242 jaggar: aa lava at kilauea 



metres in excess of the sinking of the bench magma (lake bot- 

 tom), a towering uptilted bench crag on the east side of the in- 

 terior of Halemaumau subsided 9 metres in the course of twelve 

 hours, and opposite to it in the lake a low flat island rose 12 

 metres, becoming in one night a towering flat-topped, steep- 

 sided mass. Flow from beneath the crag was compensated by 

 upfiow beneath the island, the distortion affecting, not the lava 

 lake, but the lake bottom. At that time the lake near this 

 island was possibly not more than 5 metres deep, proved by 

 subsequent uncovering of the bottom locally. Simultaneously 

 other parts of the bench subsided and island features rose, cor- 

 roborating by survey the equilibrium relation shown by this 

 extraordinary east island. 



Rising began on March 20, 1917, and on March 24 the writer 

 crossed the now solid arch of crust which had formed by con- 

 gelation and overflow between the east shore of the lake and 

 the island, and examined the base of this monolith. It proved 

 to be aa or block lava. This is the first identification of actual 

 aa under formative conditions in Halemaumau pit known to 

 the writer, though aa exists as rare flows on the floor of the 

 greater crater of Kilauea. It occurs also as graduation forms 

 of the dominant pahoehoe, notably on the west lip of over- 

 flow, or rampart of Halemaumau, last actively overflowing when 

 the lava in the pit reached its highest and receded in 1894. 



This east island has been figured.' 2 The depression of the 

 lake below the rim of the pit at the time of the sudden lift of 

 the island was about 29 metres. The flat elongate islet from 

 which it arose had been 66 metres long on February 14, but only 

 the southern half of this was raised into the new tabular mass, 

 the remainder forming a tumble of pinnacles which extended it 

 northward. The tabular elevation was 25 metres across the 

 top and subcircular. The top was bounded by a vertical cliff 

 3 metres deep showing horizontal layers of pahoehoe made by 

 lake flooding dunng fluctuations of the lake level, while the 

 island was low and at or near the lake level. Below was the 



-- Bull. Hawaiian Vole. Obs.. Feb., 1917. 



