Encyclopedic Entry

Like many boundaries, the thin line between the Earth's atmosphere and outer space is constantly changing.

Image ISS023-E-58455, courtesy the Image Science & Analysis Laboratory, NASA Johnson Space Center, http://eol.jsc.nasa.gov

Maritime Boundary
A maritime boundary divides the ocean into areas controlled by different governments or no governments at all. The United Nations Convention on the Law of the Sea establishes a maritime boundary no more than 200 nautical miles (370 kilometers/230 miles) from a nation's coastline.

Personal Boundary
Personal boundaries are the physical and emotional boundaries a person establishes around himself or herself. Different people have different boundaries: Some people reject most physical contact, such as a handshake, upon greeting. Other people embrace when they meet.

Boundary Survey
A boundary survey establishes the exact property lines of a parcel of land. Boundary surveys are carried out by surveyors and engineers using historical records, field observations, and careful measurement.

A boundary is a real or imaginary line that separates two things. In geography, boundaries separate different regions of the Earth. There are many different types of boundaries.

Physical Boundaries

The most obvious type of boundary is a physical boundary. A physical boundary is a naturally occurring barrier between two areas. Rivers, mountain ranges, oceans, and deserts can all serve as physical boundaries. Many times, political boundaries between countries or states form along physical boundaries. For example, the boundary between France and Spain follows the peaks of the Pyrenees mountains, while the Alps separate France from Italy.

The Strait of Gibraltar is the boundary between southwestern Europe and northwestern Africa. This narrow waterway between the Atlantic Ocean and the Mediterranean Sea is an important political, economic, and social boundary between the continents.

Rivers are common boundaries between nations, states, and smaller political units such as counties. The Rio Grande forms a large part of the boundary between Mexico and the United States. The Mississippi River is the defining boundary between many of the states it winds through, including Iowa and Illinois, Arkansas and Tennessee, and Louisiana and Mississippi.

Another type of physical boundary lies below the Earth’s surface. The Earth’s shell, or crust, is made of thick slabs of rock called tectonic plates. There are seven major tectonic plates and many smaller ones. These plates are constantly moving.

Interaction between tectonic plates creates activity on their boundaries. Sometimes, the plates spread apart from each other, creating ocean trenches and, eventually, continents. This is called a divergent boundary. Sometimes one plate slides underneath the other, creating volcanoes and earthquakes. This is called a convergent plate boundary. Sometimes the plates grind past each other, creating earthquake fault lines. This is called a transform fault or transform boundary.

The movement between the massive Pacific plate and the plates that border it creates all three types of boundaries. This tectonically active area is called the Ring of Fire. The divergent boundary between the Cocos and Nazca plates creates the Galapagos Ridge, off the coast of South America. The convergent boundary between the Pacific and Australian plates makes the island nation New Zealand very volcanically active. The transform fault between the Pacific and North American plates makes the U.S. state of California prone to earthquakes.

Political Boundaries

Political boundaries are the dividing lines between countries, states, provinces, counties, and cities. These lines, more often called borders, are created by people to separate areas governed by different groups. Sometimes, political boundaries follow physical boundaries, but most of the time you can’t see them. Most maps show political boundaries.

Political boundaries change over time through wars, treaties, and trade. After World War II, the map of Europe was almost completely redrawn. Germany’s eastern border was moved farther west, and the country itself was later divided into East and West Germany.

In 1803, the United States bought 2,147,000 square kilometers (828,800 square miles) of land in a treaty with France. This land, the Louisiana Purchase, expanded the size of the U.S. to include the areas that are now Arkansas, Missouri, Iowa, Oklahoma, Kansas, Nebraska, and parts of Minnesota, North and South Dakota, New Mexico, Montana, Wyoming, Colorado, and Louisiana. The western boundary of the U.S. moved from the Mississippi River to what is now Yellowstone National Park.

An important type of political boundary in the United States is the boundary of a congressional district. A congressional district is an area that elects a representative to the U.S. House of Representatives. After the U.S. Census, which is taken every 10 years, the population of a state may grow or shrink enough to gain or lose a representative in the House. When this happens, congressional district lines are redrawn in a complicated and controversial process called redistricting. The boundaries between congressional districts may unite or divide economic, social, or ethnic neighborhoods.

Other Boundaries

Political boundaries are just one type of artificial, or man-made, boundary. Other boundaries created by people include linguistic, economic, and social boundaries.

Linguistic boundaries form between areas where people speak different languages. Often, these boundaries match political boundaries. For example, the predominant language in France is French, and the predominant language in Germany is German.

In India, 122 different languages are spoken by more than 10,000 people each. The Indian government recognizes 22 of these as “official languages.” People who speak these languages are generally split up into different geographic regions. Inability to speak a neighboring region’s language can cause difficulties and tensions between people and businesses.

Economic boundaries divide people with different incomes or levels of wealth. Sometimes these boundaries fall on national borders. The border between the developed country of the United States and the underdeveloped country of Mexico is an economic boundary as well as a political one.

Sometimes, economic boundaries fall within a single country, and even within a single city. For example, Beverly Hills, in Los Angeles, California, is a wealthy neighborhood with internationally recognized universities and hospitals. Watts, also in Los Angeles, is a low-income neighborhood whose residents struggle to access the excellent education and health care available only a few kilometers away.  

Natural resources also play a role in economic boundaries. People who settle in areas rich in resources—whether it is underground oil or fertile soil—are more likely to become wealthy, while people who live in areas without many resources stay poor. People are also willing to pay more to live in areas with access to natural or economic resources: beautiful views, excellent schools, hospitals, and convenient access to shopping facilities.

Social boundaries occur where social differences lead to unequal access to resources and opportunities. Some of these boundary issues include race, gender, religion, and physical abilities. In some places, women may not have access to certain jobs or be allowed to travel in certain areas. In Saudi Arabia, for instance, all women must have a male guardian. This guardian’s approval is required for women to travel, seek health care, manage personal finances, marry, or divorce. This social boundary discourages many women from seeking leadership positions in business or government.

People of different races may be voluntarily or forcibly segregated into different neighborhoods. In Bahrain, political leaders have outlined plans to force the country’s Southeast Asian population to move to parts of the country where they will not live in communities with ethnic Bahrainis. Because most of Bahrain’s Southeast Asian population is made up of immigrant laborers, this social boundary is also an economic one.

Social boundaries can also form along religious lines. The nation of Sudan has many distinct religious social boundaries. Northern Sudan is mostly Muslim, southwestern Sudan is mostly Christian, and southeastern Sudan has more followers of animism than the other two regions. Sudan suffered more than 20 years of civil war, and South Sudan voted to secede from Sudan as a separate nation in 2011.

Linguistic, economic, and social boundaries are not as sharply defined as political and natural boundaries. These types of boundaries are often transition zones.


Term Part of Speech Definition Encyclopedic Entry



religious belief that there are spirits throughout nature.



natural or artificial line separating two pieces of land.

Encyclopedic Entry: border



line separating geographical areas.

Encyclopedic Entry: boundary



people and culture focused on the teachings of Jesus and his followers.



large settlement with a high population density.



one of the seven main land masses on Earth.

Encyclopedic Entry: continent



questionable or leading to argument.

convergent plate boundary


area where two or more tectonic plates bump into each other. Also called a collision zone.



political unit smaller than a state or province, but typically larger than a city, town, or other municipality.

Encyclopedic Entry: county



rocky outermost layer of Earth or other planet.

Encyclopedic Entry: crust



area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.

Encyclopedic Entry: desert

developed country


a nation that has high levels of economic activity, health care, and education.

divergent boundary


area where two or more tectonic plates are moving away from each other. Also called an extentional boundary.



the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.



having to do with money.



a crack in the Earth's crust where there has been movement.



able to produce crops or sustain agriculture.



budget, or money available for a specific project or goal.



physical, cultural, and social aspects of sexual identity.



study of places and the relationships between people and their environments.

Encyclopedic Entry: geography



to make public-policy decisions for a group or individuals.

House of Representatives


federal branch of Congress in the United States, with state representatives elected every two years.



person who moves to a new country or region.



wages, salary, or amount of money earned.



having to do with language or speech.

Louisiana Purchase


(1803) land bought by the United States from France, extending from the Mississippi River to the Rocky Mountains, and Canada to the Gulf of Mexico.



symbolic representation of selected characteristics of a place, usually drawn on a flat surface.

Encyclopedic Entry: map

mountain range


series or chain of mountains that are close together.



having to do with Islam, the religion based on the words and philosophy of the prophet Mohammed.



political unit made of people who share a common territory.

Encyclopedic Entry: nation

natural resource


a material that humans take from the natural environment to survive, to satisfy their needs, or to trade with others.



an area within a larger city or town where people live and interact with one another.

Encyclopedic Entry: neighborhood



large body of salt water that covers most of the Earth.

Encyclopedic Entry: ocean

ocean trench


a long, deep depression in the ocean floor.

Encyclopedic Entry: ocean trench

official language


language adopted by the government of a nation or other political unit.



fossil fuel formed from the remains of marine plants and animals. Also known as petroleum or crude oil.



the very top.



leading or most influential.



vulnerable or tending to act in a certain way.



arbitrary grouping of people based on genetics and physical characteristics.



a system of spiritual or supernatural belief.



someone or something who acts in place of a group of people.

Ring of Fire


horseshoe-shaped string of volcanoes and earthquake sites around edges of the Pacific Ocean.

Encyclopedic Entry: Ring of Fire



large stream of flowing fresh water.

Encyclopedic Entry: river



top layer of the Earth's surface where plants can grow.



political unit in a nation, such as the United States, Mexico, or Australia.

tectonic plate


large, moveable segment of the Earth's crust.



buying, selling, or exchanging of goods and services.

transform boundary


site of tectonic plates sliding next to each other in opposite directions. Also called a transform fault.

transform fault


boundary between two tectonic plates, where the plates are moving horizontally or vertically in opposite directions, not against or away from each other. Also called a conservative plate boundary.

transition zone


area between two natural or artificial regions.



official agreement between groups of people.

underdeveloped country


country that has fallen behind on goals of industrialization, infrastructure, and income.

U.S. Census


count of everyone in the U.S., conducted every 10 years.

Encyclopedic Entry: U.S. Census



an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.



large-scale armed conflict.



amount of money or other valuable materials.

World War II


(1939-1945) armed conflict between the Allies (represented by the United States, the United Kingdom, and the Soviet Union) and the Axis (represented by Germany, Italy, and Japan.)


Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.


Kim Rutledge
Melissa McDaniel
Diane Boudreau
Tara Ramroop
Santani Teng
Erin Sprout
Hilary Costa
Hilary Hall
Jeff Hunt


Tim Gunther
Mary Crooks, National Geographic Society


Kara West
Jeannie Evers

Educator Reviewer

Nancy Wynne


Caryl-Sue, National Geographic Society


Dunn, Margery G. (Editor). (1989, 1993). "Exploring Your World: The Adventure of Geography." Washington, D.C.: National Geographic Society.

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