104 J. D. Dana — Explosive Eruptions. 



projectile vapors, and thus to produce the upward throw of 

 cinders and the pericentric deposition and stratification which 

 are the marked features of a lateral cone. Xo such shape or 

 structure can come from the simple discharge of a lava-stream 

 into the sea, however rapid its progress ; for this merely puts 

 the fragments that are made at the disposal of the waves or 

 currents along the coast, and the heaps piled up will be such 

 as the action of waves and currents may make elsewhere. It 

 cannot imitate successfully the pericentric work of the volcano. 

 For this work, a center of ejection, acting for successive days 

 or weeks, is required. 



2. EXPLOSIVE ERUPTIONS. 



All the eruptions of Mt. Loa and Kilauea within rate last 65 

 years, the period of actual history, have been, as has been stated 

 of the ordinary kind, that is quiet onflows. At each, the lavas 

 of the crater have simply quit work and sunk out of sight ; and 

 the discharge thus begun, with the consequent down-plunge of 

 the undermined floor, was nearly all there was of eruption so 

 far as the crater was concerned. 



But traditional history gives hints of an eruption in 1789 — 

 a century back within a year — of another kind ; and the results 

 are visible over the region around the crater of Kilauea, as 

 already described (xxxiv, 359). Similar evidences exist of an 

 explosive eruption in the summit crater, as may be inferred 

 from the descriptions of Mr. Brigham (p. 23) and J. M. Alex- 

 ander (p. 31), as well as the earlier of Captain Wilkes," and also 

 in that of final alai. 



In the cases here referred to, the ejected material includes 

 solid masses of the basalt, much of it very compact, and some 

 of the blocks 50 to 100 cubic feet in size. For such work, 

 instead of a cessation of the ordinary projectile action of the 

 crater and a quiet discharge of the lavas when the eruption 

 began, there must have been an enormous increase of projectile 

 power, with great rendings of the rocks within reach of the 

 up-thrust action. The eruption was not a quiet outflow, but a 

 catastrophic upthrow. Whether accompanied or not by an 

 outrlow of lava is unknown. 



Examples of explosive eruptions from Tarawera in Xew 

 Zealand, and Krakatoa an island just west of Java, will make 

 clear what is meant distinctively by an explosive eruption. 



In 18S6, in the Tarawera geyser region after some earth- 

 shocks, a projectile eruption of terrific violence and incessant 



* Wilkes speaks of large bowlders of a grayish basalt at the summit which had 

 apparently been ejected from the crater (p. 159). 



The precise date of the Kilauea eruption is in some doubt. 



