||White Sands National Monument Dust Storm, New Mexico, USA |
Driven by winter winds from the southwest, white dust from the White Sands dune field (image center) in New Mexico rises thousands of feet from the valley floor, up and over the snowy peaks of the Sacramento Mts. downwind (image right). White Sands National Monument lies in the 50 km-wide Tularosa valley, between the dark rocks and forested slopes of the Sacramento Mts. and the San Andres Mts. (image left center). The lower and warmer ridge line of the San Andres was without snow on this day. The striking black lava flows of the Carrizozo lava field also occupy the valley floor (image top right). The darker tones of agriculture in the Rio Grande floodplain can be seen along the left margin of the image.
The length of the dust plumes in this astronaut photograph is more than 120 km. The vigor of the winds on this day can be judged also from the fact that they are lifting the dust particles from the valley floor more than 1200 m over the mountains. Winds channel the dust through a low point on the Sacramento Mts., some 800 m below the ridge crests to north and south (image right).
In most parts of the world blowing dust is some shade of light brown or red. It is interesting to observe two colors of dust that contribute to this dust plume: redder dust from the local hillsides north of White Sands (seen by the small brown zone along the northern margin of the dust mass), and the white dust from the dune field.
The sand dunes of the Monument are white since they are composed of the mineral gypsum, a relatively rare dune-forming mineral. The gypsum is deposited during evaporation of mineral-rich waters in an ephemeral lake located in the western part of the Monument. Erosion of the deposits, together with wind transport, provides the granular material for the dunes. The dunes' brilliance, especially contrasted against the nearby dark mountain slopes, makes the dunes easily identifiable at great distances by orbiting astronauts. The white speck of the dunes is even visible in images taken looking back at Earth by crews on the way to the Moon.