This crystal is as big as a bicycle wheel. Giant crystals like it grow at White Sands, in New Mexico. Then they crumble, and the grains blow into white sand dunes.
The sand is gypsum. It's the same mineral inside the walls of your house. It started in a shallow sea that covered part of New Mexico. The sun warmed the water until it evaporated. As it dried, gypsum came out of the seawater and settled on the floor in layers. The sea came and went, again and again.
Those layers were buried and turned to rock. In places, rock laced with gypsum stacks up more than fifteen hundred feet thick. That's thicker than the Empire State Building is tall.
Much later, the crust here buckled. Then it pulled apart. Two mountain ranges rose, the San Andres and the Sacramento. The land between them dropped along faults, in places by as much as ten thousand feet. That's about seven Empire State Buildings. The sunken middle is the Tularosa Basin. On the mountain faces, the old gypsum layers show as pale stripes.
But gypsum has a weakness. It dissolves in water. Rain and snowmelt soak into the mountains and dissolve the gypsum in their rock. Some of that water runs down the slopes. Much of it travels underground. In most places, it would end up in a river, and then in the sea.
Here, it can't. The basin has no outlet, like a bathtub with no drain. And the air is very dry. Less than a foot of rain falls here in a year. The sun could evaporate over eight feet. So the water vanishes, and the gypsum stays behind.
During the last Ice Age, the climate was wetter. Runoff filled a lake that covered about sixteen hundred square miles. That's bigger than the state of Rhode Island. We call it Lake Otero. Runoff carried dissolved gypsum into it, and the gypsum had nowhere to go. Then the Ice Age ended. Lake Otero dried up. Its gypsum was left behind, in thick beds full of crystals.
A small piece of that lake is still here. It's called Lake Lucero, and it covers about ten square miles. After heavy rains, water pools there, soaks in and evaporates. The gypsum grows back into crystals, called selenite. Most new crystals grow after big floods, every ten to fourteen years. Some, buried in the mud, grow as big as bicycle wheels.
Then they crumble. Gypsum is soft, softer than your fingernail. Water seeps into cracks, freezes, and splits the crystal apart. Wind picks up the flakes. The grains bounce across the ground and knock into each other. Each scuff on a grain reflects light. That's what makes the sand so white.
But the giant crystals didn't make most of these dunes. In nineteen seventy-three, two researchers traced the sand to smaller crystals, about the size of a fingernail. Those filled the old lake beds. The big crystals were brown. The little ones matched the white sand. So the wind stripped the dried lake bed and carried its little crystals away.
The wind piled them into dunes. It blows mostly from the southwest, so the dunes march to the northeast. The main dune field was already building nearly nine thousand years ago. Today, White Sands is the largest gypsum dune field in the world. And it still moves. Some dunes in the middle travel up to thirteen feet a year. That's about six bicycle wheels, laid end to end.
Why doesn't the wind blow it all away? Water again. A foot or two under the flats between the dunes, the ground stays damp. That dampness holds the base of the dunes in place.
And the cycle hasn't stopped. Rain still dissolves gypsum in the mountains. Groundwater still carries it to Lake Lucero. Crystals still grow in the mud, crack in the frost, and blow away as sand. Lake Lucero and the flats beside it still feed new dunes.
So that crystal, as big as a bicycle wheel, is new. So is the sand. The gypsum inside both of them is about two hundred and eighty million years old. It's the same old sea, still on the move.