White Sands National Monument - Gypsum Dune-ified!
On our way through Arizona, we happened to be passing near the White Sands National Monument. We hadn't heard of this before so thanks CRaupp for putting it on our radar to look out for!
White Sands is the world's largest gypsum dunefield (side note - when Jon & I read the sign we both read it as gypsum "dune-ified" ... which is funny). It's literally Sahara Desert-style sand dunes made entirely of gypsum sand. The sand itself has a bit of a funny texture, gritty like sand but softer, kind of like coarse cornmeal. I thought it might be dusty like chalk, but it's not, just soft & gritty - and tricky to drive a motorcycle through when it's on the road!
There were people there taking photos and families with kids who bought the sliding saucers that you use for sledding, so go sliding down the dunes. Also a sand-plow to keep the road clear from the blowing dunes!
For the geology nerds (like us) reading this, the gypsum actually comes from the nearby mountains (you can see them as the dark ridge in the distance in the photos). Back in the day (like ~40 million years ago) there was ocean in Arizona. Technically there was ocean twice. It was there, then sea levels dropped so it wasn't, then they rose again, then dropped again. Anyway, each time the sea level dropped, the minerals in the ocean water were left behind forming two white layers in the rock. Then mountains formed (as they do - thanks tectonic plates), so the two layers got pushed up into the mountains. Now, since the minerals are all water soluble, when it rains in the mountains in the spring, the minerals dissolve in the rainwater and run down into the valley/basin. The basin is essentially a desert, so the spring rains form lakes in the valley but the water evaporates pretty quickly and leaves behind the minerals in crystal form - but super wimpy crystals, softer than human fingernails according to the info movie in the visitor center. So the wind in the valley erodes the crystals into sand and all of a sudden* you have big white sand dunes.
*by "all of a sudden," I mean the dunes formed over the past ~7000 years, but in geologic time that's pretty darn quick.
And now you're an expert in gypsum sand dunes!