Geographic overview

World

The surface of the Earth is approximately 70.9% water and 29.1% land. The former portion constitutes the World Ocean, the single largest feature of the planet and one that connects all places on the globe. The World Ocean is divided by the intervening continental landmasses into five major ocean basins, which are (in decreasing order of size) the Pacific, Atlantic, Indian, Southern, and Arctic. The World Factbook describes each of these five as oceans. Given the significant differences in direction and temperature of major ocean currents, as well as the effects of the major air masses above them, the Pacific and Atlantic Oceans are generally divided at the equator into the North and South Pacific Oceans and the North and South Atlantic Oceans, thus creating seven major water bodies -- the so-called "Seven Seas."

About 97.5% of the Earth's water is saltwater. Of the 2.5% that is fresh, about two-thirds is frozen, mostly locked up in the Antarctic ice sheets and mountain glaciers worldwide. If all the surface ice on earth fully melted, the sea level would rise about 70 m (230 ft).

In a 100-year period, a water molecule spends 98 years in the ocean, 20 months as ice, about two weeks in lakes and rivers, and less than a week in the atmosphere. Groundwater can take 50 years to just traverse 1 km (0.6 mi).

Earth's land portion is divided into several large, discrete landmasses called continents. Depending on the convention used, the number of continents can vary from five to seven. The most common classification recognizes seven, which are (from largest to smallest) Asia, Africa, North America, South America, Antarctica, Europe, and Australia. Asia and Europe are sometimes lumped together into a Eurasian continent, resulting in six continents. Alternately, North and South America are sometimes grouped as simply the Americas, resulting in a continent total of six (or five, if the Eurasia designation is used).

North America is commonly understood to include Greenland and the Caribbean islands and to extend south to the Isthmus of Panama. The Ural Mountains and the Ural River are usually considered the easternmost part of Europe, the Caspian Sea is the limit to the southeast, and the Caucasus Mountains, the Black Sea, and the Mediterranean to the south. Portions of five countries -- Azerbaijan, Georgia, Kazakhstan, Russia, and Turkey -- fall within both Europe and Asia, but in every instance the larger section is in Asia. These countries are considered part of both continents. Armenia and Cyprus, which lie completely in Western Asia, are geopolitically European countries.

Asia usually incorporates all the islands of the Philippines, Malaysia, and Indonesia. The Pacific islands are often lumped with Australia into a "land mass" termed Oceania or Australasia. Africa's northeast extremity is frequently delimited at the Isthmus of Suez, but for geopolitical purposes, the Egyptian Sinai Peninsula is often included as part of Africa.

Although the above groupings are the most common, different continental dispositions are recognized or taught in certain parts of the world, with some more heavily based on cultural spheres than physical geographic considerations.

Based on the seven-continent model, and grouping islands with adjacent continents, Africa has the most countries with 54. Europe contains 49 countries and Asia 48, but these two continents share five countries: Azerbaijan, Georgia, Kazakhstan, Russia, and Turkey. North America consists of 23 sovereign states, Oceania has 16, and South America 12. In total, the United States recognizes 197 countries.

countries by continent: Africa (54): Algeria, Angola, Benin, Botswana, Burkina Faso, Burundi, Cabo Verde, Cameroon, Central African Republic, Chad, Comoros, Democratic Republic of the Congo, Republic of the Congo, Cote d'Ivoire, Djibouti, Egypt, Equatorial Guinea, Eritrea, Eswatini, Ethiopia, Gabon, The Gambia, Ghana, Guinea, Guinea-Bissau, Kenya, Lesotho, Liberia, Libya, Madagascar, Malawi, Mali, Mauritania, Mauritius, Morocco, Mozambique, Namibia, Niger, Nigeria, Rwanda, Sao Tome and Principe, Senegal, Seychelles, Sierra Leone, Somalia, South Africa, South Sudan, Sudan, Tanzania, Togo, Tunisia, Uganda, Zambia, Zimbabwe

Europe (49): Albania, Andorra, Austria, Azerbaijan*, Belarus, Belgium, Bosnia and Herzegovina, Bulgaria, Croatia, Czech Republic, Denmark, Estonia, Finland, France, Georgia*, Germany, Greece, Holy See (Vatican City), Hungary, Iceland, Ireland, Italy, Kazakhstan*, Kosovo, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Moldova, Monaco, Montenegro, Netherlands, North Macedonia, Norway, Poland, Portugal, Romania, Russia*, San Marino, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey*, Ukraine, United Kingdom (* indicates part of the country is also in Asia)

Asia (48): Afghanistan, Armenia, Azerbaijan*, Bahrain, Bangladesh, Bhutan, Brunei, Burma, Cambodia, China, Cyprus, Georgia*, India, Indonesia, Iran, Iraq, Israel, Japan, Jordan, Kazakhstan*, North Korea, South Korea, Kuwait, Kyrgyzstan, Laos, Lebanon, Malaysia, Maldives, Mongolia, Nepal, Oman, Pakistan, Philippines, Qatar, Russia*, Saudi Arabia, Singapore, Sri Lanka, Syria, Tajikistan, Thailand, Timor-Leste, Turkey*, Turkmenistan, United Arab Emirates, Uzbekistan, Vietnam, Yemen (* indicates part of the country is also in Europe)

North America (23): Antigua and Barbuda, The Bahamas, Barbados, Belize, Canada, Costa Rica, Cuba, Dominica, Dominican Republic, El Salvador, Grenada, Guatemala, Haiti, Honduras, Jamaica, Mexico, Nicaragua, Panama, Saint Kitts and Nevis, Saint Lucia, Saint Vincent and the Grenadines, Trinidad and Tobago, United States

Oceania (16): Australia, Cook Islands, Fiji, Kiribati, Marshall Islands, Federated States of Micronesia, Nauru, New Zealand, Niue, Palau, Papua New Guinea, Samoa, Solomon Islands, Tonga, Tuvalu, Vanuatu

South America (12): Argentina, Bolivia, Brazil, Chile, Colombia, Ecuador, Guyana, Paraguay, Peru, Suriname, Uruguay, Venezuela

Three of the states described above – France, Netherlands, and the United Kingdom – consist of smaller political entities that are referred to as countries. France considers French Polynesia an overseas country; the Kingdom of the Netherlands refers to all four of its constituent parts (Netherlands [proper], and the islands of Aruba, Curacao, and Sint Maarten) as countries; and the United Kingdom is composed of the countries of England, Wales, Scotland, and Northern Ireland. While not recognized as sovereign states, The World Factbook does include descriptive entries for the French and Dutch island "countries."

the World from space: Earth is the only planet in the solar system to have water in its three states of matter: liquid (oceans, lakes, and rivers), solid (ice), and gas (water vapor in clouds). From a distance, Earth would be the brightest of the eight planets in the solar system; this luminous effect would be because of the sunlight reflected by the planet's water.

Earth is also the only planet in the solar system known to be active with earthquakes and volcanoes due to plate tectonics; these events form the landscape, replenish carbon dioxide into the atmosphere, and erase impact craters caused by meteors.

Earth has a slight equatorial bulge -- a difference between its equatorial and polar diameters -- because of the centrifugal force from the planet rotating on its axis. The equatorial diameter is 12,756 km, but the polar diameter is 12,714 km; this results in the Earth's circumference at the equator being 40,075 km, while the polar circumference is 40,008 km.

the physical World: the Earth is composed of three layers: the outer lithosphere (with its crust and uppermost solid mantle), the mantle (the thickest layer, with distinct upper and lower layers), and the core (with an outer liquid core and inner solid core). Researchers have discovered that a transition zone (at a depth of 410-660 km, or 255-410 mi) between the upper and lower mantle may well be hydrous (water-bearing). But this hydrous rock would neither feel wet nor drip water. At that depth, high temperatures and pressures structurally transform a mineral called olivine into another mineral, ringwoodite, which can incorporate water, but not in a liquid, solid, or gas form. The high temperatures and pressures cause: 1) water molecules to split, creating hydroxide ions (hydrogen and oxygen atoms bound together), and 2) create structural changes in ringwoodite so that it is able to contain hydroxide ions. Ringwoodite can hold 1 to 3% of its weight in hydroxide ions. Considering that the 250 km-transition zone is about 7% of the Earth's mass (the crust is only about 1%), it could include at least as much (if not several times more) water than all the Earth's oceans.