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 part 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 have historically been considered somewhat culturally distinct, most geographers treat them as two separate continents.

Continents Started to Form 4 Billion Years Ago

Earth is made 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 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 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 4 billion years ago, soon after Earth formed. At that time, a huge ocean covered Earth. Only a small fraction of the crust was made 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 extreme 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 below the Equator. Millions of years of constant tectonic activity slowly changed their positions. By 240 million years ago, almost all the world's land was joined into a single supercontinent.

About 200 million years ago, the supercontinent began to break apart. The pieces 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 an inch a year. The continents also continue 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 Himalaya mountain range was 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 Himalaya to continually grow taller.

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. There are two types of ecology in North America. One is the Nearctic region, which includes Canada and most of the United States and Mexico. Animals like bison (Bison bison), moose (Alces alces) and the California condor (Gymnogyps californianus) live there. The other ecology is the Neotropical region, which extends south from southern Mexico. Animals like llamas (Lama glama), tapirs and vipers live there.

The United States, Canada, Mexico, Belize, Costa Rica, El Salvador, Guatemala, Honduras, Nicaragua and Panama are all in North America. Greenland is also geographically part of North America, even though Denmark partly controls it.

European colonizers had a profoundly negative impact on Indigenous cultures, destroying many of the groups that once lived in North America. Despite this, more than 500 Indigenous nations remain, including the Inuit of Arctic Canada and Alaska, the Iroquois of the United States and the Nahua of Mexico.

People have migrated to North America from around the world for thousands of years. This migration continues today, largely because of migration to the United States. Since the United States became a country, more than 86 million people have immigrated there.

South America

South America is connected to North America by the narrow Isthmus of Panama. The two continents were not always connected, and only came together 3 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 Basin, including the Amazon River dolphin (Inia geoffrensis) and the blue-throated macaw (Ara glaucogularis).

Twelve countries are located in South America. South America was colonized by Spain and Portugal, which destroyed many of its Indigenous cultures. Still, some Indigenous nations, such as the Guarani in Brazil, continue their traditional ways of life and fight against oppression.

Europe

Europe is the sixth-largest continent, containing just 7% 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 best-known 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.

Europeans colonized land on every continent except Antarctica. They extracted the natural resources from those areas, often destroying the Indigenous people and their cultures in the process. This led to greater wealth for Europe, but the Indigenous population was left with political instability and wealth disparity, which still affects them to this day.

Human development in Europe also harmed animal life there, driving many animals to extinction or close to extinction. Some of the species that have survived are the Eurasian lynx (Lynx lynx), the Golden eagle (Aquila chrysaetos) and Mediterranean tortoises (Testudo graeca).

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 6,600 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 (Panthera leo), 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, Djibouti and Somalia will break off from the rest of the continent.

Africa is home to 56 countries, yet it accounts for only 18.3% of the world's total population. During the era of colonization, more than 12.5 million Africans were kidnapped and enslaved. They were taken to colonies in the Americas as part of the transatlantic slave trade. Nearly all of Africa was colonized by Europe. In the mid-20th century, African leaders broke free of this rule. However, Indigenous groups have had to fight for equal rights. The Maasai in Tanzania, for example, are fighting for land that is being taken away to use for agriculture and game reserves. The Maasai raise cattle as part of their traditional culture and need the land for grazing.

Asia

Asia is the largest continent, stretching from the eastern Mediterranean Sea to the western Pacific Ocean. About 60% of Earth's population lives in Asia. More than a third of the world's people live in China and India alone. Asia has the world’s highest population of Indigenous people of any continent. But some groups, like the Hmong of Southeast Asia, have faced generations of persecution.

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. They have some of the world’s most desired natural resources, like oil. European countries used colonization as a tool to extract these resources. Today, foreign powers compete for political influence to continue to have access to them.

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. Many different animals live in these climates, like the King cobra, Asian elephant (Elephas maximus) and Oriental scops owl (Otus sonia).

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 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 of thousands of islands throughout the Central 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 under the Pacific Ocean. This vast body of water is larger than all 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 under 9 million square kilometers (3.5 million square miles). Australia and Oceania is the most lightly populated continent, with a population of fewer than 40 million. Most of the population lives in coastal cities. The two major Indigenous groups are the Aboriginal and the Torres Strait Islander. Both have resisted colonial and modern influence and control to continue their traditional practices.

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 (Phascolarctos cinereus), the platypus (Ornithorhynchus anatinus) and the Tasmanian devil (Sarcophilus harrisii).

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. They coexist with penguins, albatrosses, seals and other animals that depend on the sea to survive.

The climate of Antarctica makes normal human life impossible. Temperatures plunge to lower than -73 degrees Celsius (-100 degrees Fahrenheit). The continent is almost completely covered with ice. This ice layer is sometimes as thick as 3.2 kilometers (2 miles) deep.

Antarctica does not have any countries.