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PACIFIC SCIENCE, VoL XIX, July 1965 
magma from the cupolas may allow the collapse 
of pit craters in and near the caldera and along 
the rift zones. Eventually the top of the moun- 
tain splits open and some of the fluid tholeiitic 
magma rises to the surface, allowing the volcano 
to detumesce. The force that drives the magma 
to the surface is primarily the hydrostatic pres- 
sure on the magma body resulting from the 
weight of the overlying rocks, but as the magma 
gets very close to the surface there is added to 
this the expansive force of the gas that is 
separating from solution. Eruptions of this 
type are confined to the summit region. 
As the mass of ultra-dense material in the 
core of the volcano grows, from time to time 
its weight becomes sufficient to cause a slight 
pushing apart of its confining walls in the 
lower part of the volcanic structure. This results 
in a splitting open of the volcano as a whole, 
including one or both of the rift zones and 
the summit region. Magma drains outward into 
the rift zones, and commonly flank eruption 
results. The volcano detumesces, and if disten- 
sion of the summit region has been sufficient 
to allow sinking of the wedge-shaped caldera 
block true caldera collapse ensues. At other 
times, without any appreciable downfaulting of 
the caldera block, the detumescence may be 
simply a gentle over-all sinking detectable only 
by instrumental methods (as it commonly is 
in the case of summit eruptions ) ; or it may be 
accompanied by marked collapse only at points 
where the underlying magma body approaches 
or reaches the surface, such as the collapse of 
Halemaumau crater during the subsidence of 
1924 or the basining of the caldera floor in 
1868 and 1894. Sinking of the caldera block 
depends on distension of the summit more than 
on draining away of the underlying magma, 
since the heavy rock of the caldera fill can sink 
indefinitely into the less dense magma. How- 
ever, sinking of the caldera is generally accom- 
panied by lateral movement of the underlying 
magma into the rift zones because the same 
lateral displacement that stretches the summit 
region enough to allow sinking of the caldera 
block also opens the rift zones, and room for 
the sinking is largely provided by drainage of 
magma into the rift zones. Some magma is 
squeezed out in the summit region by sinking 
of the overlying rocks, but the denseness of 
the caldera-filling rocks and the wedge shape 
of the sinking block keeps the underlying coni- 
cal subsidence fractures tightly dosed and 
largely prevents the rise of magma through 
them. Discrepancies between the volume of 
summit sinking, including caldera collapse, and 
the volume of lava extruded in subaerial erup- 
tions (both flank and summit) are accounted 
for partly by intrusion into the rift zones, prob- 
ably partly by submarine eruption, at times 
partly by squeezing of magma upward into fis- 
sures in overlying rocks, and partly by space 
provided by the slight sinking of the top of the 
ultra-dense core as a result of spreading of its 
lower portion. 
The eruption is brought to an end by drain- 
age of the easily-eruptible magma down to the 
level of the opening of the fissures, but the 
eruption may be prolonged by rise of additional 
magma from deep levels, as appears probably 
to have been the case during the 1959 eruption 
in Kilauea Iki (Richter and Eaton, I960). 
Afterward the volcanic structure is sealed by 
partial or complete congealing of magma in 
the fissures, and the whole cycle repeats itself 
as more ultra-dense magma rises from the man- 
tle and more tholeiitic magma accumulates in 
the shallow magma reservoir. 
REFERENCES 
Anderson, E. M. 1936., The dynamics of the 
formation of cone-sheets, ring-dykes, and 
cauldron-subsidences. Roy. Soc. Edinburgh 
Proc 56(2): 128-163. 
CLOGS, E. 1955. Experimental analysis of frac- 
ture patterns. Geol. Soc. Am. Bull. 66:241- 
256. 
Clough, C T., H. B. Maufe, and E. B. Bai- 
ley. 1909. The cauldron-subsidence of Glen 
Coe and the associated igneous phenomena. 
Geol. Soc. London Quart. J. 65:611-674. 
Daly, R. A. 1914. Igneous Rocks and Their 
Origin. McGraw-Hill Book Co., New York. 
563 pp. 
Eaton, J. P., and K. J. Murata. I960. How 
volcanoes grow. Science 132:925-938. 
