Pololū Valley
This summer I had the privilege of going to Hawaii. While still vast with beaches, they could not be more different from the beaches we have here in Florida. The cliffside edges, the color of the sand, and, as always, the material makeup of the sand. The typical flat, minutes long walk we take to the beach could not be more opposite that the 2000 meter hike we had to take down from the top of the cliff to reach the beach. The beach we visited in Hawaii is called Pololū Valley on the Big Island. Pololū is the northernmost of a series of erosional valleys forming the east coast of Kohala Mountain. While not every beach in Hawaii is a black sand beach, Pololū Valley met all of our expectations: the dichotomy of the black sand meeting the crashing waves, the scorching sun hitting the darker colored sand beneath our feet, and the towering cliff sides covered in greenery complementing the surrounding rocks. Black sand beaches, like this one in Hawaii, are primarily composed of basalt, a dense volcanic rock. When molten lava meets the cool ocean, it quenches rapidly, shattering into fine, glassy fragments. Over time, wave action breaks down these volcanic shards into sand-sized particles. This is an example of mechanical weathering and thermal shock, where rapid cooling induces fracture and brittleness in otherwise tough volcanic glass.
White Sand Beaches
On the other hand, Florida’s white sand beaches are made mostly of quartz (silicon dioxide) and crushed calcium carbonate from coral and seashells. Quartz is highly chemically stable, which means it resists weathering and remains bright and white even after long periods of erosion. These grains have been transported over millennia by rivers and wind, polished down to a smooth texture. What’s fascinating from a materials science standpoint is how these sands differ not just in color but also in thermal properties, hardness, and reflectivity. Black sand absorbs more heat due to its lower albedo, making it much hotter to walk on. White sand reflects sunlight, staying cooler underfoot. The angularity and composition of volcanic sand can give it a coarser texture, while quartz-rich sands are typically smoother and finer. Shaped by heat, pressure, erosion, and time, their colors tell a story not just of geography, but of the structure and behavior of materials on Earth’s surface.
References
“Basalt.” Minerals Education Coalition, www.mineralseducationcoalition.org/minerals-database/basalt.
USGS. “Coastal Erosion and Sand Composition.” U.S. Geological Survey, www.usgs.gov/special-topic/coastal-and-marine-geology/science/sand-composition-and-erosion.
