Volcano Watch — Kīlauea: Explosive eruptions

The current eruption along Kīlauea's East Rift Zone continues with passive effusion of lava. The lava is transported underground to the coast along the western edge of the flow field, where it enters the ocean after cascading over an ancient sea cliff.

The National Park Service has had to close off access to the area because of noxious fumes produced where lava is burning the road. As these conditions change from hour to hour and from day to day, check with the National Park Service before driving down the Chain of Craters Road to the active lava flows near Lae'apuki.

Kīlauea is world-famous as the "drive-in" volcano because of the passive character of its eruptions and the ease and safety of viewing the volcanic activity. Much to the surprise of those who have seen its approachable eruptions, Kīlauea also has a history of violent phreatic or phreatomagmatic explosive eruptions.

May 10 is the 71st anniversary of the most recent of these violent eruptions. On that day, 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 up to eight tons as far as 0.6 miles from the crater; these blocks still surround Halema'uma'u.

By far the most devastating historical eruption occurred at Kīlauea's summit in 1790. 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.

The surface of the volcano was scoured by hurricane-like blasts of hot gases and ash, called base surge, that formed from collapsing columns of ash. The 1790 ash columns are estimated to have been about 30,000 feet high, based on reports of visibility from Kawaihae.

There is evidence for numerous explosive eruptions at Kīlauea Volcano in the more distant past. Another 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.

The Pahala Ash formed from even larger explosive eruptions earlier in Kīlauea's history. Two one-to-two-meter thick ashes underlie the Uwekahuna Ash but overlie the thickest prehistoric ash, a reddish sequence of ash up to 85 feet thick where it is exposed in the Hilina Pali. The Pahala Ash is thus a composite of these separate ashes. The youngest is dated between about 3,500 and 4,800 and the oldest thick section is about 39,000 years old. The Pahala ash occurs in a scientific drillhole located near the Hilo airport, where it is still about four feet thick. The 39,000-year age on this thick unit is derived from the drill core.

Beneath the Pahala Ash is a sequence of even older ashes, including the Mo'o Ash, the Pohaka'a Ashes (seven layers), the Kahele Ash, and the Halape Ash. These ashes are older than 39,000 years old, although how much older is not known; all are exposed in the Hilina Pali.

The term "Pahala Ash" has been widely used to describe nearly all the thick soils on Hawai'i. The Pahala ashes on the south flank of Mauna Loa and those on Kīlauea are all from explosive Kīlauea eruptions. On the other hand, ashes near Hilo and along the Hamakua coast are from explosive eruptions of Mauna Kea.

The ash layers in Hilo, commonly called the Homelani Ash, consist of many layers from different explosive eruptions. The highly variable thickness of the ash is caused by having different ash units represented. For example, the thickest ash, commonly up to 20 feet thick, consists of ashes from Mauna Kea that are older than about 120,000 years. This layer underlies a section of the 39,000-year old Pahala Ash from Kīlauea Volcano, which lies, in turn, under several ashes from Mauna Kea that are roughly 20,000 and 10,000 years old. Depending on the age of the youngest underlying lava flow, all, or some, of these units may be present.

In the past, the entire Homelani Ash was correlated with the Pahala Ash on Kīlauea. We now know that the thickest section of the Homelani Ash is from Mauna Kea, lies beneath the Pahala Ash, and is between 120,000 and 200,000 years old. It is likely that this thick ash unit erupted between 135,000 and 165,000 years ago, when the summit of Mauna Kea was covered by glaciers, and that melt water from subglacial eruptions triggered explosive phreatomagmatic activity.