Life of a Sand Grain
THE LIFE OF A SAND GRAIN by Carl Bowser (Sept. 2018)
They surround you almost anywhere you are in Arizona. They cling to your shoes, they end up in pockets and pant cuffs, they provide a little crunch to that clam chowder you made, they color the water of streams tumbling through mountain canyons, and they wash back and forth in the waves on the shore of an ocean or lake. They are found most anywhere, and are very common. Yes, it’s the common sand grain. Scientifically defined as mineral grains that range in size from 4.8 mm (very coarse) to 0.4mm (very fine grained), sand grains not only vary greatly in size and shape, but they also vary greatly in their mineral composition. But the queen of sand grains is made up of common quartz (SiO2). If each, single grain of sand could talk, oh what a story it could tell!
Over the years, geologists have learned to read some quartz grain’s stories, but they are really stories of aggregates of grains, not individuals. Some general rules guide the shape, size, and variety of quartz grains (and other less common ones that we will talk about later). Typically, coarser sands are more angular in shape and tend to have more different neighbors (different minerals such as feldspar or iron/magnesium minerals like amphibole). As the sand makes it way to the sea, be it from a glacier, a sand storm, or more likely by river, it gets more rounded by abrasion against other grains and in the process, gets smaller and smaller. The wind and rivers are excellent sorting media and as each grain works its way to the sea, they not only become smaller and more rounded, but grains of similar size tend to sort together. Just like the grains of sand settling in a glass of water, the coarser (and heavier) grains sink faster than the smaller, lighter ones. (Try it yourself, using some sandy soil from your yard).
Thus, heterogeneity becomes a measure of the age of accumulated sand grains. Young accumulations of sand are coarser, more angular, and less well sorted, and as the grains “age” they get smaller, more rounded, and much better sorted. We describe the age of sand accumulations by their maturity, that is, how long the grains have been subject to the processes of grain erosion, transport distance, and current (or wind) sorting. The next time you are near a river pick up a handful of sand and examine it carefully. If you have one, use a hand lens or jeweler’s loupe to look closer at the grains and notice their size, rounding, and how many different kinds there are. Put some in a small plastic bag to save for later. The next time you are at the beach, get a sample from the shoreline, and another sample from the wind-blown dunes that lie higher and inland from the beach. Compare all three samples and pay special attention to the differences within and among the samples. Heck, why stop there? Do as I have, and book travel around the world to collect sand samples from the sand dunes of Namibia, Egypt, Australia, and the western U.S. and all the exotic, vacation worthy beaches of the world. I guarantee that no two samples will be the same, be they river, beach, or dune sands.
So where do these quartz grains come from? The answer comes from the very beginning of our planet’s history well over four billion years ago. Water on the early planet gathered at its surface to form its first oceans. These oceans presumably covered a large portion of the planet. Deeper, below the planet’s solidified crust lies molten material that would later solidify to become rocks as they cooled nearer the earth’s surface. These early rocks crystallized to form rocks low in silica content and higher in iron, magnesium, and aluminum, but gradually, through continued melting, re-solidifying and remelting, quartz would begin to appear in some of these rocks as products of igneous differentiation. As they evolved to form more silica rich, lower-density rocks, these, quartz-bearing rocks, then formed higher standing (floating) masses that then, ultimately emerged above the ocean’s surface to form dry land (islands, and later, continents). From these less dense highlands the first grains of quartz appeared, but still locked within the rocks. Upon exposure to crashing waves, rain, and ever present tectonic movements these rocks were broken down into their constituent minerals, and, thus, the first, sedimentary quartz grains were born. Along with their birth, the quartz fragments were joined by other rock grains composed of dark minerals (principally pyroxenes and amphibole), K-feldspar, and plagioclase were also freed, and these, main “characters” began the long and storied histories in their race to the sea to form the first (perhaps of many) sand accumulations (sedimentary rocks).
But these other minerals have a disadvantage compared to quartz, and it would have consequences. Amphiboles, plagioclase and K-feldspar grains are much more chemically active and suffer from a property that quartz doesn’t, they have easy parting zones (cleavage). Thus, as they travel the path to the sea they are not only rounded and diminished in size like quartz, but they break into smaller sized particles when they split and cleave. Poor amphibole degrades so rapidly that it’s of little consequence to all but the most immature of sand deposits. Of the two feldspars, plagioclase is the most vulnerable, and quickly diminishes in size and abundance or is weathered into other minerals. Consequently, the quartz to plagioclase ratio of sediments increase as the sands mature (age). Eventually, the K-feldspar succumbs to these processes as well, so the more evolved sediment is characterized by higher quartz to plagioclase AND K-feldspar ratios. In the world of sedimentary rocks, you might consider quartz to be the teflon of the minerals, at least relative to its other mineral companions. Today we find these mature sands (sand dunes and beach sands) mostly as quartz rich, well rounded, and better sorted. Of course, there are exceptions, but that is another story I’ll have to save for a later time.
On our dynamic earth these unconsolidated sands ultimately become hardened with burial and increased temperature and turn into sandstones, or even their metamorphic equivalent, quartzite, and so began the long, slow process of burial, uplift and re-exposure to the elements of weathering and erosion as these rocks follow the rock cycle and, again, appear at the earth’s surface. Sadly, the quartz grain, comfortably embedded with its neighboring sand grains in what it thought was its final resting place, again finds itself freed, and involved in the process of moving, again, down a stream or carried by the wind. Thus, is born a multi-cycled grain, rounded and sorted, and with even fewer other contaminant minerals, a nearly pure quartz sand. From here the story gets muddled as it is currently impossible to count the number of times a given quartz grain has made this trip. My former colleague, Bob Dott, (memorialized in last month’s blog) once addressed the problem, but, at the time, the available tools were crude, and definitive conclusions were hard to make. On rare occasions, a quartz grain remained welded to it former companion from an earlier cycle, and we might be able to conclude that it is a two-cycle grain, but recognizing cycles beyond two remains a challenge. Single, or wedded, these grains don’t reveal their histories easily, but if only they could, what stories each could tell!
Fortunately, there may be tools on the horizon to help answer the question of grain “cyclicity”. Another of these Teflon-like (resistant) minerals, zircon (ZrO2), is also highly resistant to weathering, perhaps even more than quartz, but much lower in abundance. It’s presence in sediments is important, but it requires more exacting techniques to separate them for analysis. Internally these zircons show rings, onion-skin like, and reveal the growth history of each grain. Even better these grains carry trace amounts of uranium and lead isotopes that enable us to determine its geologic age. By implication each zircon reveals its source age and history, and, thus, the ages of the rocks eroding to form these sedimentary rocks, river sands, etc. Sediments typically contains many zircon grains of different ages, and a plot of their abundance looks like a histogram with many peaks, each with different heights (abundance) and age. Careful geologic mapping, zircon dating and rock examination can tell us more about the life of these sand grains. Pioneering work on the ages of zircons in sediments is being done here at the University of Arizona Geosciences, in the lab of Dr. George Gehrels and his colleagues.
In the meantime, individual quartz grains continue their trips to the sea and back, taking their own, sweet time, some faster, some slower, and sadly, each grain is unable to remember its specific paths to the sea (and back). These nearly indestructible grains grow older and older, keeping their secrets until the next advance in science helps crack their narrative. “All right, you guys! Which one of you is the oldest? Which of you has made this trip before?. [Silence].
Hopefully you remembered about the sand grains I asked you to collect earlier. If you did, take them out and look at them again, this time even more carefully. They may have a much more interesting story to tell than you ever imagined. Earth’s clocks, but without hands.
Figure 1: Colorado River sand near Lees Ferry. Note the mix of different angularities, including some very well rounded grains (probably from eroded sandstones that have a wind-blown source, and likely from nearby sands like the Navajo sandstone.
Figure 2. Nanny Goat Beach, Sapelo Island, Georgia. Very fine grained, nearly pure quartz sands transported along the beaches from Connecticut to Georgia. Despite their long transport distance the grains are still highly angular, but also free of feldspars. Mineralogically more mature, but texturally still very immature.
Figure 3. St. Peter sandstone (Ordovician age), Dane County, Wisconsin. A very well sorted sand both texturally and mineralogically. My best candidate for a “polycyclic” sandstone.
Figure 4. Wisconsin River near its confluence with the Mississippi River. A heterogeneous mix of mature and immature sands both mineralogically and texturally. The very well rounded grains are unmistakably derived from the Ordovician, St. Peter sandstone, along with a mix of less mature grains derived from Pleistocene glacial material, derived miles upstream.