The surprising physics hiding in a single handful of beach sand Lisa Lock Scientific Editor Andrew Zinin Chief Editor Sand has a way of getting everywhere on a beach day—and for some, it can make or break the whole trip. There's the way it whips through the air just as sandwiches are being unwrapped, the way it turns blisteringly hot underfoot by midday, and the way it stubbornly clings to every piece of clothing. But the same stuff people curse at the shore is quietly becoming one of the most important materials on Earth.

And it's starting to run short. "After water, sand is the second-most-used natural resource on the planet—the world goes through roughly 50 billion tons a year," says materials scientist Shu Yang. "That's enough to build a nine-story wall around the entire globe, and we're pulling it from rivers, coasts and seabeds faster than the hundreds of thousands of years Earth took to make it." Yang, who uses sand to create more sustainable concrete and building materials, says that while places like deserts are covered in sand, "not just any sand will do." The wind-smoothed grains of dunes are nearly useless for construction, which is part of why a planet seemingly covered in sand can still face a shortage.

Penn Today caught up with Yang to get into the nitty-gritty of what's really going on beneath people's feet. Why some sand scorches your feet—and some stays cool Whether a stretch of beach is pleasant or a barefoot hazard comes down to several factors, Yang says, composition, color, grain size, moisture, how tightly the sand is packed and airflow. "Darker sands absorb more solar radiation and can become much hotter than whiter ones," she explains.

Dry sand heats up fast because the air trapped between the grains is a poor conductor, so the heat stays near the surface; wet sand stays cooler because water conducts heat well and carries it off as it evaporates. Finer grains, with more surface area touching the skin, can also feel hotter underfoot. If the air temperature is around 75°F (24°C), sand can climb above 100°F (38°C)—and on a 90°F (32°C) day, it can top 120°F (49°C), well above the roughly 111°F (44°C) at which skin begins to burn.

There's even a name for the blistered soles that follow: "beach feet." "The standard advice to walk along the wet sand near the waterline is, it turns out, sound thermal physics," Yang chuckles. No two beaches feel the same underfoot Why is one beach powdery while another is coarse? Most of it comes down to where the sand came from and how far it has traveled, Yang says.

Beaches near volcanoes or coral reefs tend toward rougher, larger grains, while shorelines pounded by constant waves get ground down into finer, smoother ones over time. That same origin story is why beach sand comes in so many colors, Yang notes. Most of it is quartz, but it can be flecked with feldspar, basalt or fragments of shell and coral.

Hawaiʻi's Papakōlea Beach is tinted green by olivine crystals; the black sand beaches of Iceland and Hawaiʻi are pulverized volcanic basalt; and the famous pink sands of Bermuda owe their blush to the red shells of tiny marine organisms called foraminifera.