174 J. D. Dana — Relations of Vesuvius to Mt. Loa. 



How far the ascensive force in the lava-column contributes 

 to the change of level in the floor of Vesuvius nobody knows. 

 The question has hitherto hardly been considered. It prob- 

 ably does its part, for, the liquid lava rises with the rising 

 floor, following it closely. 



With the column of liquid lava thus lengthened, making the 

 mountain ready for a discharge, the danger of catastrophe is 

 great for the same reasons as at Kilauea. But the danger is 

 greater than there. It is greater because the forces from 

 vapor-generation and hydrostatic pressure have a weaker 

 mountain to deal with — one that has steeper sides and there- 

 fore thinner walls to the lava-cauldron, and walls that are 

 partly cinder-made. It is greater because also of the nearness 

 of the lava-column to the sea, the distance being only four 

 miles, while in the case of Kilauea it is over nine miles and in 

 Mt. Loa over twenty ; so that at Vesuvius water from two 

 sources, the sea and the land, is close by. 



Causes that produce earthquakes may make a rent in the 

 Vesuvian lava-conduit that will let in water for an explosive 

 eruption ; but usually it opens the way, as at Mt. Loa, for a 

 comparatively quiet escape of lava, however disquieting the 

 event may be to deluged villages. 



The loss by upthrows and outflows tends to produce a sink- 

 ing or down-plunge of the floor of the crater, and some fall of 

 its walls to the new bottom, as in Kilauea. At the Kilauea 

 eruption of 1886, the outflow drew off the lavas of a lava-lake 

 half a mile in diameter ; the crust of lava that covered the 

 borders of the lake, along with portions of the walls, conse- 

 quently sunk down, and the cavity or crater left by the dis- 

 charge was half a mile across and between 500 and 600 feet in 

 depth. This is little different from the ordinary event in 

 Vesuvius, except that the loss by the discharge at Kilauea is 

 almost solely by outflow, and no high, weak-sided cone sur- 

 rounds the vent to suffer from the disaster. It is true that the 

 Kilauea lava-lake in the eruption just referred to occupied only 

 a small part of the great crater. But its diameter was as large 

 as the lava cauldron of Vesuvius has been before any of its 

 modern eruptions ; and the movements in the lake were the 

 same that would take place were all Kilauea one great lake. 



Explosive eruptions might prove more disastrous to a Vesu- 

 vian cone than to one of massive Mt. Loa style ; but not be- 

 cause the explosion has the power of blowing off the moun- 

 tain's summit — which failed to happen at Tarawera in 1885 

 although the vent was closed and is not a possibility when the 

 vent is an open one — but chiefly because a steep-sided moun- 

 tain is likely to lose more in height than a broad lava-cone from 

 the same amount of undermining. 



