August 25, 2005
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
A pioneering volcanologist narrowly beats the Reaper
Two pioneering volcanologists-Thomas Jaggar and Frank Perret-set up the first systematic monitoring station in Hawai`i's Kilauea caldera in 1911. The following year, Jaggar founded the Hawaiian Volcano Observatory, now run by the U.S. Geological Survey.
Thousands of visitors to Hawai`i National Park stop by Jaggar Museum each year, so Jaggar is relatively well known to the general public. Frank Perret, on the other hand, is virtually unknown to the general public. Yet he is well known amongst volcanologists because of his exploits during the 1929-32 eruption of Mt. Pel?e on Martinique in the Caribbean.
Like Perret, the 1929-32 eruption of Mt. Pel?e is also largely unknown to the general public. The 1929-32 eruptions were overshadowed by the preceding eruptions in 1902, which destroyed the city of St. Pierre and its 26,000 inhabitants. The death and destruction was caused by a phenomenon that was new to the then young science of volcanology: the nu?e ardente.
Nu?e ardente - a French term meaning glowing cloud - refers to a high-speed, high-temperature mixture of hot volcanic gas and rock fragments that behaves like a fluid. These mixtures are typically associated with dome-building eruptions, like the one that is currently underway at Mount St. Helens. Today, most volcanologists prefer the term "pyroclastic flow" to nu?e ardente.
The 1929-32 eruption sequence was more conducive to study than its predecessor, because it lasted longer and because it was less explosive than the 1902 eruptions. A lava dome was slowly growing at the summit of Mt. Pel?e. Intermittently, parts of the dome would collapse and generate pyroclastic flows that traveled down a valley that headed on the volcano.
In an act that is today considered lunacy, Perret constructed a small wooden observation cabin atop a low hill in a valley. From this observation post, he watched the pyroclastic flows form and travel past him, on one or both sides of his perch. Usually the flows were rather small and were confined to the valley bottom. But one day he got more than he bargained for. In his own words?
"A huge spine had grown up near the west side of the dome summit. It was unstably poised and its fall seemed imminent. Just at the close of the swiftly passing tropical twilight, in the dead calm between day and night winds, an unusual sound brought me to the cabin door. The whole mass had fallen, leaving a great scar on the dome from which poured forth an ash-cloud of inky blackness, expanding upward as it rushed down the talus slope like a nu?e ardente. At the same instant an explosion on the eastern summit shot out a second avalanche with rising cloud white as snow; two mighty parallel columns, ominous, terrifying, moving straight toward the station. I shall not attempt to describe all the sensations I felt, nor the thoughts that swept through my mind at that moment. My first thought was of instant flight. With a distance of 2.5 kilometers between me and the crater, it might be four minutes before the clouds reached me, but a moment's reflection indicated that this could not be enough to escape the wide path of the cloud. I decided to risk the protection of the frail shack. Doors, windows, cracks and holes were hastily closed and the onset awaited.
Escapes from many former perils helped to allay my fears, but there was still the thought that this might be my last. I recall a sense of utter isolation; awe in the face of overwhelming forces of nature so indifferent to my feeble self. The track of the avalanche lay to the side of the station or these lines could never have been written. The chief dangers were heat and gas from the cloud. There was still a minute left. I peered out from the rear of the station. A sublime spectacle! Two pillars of cloud a thousand feet in height, apparently gaining in speed every instant and headed straight for my shelter. As I darted within, the blast was upon me-not a terrific shock as the reader might think, but swirling gusts of ash-laden wind, bringing a pall of darkness that might indeed be felt. The dusty air entering every crevice of the shack was hot but not scorching. I felt the gases burning and parching my throat and then came a feeling of weakness (carbon monoxide?). It all lasted for half an hour, but it was nearly an hour before the feeling of suffocation was relieved by a kindly wind."
The particle concentration in pyroclastic flows decreases upward: particle-rich on the bottom, gas-rich on top. The particle-rich part stayed in the valley bottom and flowed around the observation hill. But the gas-rich part overwhelmed the observation cabin. A similar cloud was responsible for the death and destruction of St. Pierre in 1902. Perret had narrowly escaped the fate of the inhabitants of St. Pierre, and recorded his observations for science.
Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater and on the southwest side of the cone.
Lava continues to flow through the PKK lava tube from its source near Pu`u `O`o to the ocean, with very few surface flows breaking out of the tube. Small flows are visible intermittently on the steep slope of Pulama pali and on the coastal plain. As of August 25, lava is entering the ocean at East Lae`apuki. Several partial collapses of the East Lae`apuki bench have occurred during the last several weeks. One of these removed as much as 5 acres over a two-hour period. Access to the ocean entry and the surrounding area has been closed due to significant hazards. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.
There were no earthquakes felt on Hawai`i Island within the past week.
Mauna Loa is not erupting. During the week ending August 24, five earthquakes were recorded beneath the summit area. None of them were deep or long-period in nature. Inflation continues.
Updated: August 30, 2005 (pnf)