10 
Fe 2 0 3 + FeO 
Fig. 3. Scatter diagram showing the Fe 2 Os/ (Fe 2 C) 3 
-f FeO) ratio for different forms of historic basaltic 
lava from Kilauea and Mauna Loa. 
1. Littoral cone glassy ash. 
2. Aa flows feeding littoral cones. 
3. 1955 lava collected in tidal zone. 
4. Basaltic pumice. 
5. Pahoehoe lava flows. 
6. Aa lava flows. 
of the lava is thrown into the air and back on 
land by the steam explosions. Hence, the volume 
of material which flows into the sea must be 
great. Judging from historic littoral cones, 
probably more than 50 million cubic yards of 
lava must enter the sea to produce a cone of 
appreciable size. 
In Table 2 are tabulated the 12 historic lava 
flows which reached the ocean on the island of 
Hawaii. Of the nine which did not produce 
littoral cones, five poured too little lava into 
the sea. These five are the flows of 1750?, 1823, 
1926, 1955, and I960. Of these, the 1926, 1955, 
and I960 flows produced observed littoral ex- 
plosions and probably would have formed cones 
had the flows been more copious or more local- 
ized. The I960 eruption poured a sizeable vol- 
ume into the sea, but it was distributed along 
a 2-mile front. 
PACIFIC SCIENCE, Vol. XIX, January 1965 
All the littoral cones were fed by aa flows. 
Probably these lava flows are more likely to 
produce littoral explosions for two reasons : ( 1 ) 
The turbulent and fragmented character of the 
flow and the presence of much included, cooler, 
solid material, will allow ingress of sea water to 
the hot interior of the flow; this water will 
expand, form steam pressure upon heating, and 
explode. The pahoehoe flows, on the other hand, 
form an elastic crust on their surface but con- 
tinue flowing within; hence the hot mobile 
interior is effectively sealed off from contact 
with the sea water. ( 2 ) The cooler, more brittle 
aa flows tend to fragment and shatter more 
readily upon contact with water than do the 
more fluid pahoehoe flows. 
However, all large aa flows do not produce 
extensive littoral explosions and resultant cones. 
Aa lava flows differ greatly in the character of 
their flow. When the active feeding channel of 
the aa front is smooth and regular in its flow 
into the sea, littoral explosions and generation 
of steam appear to be inhibited. Macdonald 
(1954:166) pointed out that the unbroken 
liquid surface of the 1950 lava river which 
plunged into the sea prevented water from gain- 
ing access to the interior of the flow. When 
steam generation occurs only on the surface of 
the flow, the pressure required for the littoral 
explosions cannot build up. When the smooth 
surface of the flow where it entered the sea was 
disturbed, as by a floating raft of solid lava, a 
brief ash-making explosion occurred. 
Undoubtedly other factors also contribute to 
the intensity of littoral explosions. In addition 
to the volume, rate, and character of the flow 
as it enters the sea, the character of the shore 
bottom is probably important. If the flow moves 
rapidly into deep water on a steep slope, the 
force of the explosions is lessened by the greater 
depth of overlying water. 
REFERENCES 
Brigham, William T. 1909. The volcanoes of 
Kilauea and Mauna Loa on the Island of Ha- 
waii. Bishop Museum Press, Honolulu. 222 
pp. 
Jaggar, T. A., JR. 1919. Monthly Bulletin of 
the Hawaiian Volcano Observatory 7(10): 
127-159. 
