The seven continents

Look at a globe. You see lots of blue water.

You also see big shapes of land. The biggest pieces of land are called continents. There are seven of them.

If you spin a globe, you will see that most of Earth is covered by ocean. National Geographic says the ocean covers almost three-fourths of the planet, more than double the area of all the land put together. The land is gathered into a few huge pieces called continents.

Most geographers count seven continents. From largest to smallest, they are Asia, Africa, North America, South America, Antarctica, Europe and Australia. Each continent also includes the islands near it: Japan belongs to Asia, and Greenland and the Caribbean islands are usually counted as part of North America.

National Geographic defines a continent as one of Earth's seven main divisions of land, and lists them from largest to smallest as Asia, Africa, North America, South America, Antarctica, Europe and Australia. Geographers usually include associated islands: Japan with Asia; Greenland and the Caribbean islands with North America.

Together the continents cover about 148 million square kilometers (57 million square miles). That is most, but not all, of Earth's land, because a very small portion is made of islands that are not physically part of any continent. All continents border at least one ocean, and the ocean's area is more than double that of all the continents combined.

Asia is the biggest continent.

Australia is the smallest. Antarctica is covered in ice. Nobody lives there all the time.

Asia is the largest continent: National Geographic says it has almost a third of all the world's land and more than 40 countries. Africa comes second. It is more than three times the size of the United States and stretches about 8,000 kilometers from north to south.

At the other end, Europe has just 7 percent of the world's land, only slightly more than the country of Canada. Australia, the smallest continent, covers just under 8.5 million square kilometers. Antarctica is bigger than both Europe and Australia, but nobody lives there permanently.

National Geographic's figures show how unequal the seven are. Asia accounts for almost a third of the world's land and has the longest series of coastlines. Africa, second, covers more than three times the area of the United States and stretches about 8,000 km (5,000 miles) north to south. North America is third and South America fourth.

Antarctica, fifth, is larger than Europe or Australia but has no permanent human population; its residents are researchers and support staff. Europe, sixth, holds just 7 percent of the world's land, only slightly more than Canada, yet more than twice South America's population. Australia, the smallest, covers just under 8.5 million km² (about 3.5 million square miles).

Each continent is special.

Africa has a long valley where the land is splitting. South America has the long Andes mountains. Antarctica is the iciest place on Earth.

Every continent has its own landscapes. North America stretches from the Aleutian Islands to the Isthmus of Panama, which is only 50 kilometers across at its narrowest. South America is joined to it there, and National Geographic says the two came together only 3 million years ago. The Andes run the whole length of South America.

In Africa, the Great Rift Valley runs from the Red Sea to Mozambique, where the continent is slowly splitting in two. Antarctica is the windiest, driest and iciest place on Earth, with ice that in places is 3.2 kilometers (2 miles) thick.

Each continent pairs old, stable regions with younger, more active ones, National Geographic explains. Every continent has a craton, an area of ancient crust; part of the Canadian Shield holds rocks more than 3.5 billion years old. Young coastal ranges rise along the western Americas, while the Appalachians and the Urals are older ranges that are no longer geologically active.

The Andes, which National Geographic calls Earth's longest terrestrial mountain range, are fueled by the Nazca Plate grinding beneath South America. In Africa, the Great Rift Valley marks where the African Plate is dividing into the Nubian and Somali plates. In Asia, India's continuing collision keeps the Himalayas growing taller. Antarctica, almost entirely ice-covered, can double its surface area in winter as pack ice builds up around it.

Most people count seven continents.

But Europe and Asia are joined together. So some people count them as one big piece of land, called Eurasia.

Seven is the most common count, but continents are not all neatly separated by sea. Europe and Asia are really one enormous piece of land called Eurasia. National Geographic explains that they have been treated as separate continents because of people's ideas about their different cultures, and most geographers still divide them along a line from the Ural Mountains to the Caspian and Black Seas.

Names can differ too. The smallest continent is often called Australia, but it is sometimes called Oceania, to include thousands of Pacific islands. Some people also group North and South America as one continent, America. That makes six continents, or five if Europe and Asia are joined as well. So the number and names of continents depend on how people decide to count.

There is no single rule that fixes the number of continents; different counts follow from different definitions. Physically, National Geographic notes, continents are defined by their continental shelves, not their coastlines. Culturally, Europe and Asia form a single landmass, Eurasia, separated historically because of perceptions of cultural difference; most geographers keep the division along an imaginary line from the northern Ural Mountains south to the Caspian and Black Seas.

The smallest continent shows the naming problem. National Geographic's continent of Australia includes Australia, the eastern part of New Guinea and New Zealand, while the wider name Oceania adds the thousands of islands of Melanesia, Micronesia and Polynesia. National Geographic's Latin American edition notes that North America, Central America with the Caribbean, and South America are sometimes grouped as a single continent, America, giving six continents, or five if Eurasia is also counted as one. None of these choices changes the land itself.

Continents move very, very slowly.

Long ago, they were all joined together. Now they are far apart.

The continents have not always been where they are now. They ride on huge, slow-moving pieces of Earth's outer shell called tectonic plates. National Geographic says that by 240 million years ago almost all the land was joined in one giant continent, which began to break apart about 200 million years ago.

The movement has not stopped. North America and Europe are moving away from each other by about 2.5 centimeters a year, a little more than the width of a U.S. quarter. One day, Africa might even split in two along the Great Rift Valley.

Continents sit on tectonic plates, which slide over the hotter mantle beneath. National Geographic describes the oldest known pieces of continental crust as nearly 4 billion years old, built up along plate boundaries by subduction and volcanic island growth. About 480 million years ago, most continents lay scattered along or below the Equator; by 240 million years ago almost all land was joined in one supercontinent, which began to break up by about 200 million years ago.

Dates for that break-up differ between sources: the U.S. Geological Survey gives about 225 to 200 million years ago for the supercontinent Pangaea. Today, North America and Europe separate by about 2.5 cm (1 inch) per year, and National Geographic suggests that part of California could one day become an island and Africa could split along the Great Rift Valley.

Look at a world map with a grown-up.

Find South America and Africa. Do their edges look like puzzle pieces that fit together?

The U.S. Geological Survey tells how, as early as 1596, the Dutch mapmaker Abraham Ortelius noticed that the coasts of the Americas, Europe and Africa looked as if they had been torn apart. You can test it yourself.

Trace South America and Africa from a world map onto paper and cut them out, with a grown-up helping with the scissors. Slide the two shapes together. How well do the coasts fit? Then find all seven continents on a globe and name them from largest to smallest.

Repeat the observation that started the debate. USGS's This Dynamic Earth describes how Ortelius in 1596, and later Alfred Wegener, noticed the fit of the South American and African coasts. Trace both continents from the same world map, cut them out and test the fit.

Then make it more precise. National Geographic notes that continents are defined by their continental shelves, not their coastlines, so try tracing the edge of the shallow shelf from a physical map instead of the shoreline. Does the fit improve? Finally, compare the size of Europe and Australia on a globe and on a flat world map, and think about which shows their true sizes.