Mountains and Deserts

How Mountains Form

A mountain is land that rises steeply and high above the land around it. A long chain of mountains is a mountain range. Most mountains are built by the slow movement of tectonic plates, the giant pieces of Earth's outer shell. Geographers sort them into a few main types.

• Fold mountains. When two plates push together, rock layers crumple and fold upward. The Himalayas began forming about 50 million years ago, when the plate carrying India crashed into Asia. India is still pushing north, so the Himalayas are still rising.
• Fault-block mountains. When the crust cracks along faults, some blocks of rock are pushed up and others drop down. The Sierra Nevada in California and the Tetons in Wyoming formed this way.
• Volcanic mountains. Melted rock from deep inside Earth erupts and piles up into a cone. The islands of Hawaii are the tops of huge volcanoes. Many peaks in the Andes are volcanoes too. They form where an ocean plate slides under South America.

The World's Great Ranges and Deserts

Every continent has mountains and every continent has dry lands. The map shows six famous ranges and six famous deserts.

Sources give different sizes for the Sahara, from about 8.6 to 9.4 million square kilometers, because a desert has no sharp edge. Its border fades into grassland, so each mapmaker has to choose where to draw the line.

How Do We Know How High a Mountain Is?

For years, different countries gave Mount Everest slightly different heights. In 2019 and 2020, survey teams from Nepal and China both climbed to the top. They used satellite positioning (GNSS) on the summit and radar to measure how deep the snow was. Back on the ground they used careful leveling, measured gravity and worked out exactly where "sea level" is under the mountain. On December 8, 2020, the two countries announced a shared height: 8,848.86 meters, including the snow on top.

"Tallest" also depends on where you start measuring. Everest is the highest mountain above sea level. But Mauna Kea, a volcano in Hawaii, starts far below the ocean surface. Measured from its base on the sea floor, it is taller than Everest.

Mountains and Climate

Climbing a mountain is a bit like traveling toward the poles. On average, the air gets about 6.5 °C colder for every 1,000 meters you climb. That is why snow can sit on mountaintops near the equator, such as Kilimanjaro in Tanzania. It is also why a mountain can have forests at its foot, meadows higher up and bare rock and ice at the top.

Mountains also change where rain falls. When wind pushes moist air up the side of a mountain, the air cools and its water vapor turns into clouds and rain. By the time the air crosses the top and sinks down the other side, it has lost most of its water and warms up. The dry land on that side is called a rain shadow.

Death Valley, in California and Nevada, is so hot and dry partly because it is in the rain shadow of the Sierra Nevada. Its lowest point is 86 meters (282 feet) below sea level.

What Makes a Desert?

A desert is a place that gets very little precipitation, the water that falls as rain or snow. Heat and sand have nothing to do with it. In a widely used system created by the geographer Peveril Meigs, arid land gets less than 250 millimeters (about 10 inches) of rain a year. Extremely arid land can go 12 months in a row with no rain at all. About one-third of Earth's land surface is desert.

• Not all deserts are sandy. Sand covers only about 20 percent of the world's deserts. Nearly half of desert surfaces are gravel plains, and the rest are bare rock, dry lake beds and other ground.
• Not all deserts are hot. The largest desert on Earth is Antarctica, which is covered in ice but gets very little new snow each year. The Gobi, in China and Mongolia, is a cold desert with freezing winters.
• Deserts have life. Plants such as cacti store water, and many animals hide from the heat by day and come out at night.

Why Deserts Are Dry

Deserts form for different reasons, and some have more than one cause.

• Subtropical deserts. Near the Tropic of Cancer and the Tropic of Capricorn, dry air sinks from high in the sky, warms and stops clouds from forming. The Sahara is the biggest example.
• Rain-shadow deserts. Mountains block wet winds, as in Death Valley. The Gobi lies partly in the rain shadow of the Himalayas and Tibetan Plateau, and far from any ocean.
• Coastal deserts. Cold ocean currents cool the air along some coasts, so it holds little water and rarely rains. The Atacama in Chile has a cold current offshore and the Andes blocking wet air from the east. Some parts may have had no rain for centuries. The Namib in southwest Africa is another coastal desert, where fog from the sea brings much of the water.
• Polar deserts. Very cold air holds little water vapor, so the interior of Antarctica gets less than about 50 mm a year (water equivalent).

Living in Mountains and Deserts

Mountains and deserts are hard places to live, but people have found clever ways to farm, travel and find water in both.

• Terraces. In the Andes, the Inka built flat steps called terraces into steep slopes. Stone walls held the soil in place, slowed erosion and gave them more land for crops.
• Oases. An oasis is a green spot in a desert, watered by a spring, a well or irrigation. Oases are often the only places in a desert where people can grow crops and live year-round.
• Moving with the herds. Some groups, such as the Tuareg of the Sahara, are pastoralists. They move with their animals to find grazing land and water.
• Water towers. Snow and glaciers in mountains feed rivers that people far away depend on. A 2019 study found that changes in the world's mountain "water towers" could affect about 1.9 billion people who live in or downstream of them.

Deserts are fragile. When people overgraze, cut too many plants or use too much water, dry land can turn into desert. This is called desertification, and you can read more about it in People and the Environment.

Links to Other Units

• Landforms and Water: the main types of landforms and Earth's record holders.
• Climate and Biomes: the desert biome and how climate zones work.
• Natural Hazards: earthquakes and volcanoes, the forces that also build mountains.
• People and the Environment: desertification and how people change the land.
• Rivers of the World: rivers that start in the mountains.

Self-Guided Quiz: Mountains and Deserts

Ten questions. Choose one answer for each, then check your score. You can try as many times as you like.

Credits

Written by Marco for Jugami's World Geography course. The maps and diagrams are original SVG illustrations made for this page from the data in the sources. Land outlines and the areas of the mountain ranges and deserts come from Natural Earth (public domain) and are simplified. Heights and areas are rounded, and different sources give different values.