Barriers Against Lava ■ — MACDONALD 
259 
ACKNOWLEDGMENTS 
Since the 1955 eruption of Kilauea the mat- 
ter of lava barriers for the protection of Hilo 
has been the subject of frequent discussion 
with Jerry P. Eaton, geophysicist of the Ha- 
waiian Volcano Observatory, U. S. Geological 
Survey. Earlier, it had been discussed re- 
peatedly with the late Thomas A. Jaggar, 
Ruy H. Finch (my predecessor as director of 
the Observatory), and Chester K. Wentworth. 
Although minor differences of opinion exist, 
I believe we agree on essential points. 
Curtis Kamai, engineer for the Territorial 
Highway Department, through the courtesy 
of that department, worked closely with us 
during the 1955 eruption and paid special 
attention to the behavior of flows in relation 
to barriers. Discussions with him during and 
since the eruption are gratefully acknowledged . 
Doak C. Cox, geologist for the Hawaiian 
Sugar Planters’ Association Experiment Sta- 
tion, has contributed valuable discussion of 
the manuscript. 
NEED FOR PROTECTION 
Hilo Bay lies at the junction of the slope of 
Mauna Loa volcano with that of Mauna Kea 
to the north (Fig. 1). Most of the city of Hilo, 
south of the Wailuku River, is built on geo- 
logically recent lava flows from Mauna Loa. 
The very existence of Hilo Bay is the result of 
these flows, which constitute all of the broad 
promontory that extends eastward to Leleiwi 
Point. These flows cannot now be dated in 
years, but probably most of them are less than 
2,000 years old. 
Since about 1820, when our real knowledge 
of Hawaiian volcanoes begins, Mauna Loa 
has been among the most active volcanoes in 
the world. During that period it has erupted 
on an average once every 3.6 years, and the 
total lava poured out has been more than 4 
billion cubic yards. Nothing in the geological 
record indicates that this degree of activity is 
abnormal in the history of the volcano, nor is 
there reason to expect that the degree of activ- 
ity in coming centuries will differ appreciably 
from that of the last. 
The vents of flank eruptions of Mauna Loa 
are concentrated along two zones of fractur- 
ing, known as rift zones, that extend re- 
spectively east-northeastward and southwest- 
ward from the summit of the mountain. The 
northeast rift zone averages about a mile in 
width, and trends almost directly toward Hilo. 
It is marked at the surface by innumerable 
fissures in the ground, and lines of cinder and 
spatter cones built at the sites of eruptions. 
The three small cinder cones known as the 
Halai Hills, within the city of Hilo itself, ap- 
pear to lie on the prolongation of the north- 
east rift zone; but fortunately the portion of 
the rift zone below an altitude of 6,000 feet 
has been inactive for many hundreds of years. 
Eruption along the northeast rift zone has 
built a broad, rounded ridge trending toward 
Hilo. The north slope of this ridge intersects 
the south slope of Mauna Kea, producing a 
broad valley through which the Wailuku River 
and its tributaries flow eastward into Hilo Bay 
(Fig. 1). Because of this topographic configu- 
ration, all lava flows erupted from the northern 
part of the rift zone below approximately 
11,500 feet altitude are directed toward Hilo 
within a belt about 6 miles wide. Whether or 
not they reach Hilo depends largely on the 
volume of lava released during the eruption, 
and whether it is concentrated into a single 
flow or spread as several flows over the upper 
slope of the mountain. 
It is the restriction of flows approaching 
Hilo to this relatively narrow (6 miles wide) 
belt just south of the Wailuku River that 
makes feasible the protection of the city by 
diversion barriers. 
Flows issuing at points on the northeast 
rift zone above 12,000 feet altitude probably 
will turn westward in the Humuula Saddle (as 
did the flow of 1843), and consequently do 
not constitute a threat to Hilo. 
Since 1850 there have been 6 major erup- 
tions in the northeast rift zone, producing 8 
major lava flows with an aggregate volume of 
