LAVA DOMES 



869 



height is great. Though now rising to nearly 14,000 feet above sea- 

 level these volcanoes began as submarine eruptions, starting on the 

 floor of the deep sea and having a total height of 20,000 or 30,000 

 feet. The visible portion is less than a hundred miles in diameter, 

 but the actual base is probably much more than twice that. The two 

 active volcanoes are Mauna Loa, the rim of which is 13.675 feet 

 above sea-level ; and Kilauea, which is less than 4,000 feet high and 

 appears to rest on the flanks of the larger volcano. The craters, 

 or caldera, have each a circumference exceeding seven miles, being 

 irregularly elliptical in outline' with the sides descending in a series 

 of steps to the central pit, which is formed by the "frozen" surface 

 of the lava. The floor of the pit of Kilauea is a "movable plat- 

 form" of frozen lava which rises and falls with the variation in 



Fig. 231. 



View of Kilauea caldera from the Volcano House. (After But- 

 ton. ) 



the pressure of the lava beneath. The difiference in height between 

 1823 and 1884 was estimated by Button {'j:i2j) to be nearly 400 

 feet. 



"Beneath the floor of the caldera," says Button, "we may con- 

 jecture the existence of a lake of far greater proportions than those 

 which now expose a fiery surface to the sky. The visible lakes 

 might be compared to the air-holes in the surface of a frozen pond." 

 The proof for this is found in the fact that new eruptions are not 

 overflows of the open pools of lava, but break out anywhere in 

 the floor of the caldera. (Fig. 231.) 



Acid lava domes. Lavas of the acid type are, as a rule, too 

 viscous to form mountains of gentle slope, occurring more often as 

 steep-sided domes, especially if the lava is only semi-fluid. This 

 is well shown in Figure 232. where, in the Auvergne district of 

 France, a trachyte cone of highly viscid lava was extruded between 

 cinder cones. The domed character of the extravasated pustular 



