PETROLOGY, GEOCHEMISTRY, FLOW DYNAMICS, GIS MAPPING AND HAZARDS OF THE HAPAIMAMO FLOW, MAUNA LOA VOLCANO, HAWAI’I

On the island of Hawaii lava flows are a major volcanic hazard. By studying the eruptive history future hazards can be estimated and risks reduced. In this study, the Hapaimamo flow, one of the largest flows on the southwest riftzone (SWRZ) of Mauna Loa is investigated. This young picritic flow is dated at 240 years and covers an area of approximately 100 km². The flow originates low on the SWRZ from spatter ramparts at 1,880 m elevation. This ′older′ picritic flow field consists of tube-fed shelly pahoehoe and is ′cut off′ by the ′young′ Hapaimamo flow at 1,640 m elevation. Field relationships and paleomagnetic data suggest that both erupted at the same time. A complex cone structure, large channels and an extensive reticulite field mark the ′young′ flow with two major flow fields extending 20 km to the sea.

Flow velocities based on channel and tube dimensions and estimated viscosities indicate rapid emplacement and high effusion rates. At the end of the southern flow field are a series of littoral cones, another indicator of high effusion rates. Such high effusion rates would pose a significant risk to the occupants of a nearby subdivision with 26,000 homes at maximum build out the largest in the state of Hawaii. In addition the circum-island highway would be severed disrupting commerce.

The Hapaimamo flow is unusual in that it is picritic, and also contains abundant and diverse xenoliths and plagioclase phenocrysts. The xenolith population is comparable to those found in two other picritic flows on the SWRZ in an earlier study by Gaffney (2002), and provides insight into the magmatic plumbing system. Lockwood and Trusdell (pers. Communs., 2004) recognize from geologic and petrographic evidence that this segment of the rift zone is a place of secondary magma storage.

Subaerial picritic flows are rare on Mauna Loa (14%) and on the island of Hawaii in general. The majority of Hawaiian lavas contain only 7−8% MgO and are thought to erupt from a low−level magma reservoir. The questions resulting from this unusual flow are: (1) Is it a primary magma that bypassed the shallow reservoir? (2) Did it erupt from a compositionally stratified reservoir? (3) Was a shallow reservoir absent due to high eruption and magma supply rates? (4) Was this an unusually vigorous eruption, involving high eruption rates and flow velocities?