What Caused Kilauea’s Caldera to Form?
Abstract
The modern caldera at Kilauea collapsed in about 1470-1500 CE, as recognized by geologic field work and C-14 dating. Two lines of geologic evidence indicate it was very deep soon after it formed, as is also suggested by Hawaiian oral tradition (Swanson, 2008, JVGR 178, 427-431). High lava fountains immediately followed collapse, and the caldera was so deep that only very light reticulite could escape to the rim, heavier pyroclasts being trapped within the caldera. Soon after the reticulite eruption, phreatomagmatic activity began, consistent with the caldera floor resting at or even below the water table, about 600 m below today’s caldera rim. A volume of 5-6 km3 is a reasonable estimate for the early caldera with such a depth and today’s diameters of 4 by 3 km. A common model--rapid eruption of magma and consequent emptying of the summit magma reservoir--would predict a large eruption of 5-6 km3 about 500 years ago. But no evidence has been found for such an event, either on land or the submarine Puna Ridge. At least two other possibilities exist for the collapse. One is that the magma reservoir indeed emptied rapidly, and all the magma was intruded elsewhere without erupting. The east rift zone would be the most likely candidate, perhaps opening at depth during a large slip event of Kilauea’s mobile south flank. A dike 60 km long, 5 km high, and 20 m wide could accommodate such a volume. A dike of equal length but twice as high, reaching down to the base of the volcano, would be 10 m wide, about an order of magnitude wider than most measured dikes in Hawaiian shields. Such an intrusion cannot be denied but seems unlikely. A second possibility is that magma withdrawal was prolonged, not rapid, in response to slow eruption of the `Aila`au lava flow, the largest in Hawai`i in more than 1000 years (Clague et al., 1999, Bull. Vol. 61, 363-381). The flow erupted on the east side of Kilauea’s summit during a ca. 60-yr period ending in about 1470. It has typical tube-fed morphology, consistent with slow eruption, and has an estimated volume of 5-6 km3. The eruption just predates the caldera--its shield is cut by the outermost caldera fault--and in fact could have ended when the caldera formed. The timing and volume of the `Aila`au flow and the caldera collapse are notably similar and raise the possibility that the caldera dropped in response to slow emptying of the reservoir over a ca. 6-decade interval. At least three problems bedevil this idea. One is whether the summit could support itself as its reservoir emptied slowly, possibly with some sagging for years before wholesale collapse. Perhaps the reservoir was not a single chamber but instead a complex of chambers as in mines or limestone caverns, with supports that held up the summit for years at a time. A second problem is more serious. If an eruption slowly emptied the reservoir, magma supply from the hotspot must have stopped or been directed elsewhere. This scenario is debatable on a time scale of decades. A third problem is equally troubling. If the magma reservoir were emptying, then what pressure was driving the `Aila`au eruption, whose vent was higher than the reservoir and so was not fed simply by gravity? These questions should motivate future research. Only one thing is clear now. Kilauea’s caldera does not have as straightforward an origin as was once thought.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V43F2324S
- Keywords:
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- 8425 VOLCANOLOGY / Effusive volcanism;
- 8428 VOLCANOLOGY / Explosive volcanism;
- 8440 VOLCANOLOGY / Calderas