Sand dunes sprawl along the coast of northeastern Brazil, sometimes extending kilometers inland. One substantial area of coastal dunes, shown in an astronaut photograph from 2003, has been protected as a national park, but the park boundaries only enclose a fraction of the marching sand dunes. Some 135 kilometers (85 miles) east of the park, another stretch of dunes pushes inland, near the city of Luís Correia.
The Advanced Land Imager (ALI) on NASA’s Earth Observing-1 (EO-1) satellite captured this natural-color image of the dune field near Luís Correia on September 11, 2002. At their southwesternmost extent, the dunes border an inland water body, Lagoa do Portinho (image lower left). In the northwest, the dunes are interrupted by cityscape. Besides settled and watery areas, the dunes are bordered by thick vegetation (image upper left and lower right). Between the shore and the inland edge of the dune field, sand alternates with vegetated ground.
Northeastern Brazil’s climate is largely driven by the Intertropical Convergence Zone—an equatorial region where northern and southern hemisphere easterly trade winds converge. During the dry season, roughly August to November, strong winds push sands inland, forming tall, crescent-shaped dunes. A 1999 study found that some dunes migrate about 17.5 meters (57.5 feet) a year, pushed by relentless dry-season winds. During the rainy season, from January to June, wind speeds drop and precipitation peaks, depositing ephemeral water bodies between dunes in some places.
The northeast-southwest sandy stripes in this image betray the prevailing wind direction, which is generally from the east. Captured during the region’s dry season, the picture shows a mixture of brown and green in the dune-free areas. In the southeast, just beyond the edge of the dune field, the land appears as a patchwork of brown and green. At other times of year, the same area shows a mixture of lush vegetation and standing water.
NASA Earth Observatory image created by Jesse Allen, using EO-1 ALI data provided courtesy of the NASA EO-1 team. Caption by Michon Scott.
The area shown here (10 kilometers across) is a small part of the dune field which is now protected as the Len&ccelde;óis Maranhenses National Park, on Brazil’s north coast, about 700 kilometers east of the Amazon River mouth. Persistent winds blow off the equatorial Atlantic Ocean onto Brazil from the east, driving white sand inland from the 100 kilometer stretch of coast (upper margin of the image), to form a large field of dunes. The strongly regular pattern of these dunes is a common characteristic of dune fields. The basic shape of each sand mass, repeated throughout the view, is a crescent-shaped dune. In an area with a rich supply of sand such as coastal Brazil, individual crescents coalesce to form entire chains many miles long. The wind strength and supply of sand are sufficient to keep the dunes active, and thus free of vegetation, despite 1500 mm (60 inches) of rainfall annually. The dark areas between the white dunes are fresh water ponds that draw fisherman to this newly established park.
One of the main reasons that rainless regions like the Sahara Desert are interesting from the perspective of landscape science is that the work of flowing water—mainly streams and rivers—becomes less important than the work of wind. Over millennia, if enough sand is available, winds can generate dunes of enormous size, arranged in regular patterns. Long, linear dunes stretch generally north to south across much of northeast Algeria, covering a vast tract (~140,000 square kilometers) of the Sahara Desert known as the Erg Oriental. Erg means “dune sea” in Arabic, and the term has been adopted by modern geologists. Spanning this image from a point on the southwest margin of the erg (image center point: 28.9°N 4.8°W) are a series of 2-kilometer-wide linear dunes, comprised of red sand. The dune chains are more than 100 meters high. The “streets” between the dunes are grayer areas free of sand.