White Sand Beaches

I got a question from Jane who, while reading Marker Bed, became intrigued by the reference to quartz sand on page 29 of the print versions. (By the way, if you have not yet read Marker Bed, click here to go to the Amazon website and order your copy.)

The quote she was concerned with came in the first GeoFantasy sidebar (Site Zero), when I was discussing how water — truly a magical substance — is capable of dissolving anything and everything it touches.

Different substances dissolve at different rates; put quartz sand in one glass of water and sugar into another and you’ll get the general idea.

Jane’s question extended this obvious physical truth into the real world: “So how come the sand on most beaches along the east coast is white, but the sand on the beaches along the west coast is gray?”

A simple question! Fortunately — and as usual — the answer is equally simple.

Thanks to work by N. L. Bowen (and others) in the early 1900s, there is a fundamental principal in geology — Bowen’s Reaction Series — that describes the order in which silicate minerals crystallize from a magma when it cools to make an igneous rock. (“Clever boy to have named it after himself,” said the troublemaker behind Strickler’s Laws of GeoFantasy. Click here for more information on Bowen’s from the “Ask A Geologist” section on my educational website.)

One of the things I always try to stress in class is that Bowen’s Reaction Series is incredibly important. Not just in describing the order of crystallization in igneous rocks, but in how chemical weathering attacks those rocks when they are exposed at the surface.

The short version is all too easy: just like you and me, minerals like to be comfy, and if they aren’t, they’ll wiggle and waggle until they are (this is the fundamental driver behind metamorphic rocks, but more on them later).

The more mafic minerals found toward the top (and hotter) end of Bowen’s are further from their comfort zone when exposed to the lower temperatures (and pressures) found at and near the surface. This makes them really unhappy, and therefore more unstable (as happens to all of us). The result is that they will chemically weather more quickly into clay and other secondary minerals which are happy in the lower temperature and pressure environment at the surface.

The more stable felsic minerals found on the lower, cooler end of Bowen’s includes quartz (which is at the very bottom). Therefore, one would expect that, after all the other silicate minerals have been weathered into clay, what you would have left when the sediment load of the rivers finally makes it to the beach would be quartz, and so we end up with white, quartz-rich sand.

But that doesn’t get us to an answer to the second part of Jane’s question: why is the sand on both coasts of the United States different? This boils down to something we’ve already talked about in earlier posts when discussing “active continental margins” as opposed to “passive continental margins.” (Please refer to “What’s the difference between an active and passive continental margin?” on my educational website, or the blog posts “Climate Change and Sea Level Rise” and “Earthquakes, Seismic Waves, and the New Madrid Fault” for more on continental margins.)

On America’s east coast — a passive continental margin — the land is low relief (nearly flat) on both sides of the beach. As such, the rivers are long, and the sediments they carry have enough time for most everything to chemically weather to clay… except for, guess what, the quartz (which, as we already know, is at the bottom of the Bowen’s Reaction Series, lowest in temperature, and closest to its comfort zone).

Sure, quartz will also eventually weather to surface-temperature secondary minerals, just not as quickly as everything above it on Bowen’s. So, as is worth noting a second time, by the time the sediments get to the beach, most everything else has altered to clay, and only the quartz remains.

With that being said, we should probably consider two more things which contribute to the white sand beaches of Florida and the Carolinas.

The first relates to the source rocks: there is simply more quartz-rich granitic crust being exposed to weathering east of the Rockies than there is to the west. (If you need a culprit, look again at the “Earthquakes, Seismic Waves, and the New Madrid Fault” blog post to get a short discussion of this variation in lithology on the opposite coastlines of North America.)

The second has to do with the shape of the earth on both sides of the beach. Not only is the land upstream from the beach relatively low relief with long river systems (consider the Mississippi), but so is the land on the other side of the beach that turns into the broad Continental shelves.

Because of this, and especially on the southern end of the coastline, the subsea topography (and temperature) is perfect for the creation of not just coral reefs, but all kinds of critters that use calcium carbonate from the seawater to build their shells and such. These carbonate remains also get beaten up, reduced to the size of sand particles, and, along with the quartz, contribute to the region’s glorious white sand beaches.

(B.T.W.: if you are interested in trying to determine if the sand is quartz or carbon-based shell fragments, it’s also really easy. The first test is simple: just look at the sand (preferably with a magnifying glass). If it looks like shell fragments — as in the image above — it’s probably carbonate. If you can’t see any shell fragments and need another way to test it, take some of the sand and stick it in a little glass of hydrochloric acid. Don’t have any acid? Try using vinegar. Either way, if it passes the fizz test and/or starts to dissolve, it’s likely calcium carbonate.)

Along the West Coast, which is an active margin, uplift due to subduction and other tectonic forces keeps the land and relief much greater, and the river systems much shorter. I mean, really, there are only two rivers that empty into the Pacific that make it past the Sierra and Cascade mountain ranges: the Columbia and Klamath (which barely makes it at all). Other than the Colorado (arguably a unique case), the rest of them stop far short of the distance needed to chemically weather the non-quartz minerals to clay.

Put it together with the more mafic source rocks, and when the sediments get to the beach, not only is there less quartz to start with, there are all these partially weathered feldspars, olivine, pyroxene, biotite, and other more mafic minerals contaminating the sand.

In the above image, the black streaks are magnetite crystals that are concentrated by their relatively higher density (check out an earlier post cleverly titled The Magic of Density). Magnetite is not a common silicate mineral, but the mafic and ultramafic source rocks in the Rogue River drainage basin upstream of Gold Beach have oodles and scads of it. Surrounding the magnetite in our image is a faint green halo of olivine, which is found at the very top of Bowen’s Reaction Series. Olivine also has a higher density, and is common in the Rogue’s source rocks.

The magnetite content of the beach sand can lead to a lot of fun! Next time you take your kids or grandkids to a southern Oregon beach, take along a magnet. You can separate buckets of this stuff in a very short amount of time out of the dry sand above the high tide mark.

Also bring along a Sharpie and a white paper plate. Add some eyes and nose and a mouth, pour on some of the magnetite, and then use a pencil magnet underneath the paper plate to pull the magnetite grains around to make hair, eyebrows, mustaches, and beards, just like in the Wooly Willy game. I guarantee your kids will love it (and get a serious science lesson about chemical weathering and the short river systems found along active continental margins).

You’ll be a hero!