It is everywhere. On the streets, your feet and in your sheets. In your car and your carpet. Sand. One of the most useful resources on Earth and the foundation of our waveriding dreams. Concrete, glass and microprocessors, sandbars and beaches: sand is ever-present in our lives. Where does it come from? Why is it different across the globe? How does it impact the formation of waves?
In geologic terms, “sand” refers to a certain size of particle in sediment: smaller than gravel and larger than silt. A sand particle measures between 1/16mm and 2mm in diameter. The main mineral components of beach sand include: quartz, feldspar and hornblende. All three are silicate minerals. These minerals, with their different combinations of silicon and oxygen, make up 90% of the Earth’s crust.
Quartz is the most abundant beach sand mineral because it is resistant to chemical weathering and thus able to withstand the tumultuous journey from mountain to beach. Rock forms in a variety of ways inside and out of Earth. Sediments layer to form sedimentary rock. Liquid magma cools inside and lava hardens outside Earth to form igneous rock. Forced upward by plate tectonics, rocks are exposed to wind and rain, freeze and thaw, plant roots and battering waves. Weathering slowly works boulders to cobbles to gravel to sand to silt and then to clay.
Erosion moves the sediments and deposition drops them off in a new location. In California, boulders cleave off granite mountains to the east. Rivers carry chunks of the mountain downstream, continuing to break it down along the way. Rivermouths flow into the Ocean, depositing cobbles and sand on the beach. California beach sand is a mixture of these inland minerals and the erosion of the marine terrace (sea cliff). Wave action along the cliff bottom weathers large masses of the sedimentary rocks. The rocks fall to the beach and are gradually worked into sand. California beach sand also contains a living ecosystem and various other detritus.
Because there are many different rocks and minerals, there is a great variety of sand across the world’s beaches. Differing landforms and ecosystems create different sand. Hawaiian beach sand is a mixture of eroded volcanic rock, shell fragments and parrotfish waste. They eat coral that passes through their digestive tract and is then deposited as sand. Sand partially composed of the remains of living organisms is called biogenic sand. Many of the world’s tropical beaches contain fragments of coral and shell.
All Ocean waves break over some amount of sand. Point breaks often have rivermouths nearby to nourish the sandbars that wrap around their shores. Reef breaks become covered by sand during a long flat spell. The next swell removes sand from the reef to expose the proper wave-making bathymetry. Beaches change shape frequently as sand is moved by swells, storms and longshore currents.
Beachbreaks are synonymous with shifting sand. The formation of sandbars is a complex process. A bedform is created when a fluid flows over a moveable surface; in this case water waves over the sandy seafloor. It is the same process as wind blowing over a sand dune. Ripples form in the surface, sand falls out of suspension in the water and continues to build up the sandbar. Incoming swell shoals on the sandbar and a breaker forms. The water flowing back out to sea becomes a rip current that reshapes the sandbar. When conditions align, perfectly shaped waves can result.
The shape of the beach and the type of the waves that break there are often dependent on the size of the sand grains. Wide flat beaches are formed from smaller grained sand than steeper beaches. North Carolina coastal geologist, Gregory Rudolph, puts it this way, “If you fill a bucket full of mud and pour; it will essentially ooze out and your pile of mud will look like a pancake. Fill that same bucket with gravel and you’ll have a pile that is almost as high as the bucket itself.” Much to bodysurfer’s delight, steep beaches often create hollow, womping shorebreak waves. Swell energy comes out of deeper water to load up and plunge powerfully on the coarse sand. There is something special about washing up and back down a steep beach fully covered in soft sand.
Vital to beach communities and even controversial in some, sand is pervasive in our lives. Embrace the sand in your ears. Cherish an afternoon spent basking on the beach. Appreciate the geology that forms our coastlines and shapes our waves. Eventually, every grain of sand on our beaches will return to the internal furnace of the Earth to become magma and then mountains again. What waves will break on that next generation of sandbars?