Why are the dunes around Sossusvlei orange-red?

The iconic orange-red color of the sand dunes surrounding Sossusvlei is caused by a thin coating of iron oxide on the individual grains of sand.1

Over millions of years, wind-blown sediment from the Namib desert's interior, the coast, and dry riverbeds—such as the Tsauchab—has accumulated to form the massive dunes of the Namib Sand Sea. As these sand grains have been exposed to the elements over vast periods, the iron oxide coating has developed, resulting in the distinctive, vibrant hues that characterize this African landscape.1