| This footprint and others preserved in the muddy ash deposits of Kilauea's explosive eruption in 1790 are believed to be those of Hawaiians who survived the hot explosion cloud. (Photograph by James F. Martin, National Park Service.) | Infrequent explosive activityExplosive eruptions that deposit large volumes of pyroclastic debris over large areas -- like the May 1980 eruption of Mount St. Helens -- are rare at Hawaiian volcanoes. The term "pyroclastic" -- derived from Greek pyro (fire) and klastos (broken) -- is a general term to describe all types of fragmented new magma or old solid rock ejected during explosive eruptions. Less than 1 percent of Hawaiian eruptions have been violently explosive, based on the scarcity of pyroclastic deposits. In contrast, some volcanic chains formed along the convergent boundaries of the Earth's tectonic plates contain 90 percent or more pyroclastic material.
In 1790, a series of major explosive eruptions, which probably lasted a few days
to a few weeks, deposited a blanket of pyroclastic debris up to 30 feet thick in
and around Kilauea summit.
|
At the time of these eruptions, a band of about 250
Hawaiian warriors, led by Keoua, chief of the Puna district in eastern Hawaii,
was marching across the summit region of Kilauea to battle the army of a rival
chief, Kamehameha. Some of Keoua's warriors were caught in a hot, high-velocity
explosion cloud, composed mainly of volcanic steam and gases but little ash. The
hot gases seared the warriors' lungs, killing about 80 of them by suffocation.
Footprints preserved in the muddy ash deposits of the 1790 eruption are thought
to be those of the surviving warriors; these still can be seen by hiking the
Mauna Iki (Footprints) Trail in Hawaii Volcanoes National Park. Had the Hawaiian
Volcano Observatory been at its present location on the summit of Kilauea in
1790, it almost certainly would have been destroyed.
A much less energetic explosive eruption took place at Halemaumau Crater in May 1924. Three months before the eruption, the long-lived lava lake in Halemaumau played actively about 150 feet below the crater rim. Beginning in February, the lake surface began to drop rapidly, and soon the lake drained entirely to expose the crater floor. Throughout March and April, the crater floor further subsided, apparently in response to magma moving from the summit reservoir into the east rift zone. By May 6, Halemaumau's floor was more than 600 feet below the rim. |
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