NGAURHOE'S ACTIVITY

Taupo Times, Volume III, Issue 140, 1 October 1954, Page 10

NGAURHOE'S ACTIVITY

NATURE OF HOT AVALANCHES The occurrence of "hot avalanches" as a feature of Ngauruhoe's volcanic activity in 1949, and during the present eruptive phase, has been referred to more than once in press reports of observations rnade at the mountain by geologists. These hot avalanches beiong to a class of volcanic phenomena first recorded by scientific observers in 1902, when a series of eruptions occurred in the West Indies. The climax of one of these eruptions, from a volcano known as the Soufriere, consisted of the outburst of a great black cloud of gases and incandescent dust, which rushed rushed down the side of the mountain, burning and destroying everything in its path. Withip the area covered by the cloud all vegetation was destroyed and^ all animals and human beings killed, except those in tightly closed buildings. The general character of the eruption of the second mountain, Mt. Pelee, in Martinique, was almost precisely similar. The loss of life was very much greater, since a similar cloud rolled straight down over the town of St. Pierre, which was practically blotted out of existence in a momment, about 30,000 inhabitants perishing. The formation of these "nues ardentes," or "glowing clouds," as the French geologists named the hot avalanches, has been described as due to viscous magma, or lava, in the crater, "boiling over" in the manner so familiar in the domestic kitchen in the case of milk. The hot avalanche which destroyed the town of St. Pierre in 1902 had a temperature as it passea through the town below the melting point of copper (1058 degrees Centigrade), but sufficiently high to melt bottle glass, so that its temperature was obviously between 650 and 700 degrees Centigrade.

The following graphic description of a hot avalanche was given by two geologists, Messrs. Anderson and Flett, who witnessed it from the sea two months after the destruction of St. Pierre:— "In an incredibly short time a red-hot avaanche swept down to the sea. It was dull red, with a billowy surface reminding one of a snow avalanche In it were larger stones which stood out as streaks of bright red, tumbling down and emitting showers of sparks. , In a few minutes it - was over. It was difhcult to say I how long an interval elapsed between the time when the great glare shone on the summit and the incandescent avalanche reached the sea. Possibly it occupied a couple of minutes. It could not have been much more. Undoubtedly the velocity was terrific." \ The nature of these eruptions is described as follows: — "As soon as the throat of the volcano is thoroughly cleared (by preliminary explosions) and the climax of the eruption is reached, a mass of incandescent lava rises and wells over the lip of the crater in the form of an avalanche of red-hot dust. It is a lava blown to pieces by the expansion of the gases it contains. It rushes down the slope of the hill, carrying with it a terrific blast which mows down everything in its path. The mixture of dust and gas behaves in many ways like a fluid." The ash particles and fragments are still giving off gas, which cushions friction between particles, and

the avalanche carries also blocks and fragments ot unexploded lava rock. The mobility of the hot avaanche is produced by the force of gravity on steep slopes, in addition to any motion given by explosion, and its speed is such that escape would be impossible if anyone were in its track.