Landforms of the Sahara Desert in northern Africa include dunes, plateaus, plains, mountains, dry valleys and lakes, salt flats, and oases. The desert covers about 25 percent of Africa, or 3.3 million square miles, and is roughly equivalent to the size of Brazil.
Sand dunes, known as ergs, cover about 20 percent of the Sahara. They can stretch for dozens of miles and reach heights of 1,000 feet. The ergs cover most of Libya and Algeria and parts of Nigeria and Mali. Dunes contain a lot of salt, which inhabitants of the desert mine and sell.
Plains of gravel and sand, called regs, cover 70 percent of the desert's surface. Gravel can be white, black or red. In prehistoric times, the plains had been sea and riverbeds but now have little water.
Elevated plateaus of stone and rock, or hamadas, reach heights of 11,000 feet and include the Atlas Mountains of northwest Africa, the Tibesti Mountains of northern Chad and southern Libya, and the Ahaggar Mountains of southern Algeria.
Oases, green areas with water, cover only about 800 square miles, but 75 percent of the Sahara Desert's people live in them. Oases' fresh, clean water comes from springs and wells. Some oases have irrigation systems from the huge aquifers or underground basins that are a legacy of the time prior to 10,000 years ago, when the Sahara was green and had regular rains.