| Center: Molten lava being shredded by littoral explosions upon entry into the ocean during the1969-71 Mauna Ulu eruption. (Photogr aph by Donald W. Peterson.) Right above: Pu'u o Mahana, a prehistoric littoral cone of Mauna Loa, is the site of the Big Island's green-sand beach. (Photograph by J.D. Griggs.) Right below: Close-up of the green sand, which obtains its color from wave- concentrated grains of the green mineral olivine. (Photograph by Robert I. Tilling.) |
The pyroclastic deposits exposed at Kilauea indicate that about two dozen major
explosive eruptions have occurred during the past 70,000 years. Mauna Loa
apparently has had less frequent explosive eruptions during the same time
interval. Judging by their distribution and thickness, Kilauea's prehistoric
pyroclastic deposits had to be produced by explosive eruptions at least as
powerful as the 1790 eruption and, in some cases, several times stronger.
A special type of explosive activity, called a littoral explosion, occasionally results when lava flows enter the ocean. Seawater comes into contact with the hot inner part of the lava flow and flashes into steam, triggering an explosive spray of fragments derived from both the solidified outer part of the lava flow as well as its still-molten core.
Because of their seashore locations, most small
deposits from littoral explosions are quickly removed by erosive action of the
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ocean surf.
Larger deposits, however, are more permanent and form littoral cones,
such as the 100-foot-high Pu'u o Mahana, near the south tip of the Big Island,
formed during a prehistoric Mauna Loa eruption.
Pu'u o Mahana is the site of
Hawaii's famous "green sand beach," composed of the shiny green mineral olivine
(a magnesium-iron-silicate) eroded from the littoral cone and concentrated by
wave action. The conspicuous occurrence of olivine, sometimes also called
"Hawaiian diamond," in the pyroclastic deposits that form the prominent cone that
is Honolulu's landmark reportedly prompted early visitors to name it "Diamond
Head." Peridot, a gem-quality variety of olivine, is the birthstone for the month
of August.
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