Hardcastle. — On the Tarawera Eruption. 281 



be sufficiently sustained — if the pressures are equal to the pro- 

 duction of the temperature required, and the series is not 

 extended over too long a time. But fusion of a considerable 

 quantity of rock might be effected by a single movement, under 

 conditions which may exist in some spaces in the crust from 

 time to time. 



In the case of the Tarawera eruption, the evidence seems 

 to show that there had been slight crust movements going 

 on, intermittently, for some time (indicated by slight earth- 

 quakes); and as the earthquakes which immediately preceded 

 the eruption were not violent, (not indicative of enormous 

 crushing effect,) we may conclude that a preliminary series 

 of attacks had been made upon a region of rock beneath the 

 mountain, possibly raising its temperature to a high point, 

 yet short of that necessary to form steam under the pressure 

 due to the depth of the field of action; that on the night of 

 the eruption, further movements, slight in themselves (as in- 

 dicated by slight earthquakes) but critical in direction and 

 the amount and nature of the deformation produced, raised 

 the temperature of some portion of the deformed region so 

 much that it was fused, and the water in this and adjacent 

 portions was enabled to expand into steam, and that the steam, 

 finding an old or a new fissure by which to commence its 

 escape, quickly enlarged this into a wide rift, up which it 

 bore millions of tons of crumbled, rocky matter. Heavier 

 earthquakes followed the outbreak, caused, not by explosions 

 of steam, but by crust movements becoming more extensive, 

 conceivably facilitated by the removal of portions of the pre- 

 viously resistent rocks by ejection, or by the fusion and squeezing 

 out of the way of a portion, or by both means together. 



The eruption from the plain commenced later, and a 

 difference is noted by Professor Hutton between the ejections 

 from the craters on the mountain and those on the plain — 

 fused rock being discharged from the former, none from the 

 latter. I would suggest that the explanation of this difference 

 is this: that more powerful pressures were required to carry 

 on the work of contraction beneath the load of the mountain 

 than beneath the plain, and the effective exercise of the higher 

 pressures developed a higher temperature. The difference in 

 the time of the outbreaks may be accounted for in this way: 

 Previous crust movements in the same direction may have 

 proceeded in some degree beneath the plain, while being re- 

 tarded beneath the mountain by its weight. The pressures 

 directed under the mountain accumulated until they overcame 

 the extra resistance, and that so effectually that renewed and 

 sudden strain was thrown upon the related rocks under the 

 plain (related as to liability to compression), which strain they 

 were unable to resist. Just as if one were pulling a carpet 



