fell on and fatally injured a Mr. Taylor who approached too close to the crater to take a photograph; he was the only person killed by eruptive activity in Hawaii in this century. The photograph shows the 6,000-foot tall column of ash generated by the explosion at 11:15 a.m. that killed Mr. Taylor. The light cloud just above the ground is a small outrush of hot gas called a base surge.

At the beginning of 1924, Halemaumau Crater was an oval pit about 1,740 feet across with a lava pond about 165 feet below the rim. At the end of the 1924 series of explosive eruptions, Halemaumau was about 3,150 feet across and 1,300 feet deep. The explosive eruptions apparently were caused by groundwater entering the magmatic system following rapid drainage of magma from the summit reservoir. The magma that drained from the summit migrated below ground down the East Rift Zone past the coastline and either erupted or intruded into the rift zone below sea level. There was considerable ground cracking and subsidence near Kapoho associated with a large swarm of earthquakes as the magma moved down the East Rift Zone.

An even more devastating explosive eruption occurred at Kīlauea's summit in 1790. This eruption is famous because it killed about a third of chief Keoua's warriors, who were in transit around Kīlauea to Kau to oppose the dominant chief, Kamehameha. In a real sense, this eruption may have changed Hawaiian history. It was by far the most devastating historical eruption in Hawaii and was somewhat similar to the eruptions at Mount St. Helens in 1980 in having enormous ash columns. The eruptions began with hydromagmatic (mixtures of water and magma) explosions of fine, hot particles of volcanic glass and ended with phreatic explosions that ejected small to large fragments and blocks of pre-existing rocks.

Keoua's warriors were probably killed by a hurricane-like blast of hot gases, called a base surge, that formed from a collapsing column of ash. Columns of ash that produce base surges are generally tall, and Thomas Jagger, the director of the Hawaiian Volcano Observatory at the time [sic], calculated that these ash columns were about 30,000 feet tall, based on reports that they were visible from Kawaihae.

The summit caldera of Kīlauea probably formed, or was modified, during this eruption. The deposits of ash produced by this eruption are locally up to 30 feet thick and can be traced continuously more than 12 miles from the summit. These deposits are still well displayed as bedded ash exposed in cracks where the Southwest Rift Zone intersects Crater Rim Drive in the National Park and as thin deposits along the Footprints Trail in the Kau desert. The footprints, which are exposed in the ash deposits between 4 and 5.5 miles southwest of the summit, occur in two layers, with the lower set all headed away from Kīlauea's summit and those in the upper layer mainly headed towards the summit. It is possible that the lower set of footprints record the escape of Keoua's surviving warriors immediately following the base surge, and the upper set of footprints record their return several days to weeks later.

We have little means of long-term forecasting such explosive eruptions. However, we should have ample short-term warning if such an event were to occur in the future. In both 1790 and 1924, sudden, large-scale magma withdrawal from the summit reservoir, associated with massive eruptions on the lower East Rift Zone, led to rapid subsidence of the summit. This magma withdrawal apparently allowed groundwater to enter the magmatic system, which, upon heating, was converted to steam that then powered the explosive eruptions. The most devastating part of both eruptions followed a period of large-scale magma withdrawal from the summit, numerous earthquakes, and smaller steam and hydromagmatic or phreatic explosions.

Of the relatively few active volcanoes in the United States, Kīlauea poses the second greatest risk because of its documented history of explosive eruptions and the size of the population that could be affected; only Mount St. Helens poses a greater risk.

### Volcano Activity Update

Recently, the episode 51 vents on the west flank of Puu Oo, which were inactive for six days starting Tuesday April 28 at about 11:30 a.m., quietly came to life again on Monday at about noon. During the six-day non-eruptive period, there were numerous small earthquakes beneath the summit and the upper and the middle East Rift Zones. These earthquakes accompanied significant inflation of the summit as magma was once again stored beneath the summit. Since the episode 51 vents began erupting again, the summit inflation is unchanged to slightly more inflated, indicating that the amount of magma supplied to the summit from below and the amount erupting are comparable.

The episode 51 vents had about 50-foot fountains on Thursday morning and the lava was mainly flowing to the south and southwest. The lava lake inside the Puu Oo cone, after being as high as 120 feet below the lowest point on the rim of the cone, has dropped to nearly 200 feet below the rim.

At 8:46 p.m., on May 2, a magnitude 3.3 earthquake occurred 6 miles deep beneath the south flank of Kīlauea Volcano. A second, much stronger earthquake with magnitude 3.9, jarred the same area at about 10:13 p.m. on May 4. This earthquake was about 6.3 miles deep and was located near Poliokeawe Pali in Hawai`i Volcanoes National Park. A third earthquake, with magnitude 3.2, occurred in the same area of Kīlauea's south flank at a depth of 4.8 miles at 4:32 p.m. on May 6. All three of these earthquakes were related to seaward movement of the south flank of Kīlauea Volcano.