122 The Tarawera Outbreak 



We can understand why Cotopaxi erupts rarely, because 

 we know that the lip of its crater is placed more than 

 19,000 feet above the level of its reservoir, and we allow for 

 the enormous resistance which must be overcome before the 

 column of lava can be forced up to the top of so lofty a 

 chimney. But at Tarawera there was no chimney — the vent 

 was an open chasm in the valley floor. 



Again, we can understand a volcano in the Solfatara 

 stage not erupting, for when in that stage the water has run 

 short, or the fires have got low. But here steam was present 

 in such abundance as to lead Dr. Hector to describe the 

 affair as " a purely hydro thermal phenomenon." 



And, again, we could understand it if, with an abundance 

 of steam and with an easy exit, there was no lava present. 

 But we know that there was an ample supply here, because 

 every uprush of steam loaded the air with obsidian, which 

 is the froth of bubbling lava. Why, then, I repeat, was no 

 lava erupted at Tarawera ? 



To make the explanation clear, let us remember our former 

 simile. I said that a volcano was a natural steam-engine. 

 Now, a steam-engine will not work well, will not generate a 

 high horse-power, however well supplied with water and 

 fire, if its boiler have a leak ; and if the leak be a large one, 

 it will wholly disable the engine, so that its steam, instead 

 of being utilised, will escape idly in great white clouds, and 

 with a prodigious roar. Such, then, exactly represents the 

 course of events at Tarawera. The subterranean steam 

 burst through the crust that confined it. The crust had no 

 weak spot in it, such as the small circular plug which is 

 blown out of the vent when Vesuvius, Etna, Hecla, or 

 Tongariro erupt. It had, therefore, to break through a 

 sound crust, and it did so with a long rent. 



Where there is an ordinary crater-vent, such as these 

 volcanoes named have, its size just suffices to give gradual 

 relief to the high-tension vapour imprisoned below, but it is 

 not large enough to permit of a free escape. Consequently, 

 the interstitial steam has a struggle to reach the vent. It 

 ■expands as it reaches the region of lessening pressure, and 

 the expanding bubbles, as they hurr}^ through the passages, 

 crowd before them a great volume of liquid aerated lava, 

 which thus boils over the lip in exactly the same way 

 that water or milk will do. But the Tarawera vent was 

 not a circular crater of limited size, but a fissure eight or 

 nine miles long, and, in places, one mile wide. Such a vast 



