Ar Rub al Khali Sand Sea, Arabian Peninsula is featured in this image photographed by an Expedition 27 crew member on the International Space Station. The Ar Rub al Khali, also known as the "Empty Quarter", is a large region of sand dunes and interdune flats known as a sand sea (or erg). This photograph highlights a part of the Ar Rub al Khali located close to its southeastern margin in the Sultanate of Oman. Reddish-brown, large linear sand dunes alternate with blue-gray interdune salt flats known as sabkhas at left. The major trend of the linear dunes is transverse to northwesterly trade winds that originate in Iraq (known as the Shamal winds). Formation of secondary barchan (crescent-shaped) and star dunes (dune crests in several directions originating from a single point, looking somewhat like a starfish from above) on the linear dunes is supported by southwesterly winds that occur during the monsoon season (Kharif winds). The long linear dunes begin to break up into isolated large star dunes to the northeast and east (right). This is likely a result of both wind pattern interactions and changes in the sand supply to the dunes. The Empty Quarter covers much of the south-central portion of the Arabian Peninsula, and with an area of approximately 660,000 square kilometers it is the largest continuous sand desert on Earth. The Empty Quarter is so called as the dominantly hyperarid climate and difficulty of travel through the dunes has not encouraged permanent settlement within the region. There is geological and archeological evidence to support cooler and wetter past climates in the region together with human settlement. This evidence includes exposed lakebed sediments, scattered stone tools, and the fossils of hippopotamus, water buffalo, and long-horned cattle.
Sandy Cape and Fraser Island, Australia are featured in this image photographed by an Expedition 17 crewmember on the International Space Station. Fraser Island, the world’s largest sand island, includes Great Sandy National Park and is located along the coastline of Queensland, Australia. The island was designated a World Heritage site in 1992, in part due to its outstanding preservation of geological processes related to sand dune formation. According to scientists, the island’s dune fields preserve a record of sand deposition and movement related to sea level rise and fall extending back over 700,000 years. In addition to sand dunes, the island also preserves an interesting range of vegetation — including vine rainforest, stands of eucalypt trees, and mangroves — and diverse fauna including crabs, parrots, sugar gliders and flying foxes. This view highlights the northernmost portion of the island, known as Sandy Cape. Active white sand dunes contrast with dark green vegetation that anchors older dune sets. Irregular patches of sand dunes surrounded by vegetation are known as sand blows (or blowouts), formed when the vegetation cover is disturbed — by wind, fire, or human activities. The exposed underlying sand can then move and form new dunes, sometimes at rates of up to one meter per year. Coastal sand dune fields — such as the one located along the eastern side of Sandy Cape (center) — will remain active until anchored by vegetation, or until no more sand is available to form new dunes.
Great Sand Dunes National Park and Preserve, Colorado is featured in this image photographed by an Expedition 16 crewmember on the International Space Station. The Sangre de Cristo Mountains of south-central Colorado stretch dramatically from top left to lower right of this image, generally outlined by the dark green of forests with white snow-capped peaks on the highest elevations. Dun-colored dunes, covering an area of 80 square kilometers, are banked up on the west side of the mountains and comprise the Great Sand Dunes National Park and Preserve. Originally established in 1932 as a National Monument, it was reauthorized as a National Park in 2004. The park contains dunes over 750 feet (227 meters) high — among the highest in North America. Sand grains that make up the dunes are small enough to be moved along by the wind (a process known as saltation), although much of the dunefield is now anchored by vegetation. Predominant winds blow broadly to the east, so that sand in the San Luis valley (part of which appears at lower left) is driven towards and piled against the Sangre de Cristo Mts. The sand of the dunes is mostly derived from ancient exposed lakebed sediments – now the floor of the San Luis valley – formed by erosion of rocks in the Sangre de Cristo and San Juan Mountains (located to the west). The action of streams and occasional storms today returns some of the impounded sand back to the valley, where the prevailing winds begin the sand’s migration to the dunefield anew. Interestingly, the specific location of the sand field appears to be related to a locally lower altitude sector of the Sangre de Cristo Mts. Altitudes can be inferred from the distribution of snow cover on the day this image was taken. Areas to the north (Cleveland Peak and northward) of the dunefield, and to the south around Blanca Peak, are higher than the ridgeline next to the dune field where almost no snow is visible. Since winds are preferentially channeled over the lower parts of any range (hundreds of meters lower here than ridgelines to north and south), sand grains are carried up to (but not over) the low point of the range.