The Sands of Hanalei

A mother and her daughter enjoy a rainy day at the beach in Hanalei Bay last December.

We think of gorgeous Hanalei as a stable, vast half-moon bay, a two-mile crescent white sand beach and a wide plain of fishponds, river, town and taro.

But Hanalei Bay has had a turbulent geological history, and the sand helps tell the story.

There have been times when the sea levels were low and much of the bay was dry land, and other times when sea level was higher and the taro field areas were reef flats. Much of the swampy interior of Hanalei Bay was most recently reef flat when sea levels were higher 1500 to 4000 years ago.

The battle between sea and land is reflected in the sand.

“The sands here are mixed with terrigenous sediments from the Hanalei, Wai‘oli, and Waipa rivers that enter at three locations in the backshore of the bay,” says a report on the island’s coastal geology on the website of the University of Hawai`i’s School of Ocean and Earth Science and Technology.

With regard to the sand, this means that it’s made up of both marine and land-based material. Coral and shell and pieces of what’s called calcareous algae make up the marine portion; and ground up lava rocks the land portion. Terrigenous is a geologist’s term for marine sediments that come from the land.

Sand is remarkable stuff. Different on every beach, it reflects what flows in from the land, and what washes in from the sea.

If you take a handful of sand from many Island beaches, you can see the mixture of materials: bits of rock and of reef-building algae, crystals, ground coral, pieces of shell, single-celled organisms called foraminifera.

Greenish sands can be green in Hawai`i from olivine crystals ground from rocks. In other parts of the world, sands can be green from single-celled Euglena organisms.

Black sands are generally dominated by ground-up lava, or in some Big Island cases, bits of shiny polished glass formed when hot lava flash cools as it hits the ocean.

White sands elsewhere in the world might be primarily from silica, but in the Islands, they tend to be calcium-carbonate based. And that means they originate as living things like chunks of the reefs and the aforementioned foraminifera.

Jan TenBruggencate

These are odd single-celled creatures that form a shell. Some live on the ocean floor, and some live in the water column. They are common enough that their shells are found on almost all Hawaiian beaches.

Foraminifera or “forams” make up just about 5 percent of Hanalei’s sand, says Kaua‘i’s premier coastal geologist, Chuck Blay. Terrigenous material — ground up rock — makes up another 5 percent or so.

The majority of the sand on Hanalei’s beach — and indeed most of all the island’s white sand beaches — is made up of calcareous or coralline algae. We tend to think of reefs as being made of coral, but most Hawaiian reefs are dominated by these hard algae.

Aside from the forams, the beach represents the shattered fragments of both land and sea.

• Jan TenBruggencate is a Kaua‘i based writer and communications consultant.