J. D. Dana — Eruptions of Kilauea and Mt. Loa. 91 



I. Characteristics and Causes of Eruptions. 



Eruptions are of two kinds : (1) JW on- explosive eruptions, or 

 quiet outflows, seismically attended or not ; and (2) Explosive 

 eruptions, or catastrophic upthrows. Both kinds are exempli- 

 fied in Hawaiian volcanic history. There are also (3) combina- 

 tions of the two kinds in volcanic regions. 



1. ORDINARY OR NON-EXPLOSIVE ERUPTIONS. 



Kilauea and Mt. Loa are alike, as has been shown, in (1) 

 their mode of work ; (2) the southward position, in the crater, 

 of the point of greatest activity ; and (3) the general features 

 of their eruptions. But in amount of eruptive work the sum- 

 mit crater is far ahead of Kilauea, and, in fact, it leads the 

 world. Kilauea has had but one subaerial outflow of any mag- 

 nitude in the last fifty years, and that only twelve miles long. 

 Mt. Loa, on the contrary, although nearly ,13,000 feet up to 

 the bottom of the crater, has had in the same time only one of 

 its eight less than twelve miles long, and several between 

 twenty and thirty-five ; and it has reached its height without 

 a loss of eruptive power. It is reasonable, therefore, that Mt. 

 Loa should have most instruction to give about outflows. 



1. Heights and positions of the places of outbreak. 



1. The Heights. — The place of outbreak of a Mt. Loa erup- 

 tion may have any height from the summit to levels far below 

 the sea-level ; and this " far below " may be, as the map on p. 

 82 shows, 17,250 feet down before reaching the actual foot of 

 the eastern slope. The heights of known occurrence are men- 

 tioned in the table on p. 83. The completion of the topo- 

 graphical survey of Hawaii, now in progress under the govern- 

 ment, will before long give more correct figures. The height 

 of the source of the one Kilauea outflow, that of 181:0, or 

 rather of the spot where it appears to begin, is 1244 feet 

 (Wilkes). 



In each of the cases of eruption, fractures were made near 

 the summit which extended down the mountain, with only 

 small discharges along them when any, to the place of chief 

 outflow. In some cases, fissures have opened on the brink of 

 the crater and let out lavas ; but all the large outflows of mod- 

 ern time have come from points a thousand feet or more below 

 the summit. 



2. Relations between the positions of the places of outbreak 

 and the diameters of the craters. — The course of the northern 

 half of the longer diameter of the summit crater is about 

 K 35° E., and that of the southern half, about N. 20° E. or 



