ERUPTION OP MOUNT TAPvAWERA. 



187 



North Island as a southerly or south-westerly gale. The spread of 

 the deposit seems therefore to be due to the ash from lit. Tarawera 

 having been thrown into an upper stratum of air where the wind 

 was westerly, and this wind changed afterwards to the south. The 

 eruption from the plains did not begin until later, and it was spread 

 by the southerly wind only. 



Cause of the Eruption. 



The immediate cause of the eruption of Tarawera was, no doubt, 

 the reheating of the old lava-streams of the mountain which were 

 previously saturated with water. The proofs of this are : — 



(1) A complete series can be made from compact andesite, with 

 fragments of decomposed rhyolite, to the same rock highly vesicular 

 but still showing fragments of the quartz of the rhyolite. 



(2) A similar series can be made from decomposed rhyolite to 

 quartzose pumice. 



(3) The Black Crater threw out blocks of compact andesite-lava 

 which had overflowed rhyolite, the two being intimately connected 

 at the line of junction, but not passing one into the other. A 

 similar intimate connexion of scoria and pumice is seen in fragments 

 ejected from Mt. Tarawera ; and it is hardly possible that the same 

 intimate connexion between two similar rocks could have been 

 brought about in two different ways. 



(4) It follows, therefore, that the scoria and pumice are remelted 

 andesite and rhyolite lava-streams. However, some of the scoria 

 may have had a deep-seated origin, as it does not always contain 

 pumice. 



This reheating must have taken place locally in the mountain and 

 not very far from the surface, as the reheated rocks have undergone 

 atmospheric decomposition. It could not, I think, have been due to 

 crushing, because (1) the earthquakes preceding the eruption were 

 not violent, many people both at Wairoa and Eotoma not being 

 awakened until after the eruption had commenced. And (2) as 

 several millions of tons of rock have been fused and ejected, it 

 follows, according to the Rev. 0. Eisher *, that several tens of 

 ; millions of tons must have been crushed and not fused. This 

 unfused rock was certainly not ejected with the fused, and the open 

 fissure on the top of the mountain shows solid rock on each side. 

 There is therefore no evidence of this enormous quantity of crushed 

 but unfused rock, and for other well-known reasons its existence is 

 highly improbable. 



The only possible hypothesis seems to be that molten rock came 

 up from below into the mountain and heated the surface-rocks ; 

 and this is rendered more probable by the fact that Ruapehu has 

 also been lately heated up without any earthquakes having been 

 felt. The cause of the ascent of the molten rock, whether by 

 occluded vapour or by pressure caused by movement of the earth's 



* ' Physics of the Earth's Crust,' p. 230. 



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