Hill. — Taupo Plateau and Lake. 451 



Ruapehu and adjacent volcanoes ; but the area of country to 

 the east of these extensive volcanoes and covered with lava is 

 very limited in extent. Southward of Ruapehu volcanic lavas 

 disappear beyond the base-limits of the mountain, and if large 

 flows ever took place they must have spread in a northerly 

 direction. 



But it has been pointed out that the country between Tara- 

 wera and Taupo is composed mostly of rhyolitic lavas, and 

 similar lavas are found for many miles to the westward of the 

 lake at heights varying from 1,500 ft. to 3,500 ft. It may be 

 that these lavas flowed from Tongariro ; but, if so, great differ- 

 ential surface-movements must have taken place to account for 

 the varying heights of the lavas, which are extensive over the 

 country. Again, there are volcanic cones on either side of the 

 lake, just as there are traces of old craters, with crumbling walls, 

 and all these were probably formed at an earlier date than 

 Tongariro, Ngauruhoe, and Ruapehu. It was during the period 

 of activity of these latter mountains that Taupo as an area of 

 depression was formed. 



But in order to interpret the changes that actually took 

 place it is necessary to study the site of the Tarawera eruption, 

 where to-day the greatest activity is found within the volcanic 

 belt of weakness. What took place at Tarawera in 1886 took 

 place at Taupo at a period that may be measured by hundreds 

 of years. The Tarawera eruption was an explosive one, by 

 which is meant that it was brought about by superheated steam 

 being imprisoned as in a boiler, the tension being so increased 

 that the overlying rocks were unable to sustain the increasing 

 strain. 



It would seem that the first or initial step in volcanic pheno- 

 mena is of an explosive character. A shaft is formed from below 

 for the imprisoned steam or gases, and, as the pressure increases 

 when the steam is nearing the surface, the final effort is explosive, 

 a cup-like or crateral hollow being formed. The material thrown 

 out of the shaft soon forms a rim, which continues to increase in 

 height as the base becomes wider, and if the volcano continues 

 active the mound of accumulation becomes cone-like in appear- 

 ance. All this has taken place many times over within the vol- 

 canic belt, and both Rotomahana and Mount Tarawera showed 

 the explosive effect of superheated steam acting along a line of 

 weakness in the earth's crust. The crateral hollows that were 

 formed became centres of increased activity, as they were the 

 lines of least resistance to the gases and superheated waters 

 from below. But what Rotomahana was before the eruption 

 in 1886, and what it has since become, show the character of 

 volcanic changes that go on incessantly within the limits of the 



