Vents and Dikes in the Heart of the Koa'e Fault System at Kilauea
Abstract
The Koa'e fault system, an ENE-trending extensional system characterized by many grabens and thousands of ground cracks, connects the east and southwest rift zones at Kilauea. As generally defined, it is 10-11 km long and 2.5-3 km wide. Although it merges at both ends into rift zones, the Koa'e itself was thought to be barren of eruptive activity, except for two small eruptions in 1969 and 1973 that barely penetrated the eastern end of the fault system. While mapping surface ruptures associated with an earthquake swarm in the Koa'e fault system in early June 2011, two small deposits of ejecta were found 400-1000 m WSW of Ohale. The deposits predate the recent cracking and are themselves cracked, but they provide evidence of previously unrecognized eruptive activity in the recent past within the Koa'e. The ejecta overlie the uppermost ash beds in the Keanakāko'i Tephra and therefore are younger than about 1790. The ejecta in both deposits erupted from a crack that today is about 1 m wide within a crack zone that is more than 10 m wide. Most of the ejecta are of lapilli size, generally 1-4 cm in maximum diameter. Many have flat, fluidal shapes, and all are loose pyroclasts, not agglutinated. The largest deposit mantles only 200-400 m2 and has a single-grain thickness; it has been swept from smooth, hard surfaces by wind and water. The ejecta probably were thrown only a few meters into the air, because their dispersal is limited to within a few meters of the source crack, biased southward from the crack as if blown by the NE trade wind. The eruption(s) probably took place during episodes of ground cracking, dates for several of which are known; May 28-29, 1938; early December 1950; May 9-11 and July 1-3, 1963; December 24-25, 1965, May 5, 1973; and November 29, 1975. Ground cracking probably also accompanied the M7.9 earthquake in 1868. Only the 1965 and 1973 events were associated with eruptions in the adjacent east rift zone. The 1973 eruption occurred about 1 km into the fault system from the east rift zone, and two unpublished leveling profiles indicate that a dike was injected at least an additional 4 km westward from that eruption site. The 1965 cracking event was much larger, but no leveling data exist to evaluate possible associated intrusion into the Koa'e. We are examining chemical data from the 1965, 1973, and other eruptions in the east rift zone for comparison with the Koa'e ejecta and will present the results on our poster. The recognition of recent eruptions and dikes in the Koa'e is evidence that magma has penetrated into the heart of the fault system, several kilometers from both rift zones. It is reasonable that more dikes are present as well, not erupting because they outgas as magma moves laterally into the open crack system. Such dikes would help explain the lack of a negative Bouguer gravity anomaly over the highly cracked area. They could also impound groundwater, and geophysical studies show that the fault system is a barrier to seaward groundwater flow from the summit area despite all of the faults and ground cracks. The Koa'e fault system, with the two rift zones, forms the northern boundary of Kilauea's mobile south flank. It has been overlooked in many studies, in part because of its supposed lack of magmatic activity. The confirmed presence of magma within the Koa'e should help focus attention on this important element of Kilauea's structure.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.V33B2862S
- Keywords:
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- 8404 VOLCANOLOGY / Volcanoclastic deposits;
- 8414 VOLCANOLOGY / Eruption mechanisms and flow emplacement;
- 8434 VOLCANOLOGY / Magma migration and fragmentation;
- 8486 VOLCANOLOGY / Field relationships