How was the green sand beach on the Big Island, Hawaii formed? Do you know if
there is another green sand beach? do all volcanoes form olivine crystals?
Regina DeSiato
Dear Regina,
You are asking about one of my favorite places on Earth. The beach
formed by the erosion and concentration of olivine crystals derived from
the surrounding cone. The volcanic cone is Puu Mahana and it is
a tuff ring (a type of volcano formed by the interaction of magma and
shallow groundwater). The cone is associated with the Southwest Rift
Zone of Mauna Loa (although the cone is not right on the rift). Mauna
Loa flows can contain abundant olivine crystals. As ocean waves crashed
against the coast they wore away at the cone and made a small bay along
the coast. The waves also removed the lighter grains of sand (made of
volcanic ash) leaving the denser olivine crystals behind to form the
beach. I do not know of another green sand beach. Olivine is common in
basalt lava. A few crystals might be found in some andesite lava. It is
never found in dacite and rhyolite lava.
Steve Mattox, University of North Dakota
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