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The slow cooking of master plaster

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Joy Moses-Hall


by Joy Moses-Hall

Sunday, February 18, 2018

The white, silky powder extends all the way to the blue mountains on the horizon, and the daughters stand ready by the sled. One push down the hill and they are giggling and tumbling and shaking off the flakes in the 90-degree sunshine.

Wait, what?

What is this glistening hill of snow that doesn’t melt? Why are the daughters in a hot summer desert dragging a sled to ride it? Welcome to some Winter Weird Olympics?

At the Olympics going on now in South Korea, where the pursuit of glory means faster, higher, stronger — or, in the case of sledding, faster faster faster — the sledders race down high-tech mountains on high-tech blades over high-tech ice.

In the sparkling Chihuahuan desert, sledding is low-tech and it isn’t on ice. The Hall’s round green saucer sled, once dragged over the white snowy ditch-hills of Greenville winters, now knows the white sand wind spills of a New Mexico summer. At White Sands National Monument, fine white grains blanket the landscape year round, come polar or solar. This snowy sand is the substrate of Southwest summer sledding.

The grains are composed of Permian gypsum salts. More ancient than dinosaurs, worn thin by wind and weather from columns of crystal to crumbs of clast, the gypsum hails from a history of seas that rose and fell, skies that rained and dried, streams that flowed and died. The sizzling sand steeps straight from a salty, slow-cooked sea.

Gypsum is a blend of calcium, sulfur, oxygen, and water that makes alabaster cities gleam. When processed, we humans use it for the molded mass of a bone cast, and the master of plaster of Paris.

The vast White Sands gypsum dates to 255 million years ago, before dinosaurs dominated the Earth, spread along the west coast of supercontinent Pangaea, where parts of the future Texas, New Mexico, and northern Mexico wallowed under a shallow bay in a climate that eventually dried it up.

When seawater evaporates, the water vaporizes but the sea minerals are left behind. Like a stew cooking down, as the water lifts away, the minerals become concentrated. Upon saturation, crystals form of one brine after another, lining the bottom and edges of the shrinking sea. Every flood washes in new salty soup and every sunny day cooks some liquid away. Over centuries, the layer of evaporites becomes a wide bed of salt and gypsum crystals coating the underlying sediments. In New Mexico, it is a Permian powder plastering the pebbles.

For millions of years, the seas came and went. For millions of years, salt layers came and were buried. At one point, North America was split by a vast strait from top to bottom, from the Arctic to the Gulf of Mexico, with only the mountains of the west and the east breaking above the water surface.

Seventy million years ago, pressure from the west pushed the Rocky Mountains up, and New Mexico tipped up with them, levering the valley of gypsum into peaks too high to inundate by sea anymore. Rain ate away at the high-top salts, dissolving them and carrying them downhill, where they became concentrated in the sunken valley between the mountains that became a lake.

After the last ice age, the lake evaporated, leaving behind a playa of refined white gypsum crystals. Gypsum is soft, and the crystals were broken into fine flakes by wind and frost and blown into a blizzard of dunes reaching to the sky and the mountains.

One person’s powder of the Permian is another person’s plaster of Paris.

As they whoosh down the hills of faster plaster, the daughters show little interest in the gripping tale of millions of years of odyssey and chemistry that delivered awesome gypsum to their retread green sled. What’s under their feet seems not worth a tweet let alone an elite seat in the annals of Earth feats. They just don’t seem to care.

Joy Moses-Hall teaches physics and astronomy at Pitt Community College. She has a Ph.D. in oceanography and is the author of the novel Wretched Refuge. Follow @jmoseshall on Facebook.

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