184 Jaggar — Yolcanologio Investigations at Kilauea. 



flooded occasionally by the liquid, there is tendency to inces- 

 sant bnt very gradual subsidence of the flooded plains, and 

 elsewhere to uplift, especially near the inlet vents (fig. 2, sec- 

 tion). The places of flooding shift, however, and in the course 

 of a long term of rising lava in Haleraauraau pit, the net effect 

 is a subsidence of the bench region compensated by an inward 

 flow under the lake bottom saucer, keeping the latter shallow, 

 and even lifting islands above the lake surface. I shall speak 

 of the two phases as the lake magma and the bench magma. 



Consistency of Bench Magma. 



When the lava column subsided 400 feet (122 meters) sud- 

 denly as above described, from depression 300 feet (91 meters) 

 within the pit to depression 700 feet (213 meters), after months 

 of building up by repeated risings and overflowings, there was 

 revealed incandescence throughout the bench and island magma. 

 This substance fell inward by opening narrow vertical fissures 

 back from the edge of the bench, until the slabs so loosened 

 tottered and descended crumbling to the slopes below. There 

 was always revealed a bright red luminous fracture surface and 

 the larger slabs, many tons in w^eight, would flex outward 

 slightly and then fall, disintegrating to a glowing talus wdience 

 arose great billows of chocolate-brown dust. These avalanches 

 were noisy but less so than might have been expected, owing 

 to the peculiar consistency revealed by the arching out and 

 crumbling, for which the writer can think of no better simile 

 than that of hard cheese breaking. The entire absence of 

 heavy quaking at the upper rim of the pit, even when a whole 

 quadrant of the bench fell immediately below the observer, 

 showed that the attachment to the side walls was slight.' I 

 have elsewhere seen a very small remnant of an older rock 

 bench, long firmly attached, produce a strong earthquake on 

 falling. 



It was evident in this collapse of June 5 that the piled up 

 overflow strata of months previous were a uniformly incan- 

 descent and mobile stiff magma throughout, and that burial 

 preserved the inner heat of the flows and reheated the crusts. 

 These crust layers of flows are of thicknesses varying from a 

 few^ inches to several feet (fig. 17t^, foreground), like other 

 pahoehoe flows and are full of air in vesicles. 



A phenomenon, occasionally seen during such subsidence at 

 orifices in this bench magma, is an outflow which also is like 

 crumbled cheese, and incandescent, but starting out like a 

 liquid and falling like gravel or sand. It trickles out from 

 fresh vertical breaks in falling benches. This substance appears 

 to be identical with or very like aa lava on cooling, but I have 

 never had access to it for verifying the resemblance.'^^' Some of 

 this fell From the glowing wall shown in background of ^g. 15c. 



* See " Live aa lava at Kilauea," by T. A. Jaggar, Jr., Jour. Wash. Acad. 

 Sci., vol. vii, No. 9, May 1917, pp. 241-243. 



