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PACIFIC SCIENCE, VoL XII, July, 1958 
types of flows have been described elsewhere 
(Macdonald, 1953). 
The viscosity of lava flows is high. Even in 
the most fluid portion, close to the vents where 
the temperature and gas content are highest 
and the load of solid crystals and rock frag- 
ments is least, the viscosity is 300,000 to 
400.000 times as great as that of water (Mac- 
donald, 1954: 173). Farther from the vent the 
viscosity of the most fluid portion rises to a 
million and more times that of water, and the 
effective viscosity of the flow as a whole is 
still higher. The liquid has a specific gravity 
probably 2 to 2.5 times that of water. Thus the 
liquid is both heavy and viscous. On steep 
slopes the heaviness of the liquid results in 
high speeds of flow, locally up to about 30 
miles per hour, in spite of the high viscosity. 
However, such high speeds are attained only 
in the narrow feeding channels or tubes. The 
high viscosity of the lava normally results in 
slow movement of the main body of the flow. 
On the steep slopes in central Kona the first 
flow of the 1950 eruption advanced as a whole 
at an average rate of 5.6 miles per hour. How- 
ever, on slopes such as prevail on the side of 
the mountain toward Hilo the fastest ob- 
served advance of a flow front is only about 
1.000 feet per hour, and most flow fronts ad- 
vance much more slowly than that. The flows 
of 1855 and 1881, on the slope of Mauna Loa 
southwest of Hilo, advanced only a few tens 
or hundreds of feet a day on the middle and 
lower slopes of the mountain. 
In almost all instances, essentially the only 
force causing movement of the flow front is 
the component of gravity along the sloping 
surface over which the lava is moving. Be- 
cause ground slopes in Hawaii generally are 
low, the component of gravitational force 
generally is small. This, combined with high 
viscosity of the liquid, results in the observed 
slow speeds of flow. In turn, because of their 
slow movement, lava flows possess very little 
kinetic energy. Where high speeds occur, the 
moving liquid may have enough kinetic en- 
ergy to cause it to dash a few feet up slopes 
opposed to the direction of flow, or be thrown 
a few feet into the air where it encounters 
obstacles. Such occurrences are comparatively 
rare, however, and are encountered only on 
unusually steep slopes in the narrow feeding 
channels or very close to the vents. They are 
never encountered at flow fronts more than a 
very few thousand feet from the vents. Like- 
wise, the viscosity of the lava, though high, 
is not sufficiently great to permit much thrust 
on the flow front from lava behind it. Thus 
Hawaiian lava flows will not advance up hill 
to any extent, or exert any appreciable impact 
pressure against an obstacle owing to energy 
of motion in the flow. A flow front encounter- 
ing a barrier will not tend to "climb” the bar- 
rier to any important extent, nor will it strike 
against it with any violence. The lava will 
accumulate behind the barrier until an equi- 
librium level is attained, just as would water or 
any other liquid, and if the depth of the lava 
becomes great enough it will spill over the 
barrier. But essentially the only pressure ex- 
erted against the barrier is a portion of the 
hydrostatic pressure of the lava in the pool. 
Wentworth (1954) has pointed out that, al- 
though essentially a liquid, lava does not be- 
have quite like water or other familiar liquids. 
The difference results largely from the much 
greater viscosity of lava, and its tendency to 
freeze, thereby building up and tending to 
clog its channel, with consequent irregular 
overflows. This building up of the channel 
makes possible one type of diversion by aerial 
bombing, mentioned earlier. The most obvi- 
ous effect of the high viscosity coupled with 
the tendency to freeze is the piling up of lava 
to form a broad mound instead of a thin sheet, 
as water would do. The margins of flows are 
abrupt scarps several feet or tens of feet high. 
The effect is confined largely to the flow 
edges. Most flows have broad nearly level 
(though irregular) tops, determined by the es- 
sential attainment of liquid equilibrium. The 
effect of viscosity and freezing at the edge of 
the flow, allowing the flow to stand as a self- 
contained unit with steep margins, Is im- 
