Hawaiian Calderas — Macdonald 
327 
rocks beneath the Oahu calderas with densities 
approximating 3.2 gm/cc extending to at least 
the depth of the ocean floor (5.5 km). Very 
dense caldera-filling tholeiitic basalts may have 
densities up to a little more than 3, and ocean- 
ites have measured densities ranging up to 
about 3.2; but of Hawaiian rocks only the 
peridotites found as inclusions in flows have 
consistently a density greater than 3.2. 
The dense high-velocity rock has generally 
been considered to be part of the earth’s man- 
tle injected to high levels in the crust. How- 
ever, it conceivably could be cumulate material 
settled from the overlying magma in the con- 
duit and core of the volcano or lagging behind 
as the more fluid portion of the magma rose 
around it. Fragments of dunite and wehrlite 
brought up by lavas of the old-age stage of 
the volcanoes, such as the 1801 eruption of 
Hualalai (Macdonald, 1949:76; Richter and 
Murata, 1961) have textures resembling those 
of the cumulate rocks of layered intrusives 
(Wager, Brown, and Wadsworth, I960) and 
probably represent fragments of cumulate rock 
brought up from relatively shallow depths be- 
neath the volcano. The mineral assemblages are 
not particularly indicative of high-pressure 
equilibria. On the other hand, the garnet pyrox- 
enite ("eciogite”) found as inclusions at Salt 
Lake Crater on Oahu does represent a high- 
pressure equilibrium assemblage, and quite 
probably represents material brought up from 
the mantle. In chemical composition it is very 
close to tholeiitic oceanite, though somewhat 
richer in silica and poorer in alkalies (Mac- 
donald and Katsura, 1964: Table 8, col. 13), 
and may represent an oceanitic intrusive mass 
crystallized under high pressure in the upper 
mantle. Its density (2.71-2.81) and seismic 
velocity (V p = 5.52-6.06) as determined by 
Manghnani and Woollard (p. 291 in this 
issue) are too low to account for the material 
in the primary volcanic "pipes,” which is char- 
acterized by high gravity values. Moreover, 
there is no significant gravity "high” associated 
with Salt Lake Crater. 
Brief mention of the rift zones of Hawaiian 
shield volcanoes has already been made. The 
rift zones generally radiate outward from the 
summit of the shield — that is, from the caldera. 
Usually there are three distinct rift zones, with 
angles of roughly 120° between them, and 
with one rift zone less well developed than the 
others. In addition to the lines of spatter cones 
and cinder cones resulting from eruption, the 
rift zones are marked by pit craters, many open 
fissures, and by long narrow grabens. The depth 
of the grabens is generally unknown, because 
they have been partly filled with later lava. 
At depth in the older, dissected volcanoes, the 
rift zones are marked by thousands of thin dikes. 
Sections across them yield counts of more than 
600 dikes per mile. Although a few instances of 
strike-slip displacement on rift-zone fissures are 
known (Macdonald, 1956:278), the configura- 
tion of the walls of the dikes generally indicates 
horizontal opening without any appreciable dis- 
placement parallel to the fissure. There can be 
no question that the rift zones represent a very 
considerable distension of the visible part of 
the shield volcano, a distension on the order 
of 0.75-1 km. 
The Hawaiian rift zones have recently been 
explained by J. G. Moore (at a lecture before 
the Peninsula Geological Society, Stanford Uni- 
versity, January 7, 1965) as the result of land- 
sliding on a gigantic scale. Specifically, he be- 
lieves that the southern slope of Kilauea is 
sliding seaward, the fractures on which the 
movement is taking place steepening to near 
verticality to form the east rift zone, with 
graben collapse along the upper edge of the 
sliding block. The distension in the rift zone 
he attributes to the southward movement of 
the block to the south. He supposes that magma 
makes its way surfaceward along the plane of 
sliding. Essentially the same suggestion was 
made for the origin of the southwest rift zone 
of Kilauea by Stearns and Clark (1930). With- 
out at present entering into any debate on 
whether or not there is large-scale landsliding 
going on along the south flank of Kilauea, it 
appears very unlikely to me that the east rift 
zone (or any other) can have the origin sug- 
gested by Moore. The essentially vertical atti- 
tude of the dikes in the rift zones down to the 
deepest level of exposure on the deeply eroded 
islands of Oahu and Kauai, a level equal to 
more than half of the probable depth to the 
magma chamber. at Kilauea, is inconsistent with 
