PDS_VERSION_ID        = PDS3
RECORD_TYPE           = STREAM

OBJECT                = TEXT
  PUBLICATION_DATE    = 2009-11-15
  NOTE                = "Geologic setting description"
END_OBJECT            = TEXT
END

Keanakakoi Ash Member

The Keanakakoi Ash Member is the result of a hydrovolcanic eruption of Kilauea
volcano that began in 1790, C.E. The deposit now covers 50 square km in a
teardrop shape extending from the caldera and tapering to the southwest (along
the SW rift zone). Original estimates suggest it once extended over 200 square
km. It is comprised of tephra that blankets portions of the Kau Desert,
varying in depth from up to 10 m near Keanakakoi crater to discontinuous,
cm-scale reworked dunes and accumulations 10 km from the source.

The Keanakakoi Ash Formation is bracketed by nonhydromagmatic layers of
reticulite and pumice produced during high lava fountains. The formation
itself has been subdivided into several types of units (e.g., Decker and
Christiansen, 1984; Easton, 1987; McPhie et al., 1990); the units represented
in this database include units II and III of Decker and Christiansen (1984) or
units 1-10 of McPhie et al. (1990). These include well-bedded fall and surge
beds composed almost entirely of juvenile ash and lapilli, overlain by a
cross-bedded unit containing lithics and fine ash in equal amounts, along with
some localized accretionary lapilli.

The source of water for this hydrovolcanic eruption has been posited to be
either groundwater (e.g., McPhie et al., 1990) or a surface lake (e.g.,
Mastin, 1997). Thus, the probable sequence of formation for these units is as
follows:  Subsequent to high fountaining that deposited the basal reticulite
layer, either rapid lowering of the magma column or subsidence of the caldera
below the water table drove hydromagmatic eruptions. These fragmented the
magma, producing tephra fall and surge beds (Unit II or 1-4). Continued
subsidence of the caldera (or continued fall of the magma level) produced
lithic debris that blocked the vent and led to discrete explosions, base
surges and ejection of juvenile and lithic debris (Unit III or 5-10, and
layers above). The overlying reticulite layer represents a return to high
fountaining in the caldera, potentially due to a rise of the caldera floor
(Mastin, 1997).



References

Decker, R.W. and R.L. Christiansen (1984), Explosive eruptions of Kilauea
volcano, Hawaii, in Explosive Volcanism, ed. National Research Council, pp.
122-132, Natl. Acad., Washington, D.C.

Easton, M. (1987), Stratigraphy of Kilauea volcano, in Volcanism in Hawaii,
ed. R.W. Decker et al., U.S. Geol. Surv. Prof. Pap., 1350, 243-260.

Malin, M.C., D. Dzurisin and R.P. Sharp (1983), Stripping of Keanakakoi tephra
on Kilauea Volcano, Hawaii, Geol. Soc. Am. Bull., 94, 1148-1158,
doi:10.1130/0016-7606.

Mastin, L.G. (1997), Evidence for water influx from a caldera lake during the
explosive hydromagmatic eruption of 1790, Kilauea volcano, Hawaii, Jour.
Geophys. Res., 102, 20,093-20,109.

McPhie, J., G.P.L. Walker, and R.L. Christiansen (1990), Phreatomagmatic and
phreatic fall and surge deposits from explosions at Kilauea volcano, Hawaii,
1790 A.D.:  Keanakakoi Ash Member, Bull. Volcanol., 52, 334-354.

Wentworth, C.K. (1938), Ash Formations of the Island of Hawaii, 183 p., Hawaii
Volcano Res. Assoc., Honolulu, HI.