As Marco Polo crossed the Gobi Desert in the 13th century, he began to hear something -- and it seemed to be coming from the sand. When he was unable to come up with a logical explanation for the whistling, humming and booming sounds that haunted his ears, he blamed the noise on evil spirits. I mean, who else?
But Polo wasn't just hearing things. Sand dunes do sing. They also grow, migrate and reproduce. This behavior can make these otherwise colorless piles of grains seem almost lifelike.
A sand dune forms when wind blows a large amount of loose sand into an obstacle, like a rock or a tree. The heaviest grains of sand settle against the obstacle and start to form a small ridge or bump. The lighter grains deposit themselves on the other side of the obstacle, or the slip face. As long as the wind blows, the ridge of sand will continue to build until it becomes a dune. To learn more about this process, read How Sand Dunes Work.
Sand comes from rocks. When rocks erode over time, their pieces are picked up by wind, water or ice (as in glaciers). The most common ingredient in sand is quartz, also called silica. The makeup of sand will vary from place to place, too, depending on the makeup of the local rocks.
The question is, how could broken down rock come alive in the desert? Find out on the next page.