Volcano Watch — Kīlauea's eruptions haven't always been as tame as now

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.

Seventy years ago, on May 10, 1924, a violent phreatic (steam) eruption began in Halema'uma'u that sent repeated columns of ash high into the sky. The explosions continued for 18 days, with the largest occurring on May 18. The steam explosions hurled rocks as large as eight tons as far as 0.6 miles from the crater; these blocks still surround Halema'uma'u. One of these blocks fatally injured a Mr. Taylor, who approached too close to the crater to take a photograph. The explosion that killed Mr. Taylor generated a 6,000-foot-tall column of ash.

At the beginning of 1924, Halema'uma'u 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, Halema'uma'u 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 Ka'u to oppose the dominant chief, Kamehameha. This was by far the most devastating historical eruption in Hawaii. Its enormous ash columns were somewhat similar to those of the eruptions at Mount St. Helens in 1980.

The 1790 eruption began with phreatomagmatic (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 and ash, called a base surge, that formed from a collapsing column of ash. Columns of ash that produce base surges are generally tall, and the 1790 ash columns are estimated to have been about 30,000 feet tall, based on reports that they were visible from Kawaihae.

Large explosive eruptions commonly result in the formation of calderas whose diameters are related to the height of the ash columns produced. Thus, the larger the ash columns, the larger the caldera. Most, if not all, of the present-day summit caldera of Kīlauea probably formed during the 1790 eruption. The deposits of ash produced 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 Ka'u 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 headed away from Kīlauea's summit and those in the upper layer mainly headed towards the summit. The lower set of footprints may record the escape of Keoua's surviving warriors immediately following the base surge, and the upper set of footprints record their return at a later time.

There is evidence for numerous explosive eruptions at Kīlauea Volcano in the more distant past. The most recent prehistoric explosive eruption deposited a thin layer of ash about 1,200 years ago, whereas a much more extensive deposit, the Uwekahuna Ash, erupted between 2,100 and 2,700 years ago. The eruptions which produced the Uwekahuna Ash were much larger than those of 1790, but most of the deposits are buried by younger lava flows of Kīlauea and Mauna Loa.

Even larger explosive eruptions occurred earlier in Kīlauea's history. Two one-to-two-meter thick ashes underlie the Uwekahuna Ash but overlie the thickest prehistoric ash, the Pahala Ash, a reddish sequence of ash up to 85 feet thick where it is exposed in the Hilina Pali. The age of the Pahala Ash is poorly determined, but it appears to be between 30,000 and 40,000 years old. The Pahala ash occurs in a drillhole located near the Hilo airport, where it is still several feet thick. The term "Pahala Ash" has been used to describe nearly all the soils on Hawaii, but the ashes on the south flank of Mauna Loa and those on Kīlauea all appear to be from explosive Kīlauea eruptions, whereas the ashes along the Hamakua coast appear to be from explosive eruptions of Mauna Kea. Even earlier in the history of Kīlauea, additional explosive eruptions occurred which deposited eight separate ash layers now preserved only in the faults of Hilina Pali.

We have little means of long-term forecasting of explosive eruptions. However, we should have ample short-term warning if such events were to occur in the future. Of the relatively few active volcanoes in the United States, Kīlauea poses the second greatest risk because of its documented history of frequent explosive eruptions and the size of the population that could be affected; only Mount St. Helens poses a greater risk.