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PACIFIC SCIENCE, Vol. II, October, 1948 
two different phreatic eruptions which prob¬ 
ably should be chosen from among the 1790, 
the seventeenth, and the fifteenth eruptions. 
Evaluation of the time span covered by the 
eruptions of the Keanakakoi formation can be 
no better than a guess at present; but perhaps 
a guess, tempered by the impressions of rela¬ 
tive time intervals and periods of exposure that 
one gets while "living with the problem” in the 
field, is worth recording. 
The present forest cover on the northeast 
rim of Kilauea is probably the heaviest tree 
growth which has developed in any period since 
the emplacement of the upper lava flow of this 
part of the crater rim. The deposits of the 
1790 and the eighteenth eruption on this part 
of the rim are too light to have killed well- 
established trees, though each probably killed 
small ground-covering vegetation. Deposits from 
the seventeenth eruption were thick enough 
probably to have killed all vegetation as far as 
half a mile to the northeast of the rim. Judging 
from comments of early visitors, a good ground 
cover and at least a scattered stand of scrubby 
ohia were established on the immediate rim as 
long ago as 1825. Taking everything into con¬ 
sideration, my impression is that the present 
reforestation of the rim since the seventeenth 
eruption has required approximately 400 years. 
The next heaviest vegetative cover formed on 
the northeast rim was established in the time 
interval between the twelfth and the seventeenth 
eruptions. The humus layer indicates heavy 
ground cover, and molds of small ohia trees 
have been found. The "kill” of the twelfth 
eruption probably extended about the same 
distance to the northeast of the rim as did that 
of the seventeenth. This time interval may 
have been about 300 years. Much less time 
seems to be required for the intervals between 
the ninth and the twelfth, and the fifth and the 
ninth eruptions, since the humus is thin. The 
vegetative cover developed on the deposits of 
the fourth eruption consisted largely of fern, 
ohelo, and other low growth and probably 
could have developed in about 200 years. A 
similar period apparently would suffice for the 
time between the second and third eruptions. 
An estimate of perhaps 1,500 years is suggested 
for the total time covering the explosive erup¬ 
tions of the Keanakakoi formation. 
CONCLUSIONS 
Detailed study of the disconformities and 
unconformities between the beds in the pyro¬ 
clastic deposits of Kilauea crater and appraisal 
of the relative time intervals which they repre¬ 
sent, combined with study of the lithology of 
the beds, have facilitated a preliminary catalog- 
Plate 1 A. Abrupt transition from heavy forest cover to barren surface, possibly the southeast limit of 
"forest kill” by the 1790 eruption, perpetuated because local climatic conditions have inhibited reforestation. 
Looking northeast across Keanakakoi Crater (foreground) and the east end of Kilauea (left background) 
from locality P. Photograph by H. A. Powers, September, 1947. 
PLATE IB. Remnant of the mantle of 1924 bedded lithic ash, originally plus 5 inches thick, as exposed by 
the small excavation in the foreground. This area is subject to torrential rains, but the absence of an incised 
drainage net indicates that runoff erosion is subordinate in effectiveness to wind planation and undercutting. 
The present surface is planed across the bedding of 1924 ash at a low angle, and a desert pavement of residual 
larger fragments is beginning to accumulate. Photograph by H. A. Powers, September, 1947, at locality M. 
PLATE 1C. Remnant of 1924 ash, originally plus 3 inches thick in this area, showing results of wind ero¬ 
sion by undercutting along least resistant layers. Most of the remaining patches of the 1924 ash mantle are 
in slight surface depressions where moisture persists longer than on the adjacent slopes, perhaps retarding re¬ 
moval of the patches by wind erosion. Photograph by H. A. Powers, September, 1947, at locality N. 
PLATE ID. Keanakakoi formation pyroclastics mantling the inner face of the southwest wall of Kilauea 
Crater exposed by a short inflowing ephemeral stream. The bedded vitric deposits of the third and fourth 
eruptions (lower left) are truncated by an erosion surface equivalent to a good humus layer on the humid 
northeast crater rim. On the unconformity are the bedded vitric deposits of the fifth to the ninth eruptions 
(center and right center). The upper beds represent the seventeenth to 1790 eruptions mantling a major 
erosion surface from which all deposits of the tenth to the sixteenth eruptions have been removed. Photo¬ 
graph by H. A. Powers, September, 1947, at locality T. 
