A continent is one of Earth's seven main divisions of land. From largest to smallest, the continents are: Asia, Africa, North America, South America, Antarctica, Europe, and Australia.

Usually, all nearby islands are also considered to be part of a continent. Japan, for example, is part of the continent of Asia. Greenland and all the islands in the Caribbean Sea are usually considered part of North America, geographically. Only a very small number of islands are not considered parts of any continents.

Together, the continents add up to about 148 million square kilometers (57 million square miles) of land.

Continents are partly defined by culture. The continents of Europe and Asia, for example, are actually part of a single, enormous piece of land called Eurasia. However, because Asia and Europe are somewhat culturally distinct, most geographers treat them as two separate continents.

Continents Started To Form Four Billion Years Ago

The earth is made up of three main layers: the central core, the mantle, and the outer crust.

The crust and the top part of the mantle form a hard, stiff shell around Earth. This shell is broken up into huge sections called tectonic plates. These plates slide around on the molten mantle, which is so hot it remains in liquid form. Today, tectonic plates continue to slowly slide around the surface, just as they have been doing for hundreds of millions of years. Geologists believe this constant sliding helped form the continents.

The oldest known pieces of the continents began to form nearly four billion years ago, soon after Earth itself formed. At that time, a huge ocean covered Earth. Only a small fraction of the crust was made up of continental material. Scientists believe this material built up along the boundaries of tectonic plates due to something called subduction. During subduction, plates ram into each other, and the edge of one plate slides beneath the edge of another.

When heavy oceanic crust subducted toward the mantle, it melted in the mantle's enormous heat. Once melted, the rock became lighter and rose through the plate above and burst out as molten lava. When the lava cooled, it hardened into igneous rock.

Slowly, the igneous rock built up into small islands above the surface of the ocean. Over time, these islands grew bigger. When plates carrying islands subducted, the islands themselves did not descend into the mantle. Their material fused with that of islands on the neighboring plate. This made even larger landmasses, which over time became the first continents.

Scattered Chunks of Land

Millions of years ago, Earth looked very different as the continents were not where they are today. About 480 million years ago, most continents were scattered chunks of land lying along or south of the Equator. Millions of years of constant tectonic activity slowly changed their positions. By 240 million years ago, almost all of the world's land was joined in a single, huge continent that geologists call Pangaea.

About 200 million years ago, the supercontinent began to break apart. The pieces of Pangaea that began moving apart were the beginnings of the continents we know today.

A giant landmass that would become Europe, Asia, and North America separated from another mass that would split up into other continents. In time, Antarctica and Australia, which were still joined together, broke away and drifted south. The small piece of land that would become India also broke away, and for millions of years it moved north as a large island. It finally ran into Asia and slowly, the different landmasses moved to their present locations.

The positions of the continents are always changing. North America and Europe are moving away from each other by about 2.5 centimeters (one inch) a year. The continents are also continuing to break apart. In time, part of California will likely separate from North America and become an island.

Continental Features Take Shape

The surface of the continents has changed many times. The rocks that form the continents have been shaped and reshaped. Great mountain ranges have risen and then have been worn away. Ocean waters have flooded huge areas and then slowly dried up. Huge ice sheets have come and gone, shaping the land as they advance and then melt away.

On every continent, mountains continue to form. One way mountains form is through the crashing together of two tectonic plates, creating wrinkles in the crust, just as a rug wrinkles when you push against one end of it. These "wrinkles" are what we call mountains.

Asia's Himalayas were formed that way several million years ago. The plate carrying India slowly and forcefully shoved the landmass of India into Asia, which was riding on another plate. The collision of the two plates continues today, which, as a result, causes the Himalayas to grow taller every year.

North America

North America is the third-largest continent. It extends from the tiny Aleutian Islands in the northwest to the Isthmus of Panama in the south.

The West is full of young mountains, among them the Rockies, North America's largest chain. Older mountain ranges rise near the East Coast of the United States and Canada.

North America has a greater variety of climates than any other continent. It ranges from the freezing Arctic to the tropical jungles of Central America.

People sometimes mistakenly think the United States and Canada are the only countries in North America. In fact, Mexico, Belize, Costa Rica, El Salvador, Guatemala, Honduras, Nicaragua, and Panama are all also in North America. Greenland is also geographically part of North America, even though Denmark partly controls it.

South America

South America is connected to North America by the narrow Isthmus of Panama. The two continents weren't always connected, and only came together three million years ago.

South America is the fourth-largest continent. It extends from the sunny beaches of the Caribbean Sea to the icy waters near the Antarctic Circle.

The Andes is the longest mountain range on any continent and stretches the entire length of South America.

In northern South America, the Amazon River flows through the world's largest tropical rainforest. The Amazon is the largest river in the world.

South American rainforests contain an enormous wealth of animal and plant life. More than 15,000 kinds of plants and animals are found only in the Amazon River basin.

Twelve countries are located in South America.

Europe

Europe is the sixth-largest continent, containing just seven percent of the world's land. In total area, the continent of Europe is only slightly larger than the country of Canada. However, the population of Europe is more than twice that of South America. Europe has more than 40 countries and many of the world's major cities.

In the east, the Ural Mountains separate Europe from Asia. Two nations, Russia and Kazakhstan, are in both continents.

Europe's most famous mountain range is the Alps. It stretches from Albania to Austria, then across Switzerland and northern Italy into France.

Africa

Africa is the second-largest continent. It covers an area more than three times that of the United States. From north to south, Africa stretches about 8,000 kilometers (5,000 miles). It is connected to Asia by the Isthmus of Suez in Egypt.

The Sahara is the world's largest hot desert and covers much of North Africa. The world's longest river, the Nile, flows more than 6,560 kilometers (4,100 miles) from Lake Victoria to the Mediterranean Sea.

The top half of Africa is mostly dry, hot desert. The middle area has savannas, or flat, grassy plains. This region is home to wild animals such as lions, giraffes, and elephants. The central and southern areas of Africa are full of rainforests.

Africa also has mountainous areas. The peak of Mount Kilimanjaro in Tanzania is covered by snow all year long.

The continent of Africa is slowly splitting in two. In time, the land now containing the countries of Eritrea, Ethiopia, Djibout,i and Somalia will break off from the rest of the continent.

Africa is home to 56 countries, yet it accounts for only 14 percent of the world's total population.

Asia

Asia is the largest continent, and stretches from the eastern Mediterranean Sea to the western Pacific Ocean. Sixty percent of Earth's population lives in Asia. More than a third of the world's people live in China and India alone.

The continent of Asia includes many islands, some of them countries. The Philippines, Indonesia, Japan, and Taiwan are major island nations in Asia.

Altogether, there are more than 40 countries in Asia.

Asia has many different climate regions. They range from polar in the Siberian Arctic to tropical in equatorial Indonesia. Parts of central Asia, including the Gobi Desert, are dry year-round.

Asia is the most mountainous of all the continents. More than 50 of the highest peaks in the world are in Asia. Mount Everest, which is in both Nepal and China, is the highest point on Earth. It reaches more than 8,700 meters (29,000 feet) high in the Himalaya range.

Australia and Oceania

The name "Oceania" is a hint to this continent's defining characteristic: the Pacific Ocean.

Oceania is a region made up of thousands of islands throughout the Central Pacific and South Pacific. It includes Australia, which is the smallest continent in the world in terms of total land area.

Most of Australia and Oceania is in the Pacific. This vast body of water is larger than all the Earth's continental landmasses and islands combined.

Oceania is dominated by Australia. There are two other major landmasses of Oceania. One is the microcontinent of Zealandia, which includes the country of New Zealand. Much of Zealandia is formed by rocks that are underwater, with only New Zealand poking through the ocean surface.

The other large land area in Oceania is the island of New Guinea. It contains the country of Papua New Guinea; the rest of the island is ruled by Indonesia.

Oceania also includes three island regions: Melanesia, Micronesia, and Polynesia, which includes the U.S. state of Hawai'i. Australia and Oceania covers just less than 8.5 million square kilometers (about 3.5 million square miles). Australia and Oceania is the most lightly populated continent, with a population of fewer than 40 million humans. Most of the population lives in coastal cities.

Biologists who study animals consider Australia and Oceania a living laboratory. When the continent broke away from Antarctica more than 60 million years ago, it carried a cargo of animals with it. These animals developed into creatures found only in Australia, such as the koala, the platypus and the Tasmanian devil.

Antarctica

Antarctica is the windiest, driest, and iciest place on Earth. It is larger than Europe or Australia, but unlike those continents, it has no permanent human population. The only people living there are scientists.

The climate of Antarctica makes normal human life impossible. Temperatures plunge to lower than 73 degrees Celsius (100 degrees Farenheit) below zero. The continent is almost completely covered with ice. This ice layer is sometimes as thick as 3.2 kilometers (two miles deep).

Antarctica does not have any countries.