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Geology Talk, Geology













The sand on Rainbow Beach, on Queensland’s Sunshine Coast, features a myriad of colours. Image courtesy of S Newrick, Wikimedia Commons.
The sand on Rainbow Beach, on Queensland’s Sunshine Coast, features a myriad of colours. Image courtesy of S Newrick, Wikimedia Commons.

Poop sand happens!

As winter sets in down under, Bill Langer and his wife are visiting east coast beaches in the US, where (to quote Lewis Carroll) there are “such quantities of sand, indeed” – and also lots of poop!

This northern summer my wife Pam and I are going to be visiting some sandy beaches on islands in New England.

Beach sand originates from many sources and environments. In New England, most of the sand comes from rocks that were eroded into small pieces by the glaciers during the Ice Age. The different colours of the sand come from the different types of rocks that the glaciers eroded.

Outside of glaciated areas, many beaches are made of sand that forms over millions of years as bedrock erodes through slow but persistent weathering. Once freed from the bedrock, the small rock fragments travel down rivers and streams. Softer minerals decompose, while hard minerals such as quartz and feldspars remain behind.

Once they make it to the ocean, the more resistant mineral fragments further erode from the constant action of waves and tides. Eventually, the resulting sand grains are thrown up on the beach by the waves. Because quartz and feldspar are the most resistant minerals, beaches commonly are white (from the quartz) or tan (from the feldspar).

In contrast, there are some really beautiful black sand beaches in places with nearby volcanoes such as in Alaska, the Canary Islands, Hawaii, Iceland, Martinique, New Zealand, Puerto Rico and Tahiti. Hawaii also has green beaches made of a green volcanic mineral called olivine. Moreover, for a multitude of coloured sands, you should try Rainbow Beach on Queensland’s Sunshine Coast.

Instead of forming from grains eroded from rocks, oolitic sand (made from oolites) is unusual sediment that “grows” in shallow, very saline waters. An oolite resembles a tiny onion consisting of concentric layers of calcium carbonate that precipitated around a nucleus, or central core.

Figure 1. ‘Star sand’ forams have star-shaped shells.
Figure 1. ‘Star sand’ forams have star-shaped shells.
Figure 2. Some foram tests have a reddish-pink colour when they collect on the beach.
Figure 2. Some foram tests have a reddish-pink colour when they collect on the beach.

The nucleus is usually a mineral fragment, or could even be a tiny shrimp faecal pellet. Oolites form in warm tropical waters surrounding the Bahamas, the eastern Mediterranean Sea, the Persian Gulf and even in the Great Salt Lake of Utah.

Some sands are biological in origin, as they are formed from single-celled organisms called foraminifera (forams for short).

Forams have a shell or “test”, usually made of calcium carbonate, that mostly encloses their cytoplasmic body. When the critters die, the shells fall to the sea floor and eventually wash up onto the beaches.

In some places, such as Hoshizuna-no- Hama in Japan, the forams have star-shaped foram tests (Figure 1), hence the name “Star Sand Beach”.

In some other areas, such as Bermuda, the foram tests have a red tinge to them, so when they collect on the beach they give it a reddish-pink colour (Figure 2).

To top things off, some sand is made of poop – fish poop, to be a little more technical. A major poop sand specialist is the parrotfish, those blue and yellow fish that populate tropical coral reefs.

Parrotfish eat the hard calcium carbonate skeleton of the coral, the soft-bodied coral organisms (called polyps) that cover the skeleton, the zooxanthellae (algae) that live inside them, and the bacteria living inside the coral skeleton. The soft organisms, algae and bacteria are absorbed, but the chewed-up dead coral shoots straight through the parrotfish pretty much unchanged. It comes out as sand-sized pieces of coral – and a lot of it. One large parrotfish can produce upwards of 453kg (or 1000 pounds) of poop sand a year!

However, don’t worry, you don’t necessarily need to abandon your coral reef island. Poop sand is not unsanitary or dangerous in any way. As they say: “Poop sand happens!”











ABOUT THE AUTHOR
Bill Langer
Independent Research Geologist

Bill Langer is a freelance writer and retired Senior Research Geologist for the U.S. Geological Survey. Bill is now an independent researcher specialising in aggregate resources. Click here to email Bill or visit his website.
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Monday, 26 August, 2019 1:00pm
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