Barriers Against Lava — Macdonald 
Fig. 3. Lava flow filling a street in the village of 
Boscotrecase on the slope of Vesuvius during the 
eruption of 1906. Note that the masonry walls are little 
disturbed. Photo by T. A. Jaggar. 
In 1920, a flow of pahoehoe transitional to 
aa, from the southwest rift of Kilauea, en- 
countered a loose stone wall 2.5 to 3 feet high 
and 18 inches thick lying at an angle of about 
60° to the course of the flow, piled up behind 
it, and eventually spilled over it without dam- 
aging the wall (Jaggar, 1945: pi. 2). Before 
spilling over the wall the flow was diverted 
for 40 feet along its length. In 1935, a pahoe- 
hoe flow from Mauna Loa encountered a sim- 
ilar wall in the Humuula Saddle and formed a 
pond behind it until the level of the lava be- 
came high enough to spill over it, again with 
almost no effect on the wall itself. In 1954, a 
pahoehoe flow on the floor of Kilauea caldera 
surrounded an old corral on three sides, but 
did not push over its loose stone walls, which 
actually were in such poor condition that they 
were starting to tumble down by themselves. 
In 1950, a rapid aa flow on the west side of 
Mauna Loa encountered a loose stone wall 
about 3 feet high along the upper side of the 
265 
highway. The lava soon piled up enough to 
spill over the wall, but it does not appear to 
have damaged the wall, and for a distance of 
about 250 feet at the south edge of the flow it 
spread only 15 to 20 feet beyond the wall. 
Farther north the same flow continued un- 
checked down the mountainside (Finch and 
Macdonald, 1950: 4). 
An excellent example of the effect of un- 
substantial walls on fluid lava is contained in 
the following description by Jensen (1907: 
653) of the lava flow of 1905 at Matavanu, 
Samoa: 
In portions of the coastal area, as at Toapai- 
pai, where the thickness of the flow is be- 
tween 10 and 40 feet, the lava has in several 
instances flowed round buildings of stone, 
piling itself higher and higher, without crush- 
ing in the walls. Such houses are now repre- 
sented by holes, except where the flow has 
been sufficiently high to enter by the roof, or 
sufficiently liquid to . . . flow in through the 
windows. At one place, near Saleaula, where 
the lava is between 6 and 10 feet thick, a 
native house was removed before the stream 
advanced, but the spot where it stood is now 
a depression surrounded by almost vertical 
lava walls and has grass growing on the bot- 
tom. This spot was preserved by a ring of 
stones about 18 inches high, such as the 
natives make round their houses. 
The latter constitutes a remarkable extreme 
example of the ability of walls to hold back 
lava flows of depth much greater than the 
height of the wall. This characteristic will be 
discussed in more detail below. 
Mason and Foster (1953) have described 
the destruction of a tea house on the rim of 
Mihara Crater in 1951. As the lava surrounded 
the building, wooden parts were destroyed by 
fire and lava which entered through window 
openings, but the masonry walls withstood 
the pressure of the flow. 
During the 1669 eruption of Mount Etna 
in Sicily, lava flowed against the ancient city 
walls of Catania. For several days the walls 
withstood the lava and diverted it around the 
