The Structural Effects of Pyroclastic Flow
The 1951 eruption of Mt. Lamington
Last updated Monday, March 10, 1997, at 2:45 PM Copyright © 1996, 1997 Kirk Martini
The 1951 Eruption of Mt. Lamington
Background
Mt. Lamington lies near the northern edge of Papua, New Guinea, and had no known history of volcanic activity prior to January 1951, when an ash plume and seismic activity were observed beginning January 15 [Macdonald 1972]. Minor activity continued until the morning of January 21 when an explosive eruption released nueée ardentes which completely or partially devastated an area of about 250 km2 (90 mi2). The effects of the pyroclastic flow were thoroughly investigated and documented by G.A Taylor [1958], and his observations provide valuable insights into the behavior of pyroclastic phenomena.Taylor identified an inner zone of total destruction, where the kinetic force and temperature of the nueée destroyed nearly everything in the region, surrounded by a region of partial destruction, where the velocity of the nueée had reduced and the damage was primarily due to temperature [1958, p. 39]. The figure below indicates these regions.
| A Map of the regions surround Mt. Lamington, indicating the zones of complete and partial devastation. [Taylor 1958]. |
Like the pyroclastic surges of the 1902 eruption of Mt. Pelée, the zone of devastation extended approximately 10-12 km from the crater, and was strongly influenced by topography, which directed flows more strongly toward the west and south at Pelée, and toward the northeast at Lamington, due to the geometry of the crater. Within the zone of devastation at Mt. Lamington, nearly everything was flattened. There were occasional sturdy tree trunks, or some groups of tree trunks which were shielded from the flow by a topographical feature. At the settlement of Higaturu, 10 km (6 mi) to the north of the crater, only one house remained reasonably intact, and it was moved 4.5 m (15 ft) northward by the force of the flow [Taylor 1958, p. 40].
Observed Properties of the Nuée Ardente
Taylor made several important observations concerning the direction, velocity, temperature, and distribution of the nuée ardente at Mt. Lamington. The following listing summarizes his observations. Unless otherwise noted, the references in this listing are to page numbers from Taylor [1958].
Direction
Although there was a clearly predominant direction of the flow radially away from the crater, there were local deviations which were not associated with topography. The flow toppled nearly all trees in the zone of devastation, and nearly all fell in the direction away from the crater; however, Taylor observed at Higaturu an area where trees fell at right angles to and opposite the predominate flow direction; he concluded that this was the result of vortices generated when a projected tounge thrust ahead of the main front of the flow [p. 41]. Such vortices create not only local variations in direction, but also locally higher velocities, as discussed below.Kinetic Energy
In addition to the general level of destruction, the nuée ardente at Mt. Lamington left a few specific clues concerning its kinetic energy. The image below shows one such clue, a jeep which was impaled on tree trunks by the energy of the nuée.
| A jeep impaled on broken tree trunks. Based on the direction of fallen trees in area near the vehicle, Taylor concluded that this was the result of locally high velocities due to turbulence rather than high general velocities [pp. 43-44]. [Taylor 1958]. |
Taylor concluded that this effect was due to locally high velocities resulting from turbulence rather that high general velocities based on two observations: general velocities sufficient to place the jeep in the trees would have carried away building fragments that had remained in place; and the direction of fallen trees indicated local turbulence [pp. 43-44].
A more specific indicator of the kinetic energy was the flagpole at the Higaturu hospital. The flagpole was severely deformed by the force of the flow; it appears in the upper part of the image below, nearly parallel with the ground. Note the severely damaged building in the background.
| In the background is a hospital building destroyed by the nuée ardente. The upper part of the image shows a portion of a flagpole that was bent over by the force of the flow. [Taylor 1958]. |
An engineering analysis of the flagpole was initiated in order to estimate the velocity of the nuée ardente. The study included contributions from three authors: R. Dunning, L.B. Bogan, and A.K. Johnston [pp. 101-109]. This study is one of the few examples of a detailed structural analysis of the effects of pyroclastic flow. The diagram below shows the deformed shape of the flagpole.
| A diagram showing the deformed dimensions of the flagpole at Higaturu. [Taylor 1958]. |
Applying classical fluid dynamics and using material properties obtained from testing samples of the flagpole itself, Dunning concluded that the velocity was 98 m/s (219 MPH), however as Johnston points out, Dunning assumed a constant flow of pure air, neglecting dynamic effects, stratification of velocity, and the additional mass of wind-born volcanic material. The effects of such material were clear however, as Bogan's close examination of the steel tubing revealed the paint was removed from the upstream side of the pole, but remained on the downstream side, a sandblasting effect that was commonly observed on trees. Microscopic analysis of the surface revealed pitting and localized cold work of the metal, very likely the result of impact from pebbles and gravel travelling at high velocity. Taking into account the factors of dynamics, stratification, and additional mass, Bogan roughly estimated the actual velocity to be "considerably less than 200 m.p.h. [89 m/s], and maybe no more than 100 m.p.h.[45 m/s]" [p. 109].
Regardless of the density of the flow, Dunning's analysis and assumptions lead to the conclusion that the kinetic energy corresponded to a steady flow with a stagnation pressure of approximately 560 kg/m2 (114 lb/ft2); this is an extremely high load, much larger than would be induced by any wind event, except for a tornado.
Distribution of Temperature and Kinetic Energy
There were several cases of uneven distribution of the kinetic energy and temperature effects: some people survived while others around them perished; trees were found lightly charred or standing while other nearby where heavily burned or overturned. In some cases, these effects could be explained; as the flow died out it was clear that there were areas where the surge projected tounges that selectively destroyed objects such as trees and buildings, leaving adjacent ones unaffected [p. 40]. In other cases, however, the uneven distribution was 'inexplicable" [pp. 48-49].Observing patterns of charring and other temperature effects, Taylor concluded that the temperatures were much lower than those prevailing at Pelée in 1902. At Higaturu, approximately 10 km (6 miles) from the crater (approximately the same distance as St. Pierre from Mt. Pelée), temperatures were not high enough to ignite or char. These effects and other effects led to the conclusion that temperatures were in the range of 200°C lasting one to two minutes [p. 46].
Summary
The eruption of Mt. Lamington provides valuable insights into the effects of pyroclastic flow. The analysis of the flagpole at Higaturu is particularly important since it provides a rational, albeit approximate, basis for estimating the kinetic energy of a pyroclastic surge. Overriding Taylor's analysis of events at Mt Lamington is the observation that the effects of temperature, direction, and kinetic energy may have significant local variations.| Next: Assessing the Kinetic Energy Potential of Pyroclastic Flow |
Last updated Monday, March 10, 1997, at 2:45 PM
Copyright © 1996, 1997 Kirk Martini
Please send comments or questions to Martini@virginia.edu