The location of vents for lava flows erupted by Mauna Loa in the past 10,000 years may vary between the summit caldera and rift zones in a systematic way (Lockwood, 1995). Below are excerpts from Lockwood's paper, Mauna Loa Eruptive History - The Preliminary Radiocarbon Record, in which he proposes a cyclic model to explain the long-term eruptive behavior of Mauna Loa of the past several thousand years. New geologic research and radiocarbon ages for additional lava flows may result in other models or a refinement of Lockwood's cyclic model.
|Eruptive fissure high on the northeast rift zone of Mauna Loa about 10 hours after the eruption started in Moku`aweoweo Caldera (top left). Although historical eruptions of Mauna Loa have occurred from both the summit and the volcano's two rift zones, geologic mapping shows that the focus of activity alternates between the rift zones and summit area over a time span of about 2,000 years.|
Radiocarbon dating of Mauna Loa lava flows has provided one of the most refined eruptive chronologies of any volcano on Earth. An analysis of 170 well-dated prehistoric lava flows reveals variations in eruptive frequency not explicable by random sampling but instead reflecting systematic changes in time and place of eruptive activity. Pronounced decreases in eruptive activity on Mauna Loa's rift zones correlate with periods of increased lava-lake activity and shield-building at the volcano's summit. Voluminous eruptions of olivine-rich lavas low on the rift zones may have heralded the cessation of summit overflows and the resumption of increased rift zone activity. These picrites (i.e. olivine-rich lava) were possibly erupted at a time when picritic melts had risen higher than normal into shallow supply conduits [Rhodes, 1995], and their eruption low on Mauna Loa's flanks may have indicated summit caldera collapse.
Radiocarbon dating, combined with detailed geologic mapping in progress, suggests the following cyclic model for this summit-flank alternation of eruptive activity, cycles that may have been repeated numerous times in Mauna Loa's past.
A cycle begins during a period of high-standing magma at Mauna Loa's summit. This is reflected by near-continuous lava lake activity and shield-building, from which vast sheets of pahoehoe blanket the northwest and southeast flanks. The last such period of summit activity lasted for about 800 years between 1,200 and 2,000 radiocarbon years ago and was accompanied by frequent small eruptions from radial fissures on the northwest flank. This period of summit overflows may end when a large flank eruption occurs low on the flanks and robs Mauna Loa of the high-standing magma required for shield building.
The sudden lowering of the magma column associated with caldera collapse results in a major shift of surface activity as both rift zones become characterized by frequent eruptive activity (an estimated average of one rift zone eruption each 20-25 years over the past 1,000 years). Summit activity probably continues during times of increased rift activity (as it has during the 19th-20th centuries - Barnard, 1995), but all erupted lavas are trapped within the summit caldera, and no trace of this activity remains at the surface.
As stresses increase across the rift zones over time, magma again rises more easily within the edifice, and the caldera is filled. A lava lake appears at Mauna Loa's summit, pahoehoe sheets are able to overflow the caldera walls, and the cycle begins again.
Possible Future Implications
The record of repetitive cycles of rift zone activity suggests that these cycles each lasted about 2,000 years. The earliest written descriptions of Moku`aweoweo Caldera [Bernard - 1995] showed that the caldera was more than 300 m deep in 1794 but has been steadily infilling ever since. The caldera is as full as possible at this time, as caldera-floor lava flows now spill out to the southwest. Since the last period of summit overflows began about 2,000 years, Mauna Loa may be on the verge of shifting to a period of long-lived lava lake activity, shield-building, increased summit overflows, and diminished rift zone eruptions.
|Moku`aweoweo Caldera atop Mauna Loa Volcano. View is looking north-northeast across the caldera toward the northeast rift zone. Gas is escaping from the 1975 eruptive vents on the caldera floor. Mauna Kea in background.|
The sharp increase in historical rift zone activity, noted by Lipman  and conspicuous for flows younger than 500 radiocarbon years, may be a "finale" related to an impending shift in eruptive style. Such a transition will obviously impact the nature of volcanic risk on the Island of Hawai`i, as the incidence of lava flow activity increases northwest and southeast of the summit (subdivisions MKN and MKS) and decreases in the areas downslope from the rift zones (subdivisions NER and SWR).
Lipman  has shown convincing evidence to suggest that, in a long-term context, Mauna Loa may be nearing the end of its tholeiitic shield-building phase. This does not contradict the suggestion that another period of summit activity is nearing but might suggest that future shield-building episodes will be less vigorous than those of the past.References
Barnard, W. M., 1995, Mauna Loa Volcano: historical eruptions, exploration, and observations (1790-1910), in Rhodes, J.M., and Lockwood, J. P. (eds.), Mauna Loa revealed: structure, composition, history, and hazards: Washington D.C., American Geophysical Union Monograph 92, p. 1-19.
Lipman, P. W., 1980, Rates of volcanic activity along the southwest rift zone of Mauna Loa Volcano, in Garcia, M. O., and Decker, R. W. (eds.), G. A. Macdonald Special Memorial Issue, Bulletin of Volcanologique, v. 43, no. 4, p. 703-725.
Lockwood, J. P., 1995, Mauna Loa eruptive history - the preliminary radiocarbon record, Hawai`i, in Rhodes, J.M., and Lockwood, J. P. (eds.), Mauna Loa revealed: structure, composition, history, and hazards: Washington D.C., American Geophysical Union Monograph 92, p. 81-94.
Rhodes, J. M., 1995, The 1852 and 1868 Mauna Loa picrite eruptions: clues to parental magma compositions and the parental magma, in Rhodes, J.M., and Lockwood, J. P. (eds.), Mauna Loa revealed: structure, composition, history, and hazards: Washington D.C., American Geophysical Union Monograph 92, p. 81-94.