Kauai's Black Sand Beach Field Trip Report

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  • Topic: Sand, Brown, Beach
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  • Published : May 17, 2013
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Kauai's Black Sand Beach Field Trip Report

Waimea, Kauai

Kaua'i is the oldest of the Hawaiian Islands and has some of the most beautiful white sand beaches. However, there are a couple of spots on the island that are black sand beaches. The largest one is the beach at Waimea on the West Side of the island.

Waimea beach is a long stretch of beach. At one end lies the mouth of the Waimea River and the far end goes out past the pier and towards Kekaha. It had been a few years since I had been to that beach and walked along the shore, and I have to admit that although it is not one of my favorite spots I found my trips there to be relaxing. There were also many things that I noticed about the beach, which I had never paid attention to before.

For one, there is a large stone by the entrance with a metal plaque informing visitors that the site is of historical importance because Captain Cook had landed there.

One of the first things I noticed when I stepped onto the beach is that the sand shimmers and sparkles. It is more of a dark brown color and the sand is very fine and gritty. The waves crash right on the shore and the beach slopes upward. It is not a very wide beach, about fifty feet wide at the widest point and the beach slopes upward towards the backyards of the homes along the beach. Many of the trees that are growing on the boundaries of the beach homes have their roots exposed where the sand has eroded from the tides. The color of the ocean is a brownish mixture probably from the sediments that wash down from the river and the dark sand of the coastline. About five hundred yards out beyond the pier the water changes to green, gray, and blue color. The water is probably pretty deep and there does not appear to be any reef outside. That is probably the reason the waves only break at shore. As I walked along the beach I saw many things like kukui nuts in all the various processes of aging. They varied from a light brown color all the way to a dark black; many of the black shells were cracked open. There was a lot of driftwood, but no seaweed along the beach. Seedpods also litter the beach and there are numerous crab holes scattered along the coast.

Along the shore break there are small to medium sized stones and pebbles. Many are worn smooth from the constant tumbling of the waves and sand. Some of the pebbles are dark black, others are dark gray, and a few are reddish-brown in color. Many are smooth and not very porous, but there are also ones that are full of surface holes. In one of those rocks I could see little bits of greenish-yellow glasslike minerals. There were also tiny glittering pieces that sparkled in the sunlight. With the help of my magnifying glass, I was able to view the larger minerals more clearly. They may have been bits of Olivine or Apatite.

Other details I noticed about the sand was its dark brownish-black color, its fine, silty texture, and the glittering characteristics, but other minerals present too.

There was some biogenic sand, which may be bits of broken shells since they were reddish-brown in color. There was also lots of magnetite which was very small and black and could only be seen clearly with the magnifying glass. A magnet that I brought along with me was able to attract large amounts of the fine sand to it.

There were also bits of glasslike minerals, some were clear, or tinged with hues of yellow and green. Basalt fragments were present and even with the magnifying glass looked like bits of pepper. Some of the minerals may be Pyroclastics, but the pieces are so tiny that I could not see if they were vesicular.

Most of the sand, about 55% appeared to be Olivine, Apatite, and Pyroclastic in composition. Another 30% of the sand composition appeared to be Basalt fragments. The final 15% was a mixture of Biogenic and Magnetite minerals.

The sand is very fine and silt-like in composition. Grains are very fine and visible at arms length, but not...
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