Are there black sand beaches on the island of Oahu?
In Hawaii the term black sand beach has been used
only for beaches dominated by grains of black volcanic glass.
Lava flows entering the ocean chill and their glassy rinds shatter.
Waves and currents may build a beach from the black grains of glass.
There is no black sand beach on Oahu. There aren't even many
light-gray beaches on Oahu.
Black sand is only present as distinct beaches on Hawaii (Big Island), at Punaluu (Kau district), at the west and east edges of the new lava delta from the current flank eruption (Puna), and in front of the recent flow that covered most of Kalapana and the famous former black-sand beach at Kaimu (Puna). Some additional Big Island beaches have a few per cent of glass (Hookena and Kealakekua (both South Kona); also small pockets at the shoreline and in storm beaches near Cape Kumukahi, South Point, and elsewhere along the Puna, Kau, South Kona, and North Kona coasts). Many others have a trace to perhaps 1% (Hilo, Waipio, etc.).
Some other beaches in the state have a per cent or so of glass (Hana, Kihei, and some lesser places, Maui; Kalaemilo (Kalaupapa) Molokai). Makaha and Sandy (Oahu) have a trace of glass (<1%).
All of the islands have some gray-sand beaches, which are mixtures of gray to brown sand-sized grains of weathered or fresh basalt lava and white to tan calcareous grains of foraminifers and broken mollusk shells, calcareous algae, sea urchins, coral, etc. When wet, these sands can be fairly dark and dull gray, but never like the glistening black sands. Hauula (Windward) is about as dark as it gets on Oahu. Elsewhere, gray basalt-sand beaches include Lumahai and Hanalei (n. Kauai), Halawa (ne Molokai), Halulu and Hauola (Lanai), Hana, Hamoa, Kihei, Olowalu, Waiehu, etc. (Maui), Pololu, Waipio, Hilo, and various tiny beaches (Hawaii).
[Beyond the question, but to get it all out: A different kind of black sand is on the stream-mouth barrier beach of Wailau Stream, north coast of Molokai. It is fine-grained magnetite-ilmenite from the caldera of East Molokai, sorted by the pounding of the winter surf. And some Hawaiian beaches have green grains of sand-sized olivine.].
Dr. Craig Glenn, Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu, HI 96822