Deserts

Do you think of a desert as a hot, dry place where sand stretches as far as you can see? The Sahara in North Africa is like that, but most deserts are not. All deserts are dry, but some deserts are very cold. Plants and animals live in most deserts. People have learned to live in deserts all over the world.
Most deserts form because of air movements over the planet. Masses of dry air create deserts. Water gets squeezed out of the air as it passes over the mountains, and a desert forms on the other side of the mountains. Deserts can also form along some seacoasts where there are currents of cold water. There are two main bands of desert areas, one north of the equator and one south of the equator. The equator is an imaginary line that goes around Earth’s middle.

TAKING AIR WITH YOU

You can go to places where there is no air. There is no air underwater, but you can dive underwater. You can stay underwater a short time just by holding your breath. Air tanks let you stay underwater for a long time. Scuba divers wear tanks on their backs. The tanks are filled with gases that make up air. The divers breathe the gases through hoses.
There is less and less air the higher up you go. People gasp for breath at the tops of tall mountains. Airplanes must carry air. Once the airplane gets up high, air is pumped into the cabin where passengers sit. Astronauts have to take all the air they need with them—there’s no air in space!

AIR

Take a really deep breath. Feel how your chest gets bigger and bigger. Your chest gets bigger because your lungs are filling up with air. You cannot see air, but air is all around you. You can feel it when the wind blows. Earth’s atmosphere is made of air. An atmosphere is made up of the gases that surround a planet.
Air is a mixture of several different gases. The main gases in air are nitrogen, oxygen, and argon. Air also contains smaller amounts of hydrogen, carbon dioxide, water vapor, helium, and other gases. Oxygen is the most important gas for animals. Animals must breathe oxygen in order to live.
Carbon dioxide is the most important gas for plants. Plants use carbon dioxide and sunlight to make food. Plants give off oxygen. Animals turn the oxygen back into carbon dioxide when they breathe.

COAL

Coal is mostly made up of the element carbon. When carbon burns, it releases a large amount of energy as heat. That’s what makes coal such a useful fuel. Coal is a fossil fuel. That means it comes from the remains of ancient life buried deep in Earth’s crust. The coal we use today started out as plants that grew in swamps millions of years ago. When the plants died, they settled to the swamp bottom. Over time, layers of mud and rock formed. They compressed and hardened the plant material. Heat and pressure caused chemical changes. Gradually, the once-living matter became coal.

REEFS IN DANGER

Scientists fear that many coral reefs are in danger. Many reefs have had a problem called bleaching. This occurs when the zooxanthellae, the algae that live in polyps, die off and the reef changes color. Then the coral polyps die. No one knows what causes bleaching.
A starfish called the Crown of Thorns starfish is also a threat to reefs. Sometimes too many of these starfish suddenly show up on a reef. The starfish kill the coral. No one knows why there are so many starfish.
Pollution is also a threat to coral reefs. Conservationists and governments are working to protect the Great Barrier Reef and other coral reefs.

THREE KINDS OF REEFS

There are three kinds of coral reefs: fringing reefs, barrier reefs, and atolls. All three kinds grow in warm places. Coral reefs cannot form in water colder than about 68° Fahrenheit (about 20° Celsius). Reefs form in clear, shallow water. They need sunlight in order for plants and animals to live there.
Fringing reefs are close to shore. There is no lagoon or other body of water between a fringing reef and the shore. Barrier reefs are farther out in the ocean. There is usually a lagoon between a barrier reef and the shore. Barrier reefs can be huge. The largest group of reefs in the world is Australia’s Great Barrier Reef. It covers thousands of square kilometers. Atolls are really the tops of volcanoes that sank below the sea. The reef grows on the rim of the volcano. Inside the reef is a lagoon.

KINDS OF CORAL

There are hundreds of different kinds of coral. Hard coral or stony coral is the kind of coral that builds reefs. These corals have hard outer skeletons. Other corals do not have hard outer skeletons and look like fans or flowers. Some kinds of coral look like the branches of a tree. Other kinds can sting you if you touch them.