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Photograph by S.R. Brantley on August 17, 1998

View is from the caldera floor north of Halema`uma`u toward the northwest. In this section of the caldera wall, north of Uwekahuna Bluff and below a section of thin horizontal lava flows, is a massive doubly-humped body of light-colored basalt. It is about 300 m long and 25-30 m thick. An unusually high concentration of olivine crystals in the central and lower parts of the layer (15-40 percent) is evidence that these heavy minerals "settled" to the lower part of the molten-rock lense before it solidifed.

What is the source of this massive layer?

Many scientists have interpreted the solidified lava as a laccolith -- a body of molten rock that moved up into the summit area of Kilauea and then forced its way sub-horizontally into a pile of lava flows. In support of this hypothesis, scientists have argued that lava flows on the north (right) edge of the body were lifted over the intruding magma, which would indicate that the flows were deformed sometime after they had been erupted onto Kilauea's surface.

Two alternative origins are drawing increased attention. One is that the body was injected laterally from a high-standing lava lake in the caldera. Such bodies were observed on a smaller scale in the walls of Halema`uma`u just after the lava lake drained away in 1924. The second alternative is that the body is a composite of two tumuli in a lava flow. Scientists have observed tumuli as tall as about 15 m develop in the pahoehoe flow field of Kilauea's Pu`u `O`o - Kupaianaha eruption.

The range of ideas of how the body formed illustrates a common problem in geology -- how to assign a unique, definite origin to an event that was not observed.

In other parts of the west caldera wall, scientists have identified 18 vertically oriented intrusive structures called dikes. The combined width of these dikes is about 15 m and accounts for less than 1 percent of the length of the west wall.

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References

Casadevall, T.J., and Dzurisin, D., 1987, Intrusive rocks of Kilauea caldera, in Decker, R.W., Wright, T.L, and Stauffer, P.H., eds., Volcanism in Hawaii: U.S. Geological Survey Professional Paper 1350, v. 1, p. 377-394.

Murata, K.J., and Richter, D.H., 1961, Margmatic differentiation in the Uwekahuna laccolith, Kilauea caldera, Hawaii: Journal of Petrology, v. 2, p. 424-437.


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Updated: 15 September 1998