Barriers Against Lava — MACDONALD 
269 
lA) was entirely successful, and demonstrated 
one method of controlling the relative speed 
of advance of different parts of a flow front. 
Barrier 2 showed that even when the flow top 
has piled high above the barrier, and some 
spill-over is occurring, the barrier may still 
control the direction of movement of the bulk 
of the flow. The Yamada barriers demonstrated 
also the importance of a cleared corridor along 
the upper side of the barrier, to facilitate the 
advance of the flow along the barrier; the im- 
portance of placing the barrier at an acute 
angle to the course of the flow, and maintain- 
ing a continuous downgrade in the new chan- 
nel created by the barrier; the importance of 
extending the barriers laterally sufficiently far 
to be certain of catching all flows that may 
advance toward the area being protected; and 
finally, the importance of planning and build- 
ing in advance, thus avoiding the poor execu- 
tion attendant on hurried construction with 
the lava crowding the bulldozers. 
As it crossed the Yamada coffee fields, the 
lava provided yet one more lesson on lava 
barriers. In clearing the fields, bulldozers had 
pushed up great heaps of trash, 10 feet or more 
in height. These heaps consisted largely of 
trunks and branches of pandanus trees, with 
smaller amounts of other vegetable debris and 
some rocks. The lava flowed between, and 
eventually over, the heaps of loose and mostly 
light rubbish without to any important degree 
displacing them, thus again demonstrating 
the small amount of thrust exerted by lava on 
obstacles. A similar example occurred earlier 
in the eruption at the time of the outbreak at 
the edge of Kapoho village, when a heap of 
rubbish that had been pushed aside in clearing 
land diverted the flow away from a house. The 
Kapoho flow was a thin and very fluid pahoe- 
hoe flow, and might be expected to be easily 
diverted. The flow through the Yamada coffee 
fields was a very active aa flow with a moving 
front 10 to 15 feet high, and might be ex- 
pected to exert as much thrust against an 
obstacle as almost any Hawaiian flow; yet 
even it exerted so little thrust that the piles of 
loose debris in its path were essentially un- 
disturbed by it. 
The fact that lava flows follow the path of 
least resistance was demonstrated repeatedly 
during the 1955 eruption. The flow fronts ad- 
vanced much more rapidly along roads than 
through adjacent cane fields or forests. Even 
the small amount of obstruction caused by 
small and relatively scattered vegetation ob- 
viously slowed the advance of the lava. At the 
Yamada barriers, the lava covered the ground 
cleared by the bulldozers during construction 
of the walls much more rapidly than it did the 
uncleared forest areas. This fact is important 
because it indicates the great desirability of 
clearing and keeping reasonably clear a path 
500 or more feet wide along the upper side of 
a diversion barrier to aid in turning the flow 
and establishing a channel along the barrier. 
PRINCIPLES GOVERNING LAVA MOVEMENT 
Certain basic facts in the behavior of lava 
flows are of fundamental importance to the 
operation of lava barriers. These facts may be 
briefly enumerated. 
Although every lava flow has some solid 
portions, the movement of the flow is gov- 
erned by the liquid portions. The solid por- 
tions are passively dragged along by the liq- 
uid, tending to modify somewhat the be- 
havior of the liquid, principally by making it 
more viscous; but, especially in Hawaiian 
flows, these modifications are small. The fact 
of basic importance is that the flowing lava is 
essentially a liquid and for the most part be- 
haves like one. Thus lava always tends to flow 
directly down the steepest available slope, and 
to follow the path of least resistance. 
In aa flows the most fluid portion is re- 
stricted to a narrow feeding river, seldom 
more than 30 feet wide, usually situated near 
the center of the flow. The margins of active 
flows commonly are still mobile, but very 
much less so than the material in the feeding 
river. Similarly, pahoehoe flows are fed by 
narrow streams flowing through natural pipes, 
or lava tubes. The modes of advance of both 
