VOICE ONE:
You may wonder how all this sand arrived in the area. To understand that, you would have to travel back in time two-hundred-fifty-million years. An inland ocean once covered the area. The minerals calcium and sulfur were at the bottom of the ocean. Over time, the water slowly disappeared. The calcium and sulfur remained. The minerals formed gypsum rock.
Then, seventy-million years ago, the Earth's surface, or crust, pushed upward. The rocks formed two groups of mountains. Later, the crust pulled apart. The area between the mountains broke and fell down. It formed a half-circle shape of a bowl. This bowl of rock is known as the Tularosa Basin.
VOICE TWO:
About twenty-four-thousand years ago, it rained a great deal in the area. The rain filled the Tularosa Basin and formed Lake Otero. The rain and snow that washed down the mountains into Lake Otero carried gypsum with it.
Gypsum DunesLater, Lake Otero almost completely dried up. Gypsum remained. A strong wind moved into the area. It blew across the land for thousands of years. Pieces of gypsum broke off. The wind wore them away to a size small enough to pick up and carry for short distances. Wherever the wind dropped sand, dunes formed.
(MUSIC)
VOICE ONE:
The sand dunes at White Sands National Monument are unusual because they are made of gypsum. Gypsum sand is different from common sand. Most sand is made of quartz, a hard silicon crystal. Gypsum sand is made of softer calcium sulfate. It dissolves easily in water. So it is rarely found in the form of sand dunes. Most gypsum would be carried away by rivers to the sea. But the Tularosa Basin is enclosed. No rivers flow out of it. So water with dissolved gypsum has nowhere to go.
最新
2013-11-25
2013-11-25
2013-11-25
2013-11-25
2013-11-25
2013-11-25