The geology and petrology of Mauna Kea volcano, Hawaii : a study of postshield volcanism

Author:

Wolfe, Edward W., Wise, William S., Dalrymple, G. Brent, Geological Survey (U.S.),

Title:

The geology and petrology of Mauna Kea volcano, Hawaii : a study of postshield volcanism

Periodical:

U.S. Geological Survey professional paper ; 1557

Year:

1997

Pages:

viii, 129 p.

Subject:

Geology Mauna Kea Rocks igneous Mauna Kea

Summary:

The subaerially exposed lavas of Mauna Kea from a cap that conceals the underlying tholeiitic basalts of the volcano's shield stage. Geologic mapping shows that this postshield cap consists of older basaltic lavas overlain by younger hawaiitic lavas. Thus, the postshield eruptive stage consisted of basaltic and hawaiitic substages. Lavas of the basaltic substage compose the Hamakua Volcanics, which were erupted between approximately 250 and 70-65 ka, whereas those of the hawaiitic substage compose the Laupahoehoe Volcanics, which were erupted between approximately 65 and 4 ka. Hamakua lavas were erupted both from fissure vents near the summit and from point-source vents marked by cinder cones that are scattered over the flanks of Mauna Kea. Lithologic distinctions and an intercalated glacial deposit permit stratigraphic subdivision of the Hamakua Volcanics on the upper flanks of the volcano. Lower and upper volcanic units, herein named the Hopukani Springs and Lileo Spring Volcanic Members, are separated by the Pohakuloa Glacial Member. A younger glacial deposit, the Waihu Glacial Member, occurs as a lens enclosed, at least in part, within the Liloe Spring Volcanic Member. No consistent criteria exist for stratigraphic subdivision of the Hamakua lavas exposed on the lower slopes. Laupahoehoe lavas issued from numerous point-source vents, most of which are marked by scoria cones mainly on the upper flanks and summit of the volcano. Air-fall ash and lapilli from the Laupahoehoe eruptions mantle much of the upper slopes, forming scattered tephra blankets and ash dunes. Reworked ash is the dominant component of local slope deposits. A final glaciation of Mauna Kea's summit occurred between approximately 40 and 13 ka. Till and outwash deposited at that time compose the Makanaka Glacial Member of the Laupahoehoe Volcanics. Laupahoehoe eruptions occurred before, during, and after the Makanaka glacial episode. The Hamakua Volcanics consist of alkali basalt, transitional basalt, and, among the oldest lavas, rare tholeiitic basalt. Evolved basalts are common, apparently derived by crystal fractionation of olivine-controlled parental magma in reservoirs within the volcano, mostly at shallow levels. Petrologic considerations suggest that these parental magmas originated from partial melting of peridotite at approximately 30-kbar pressure and that the degree of silica saturation reflects the degree of partial melting; Alkali basalt originates from a lesser degree of partial melting, and tholeiitic basalt from a greater degree. Alkali basalt is common on the lower flanks of the volcano but rare on the upper flanks, a difference suggesting that the degree of partial melting decreased outward from beneath the center of the volcano during Hamakua time. A distinct compositional gap separates the Hamakua and Laupahoehoe lavas. The Laupahoehoe lavas consist mostly of felsic hawaiite to mugearite but include some benmoreite. They are aphyric to somewhat plagioclase phyric and, unlike the underlying basaltic lavas, commonly contain xenoliths of gabbro, peridotite, or pyroxenite. It was interpreted that these hawaiitic magmas as derived from fractionation of basaltic magma in reservoirs below the base of the crust. Little or no additional fractionation occurred during ascent. The estimated total volume of Mauna Kea is more than 30,000 km(super 3). Shield-stage lavas represent the bulk of the volcano; the volume of postshield lavas is less than 1,000 km(super3). The transition from the shield to the postshield stage occurred before approximately 250 to 200 ka, and we estimate that Mauna Kea volcanism began about 1 Ma. Decreasing magma supply provides a contest for the magmatic and volcanic evolution of the volcano. The decreasing supply was accompanied by changing lava composition, from tholeiitic basalt that dominated the shield stage, through transitional and alkali basalt that dominated the postshield basaltic substage, to hawaiitic lava of the postshield hawaiitic substage.

URL:

http://pubs.usgs.gov/pp/1557/report.pdf

Collection:

Monographs