A boundary is a real or imaginary line that separates two things. In geography, boundaries separate different regions of 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 U.S. states it winds through, including Iowa and Illinois, Arkansas and Tennessee, and Louisiana and Mississippi.

Another type of physical boundary lies below Earth's surface. 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 a very active volcanic region. 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 Germany 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, and Nebraska. It also included parts of Minnesota, North Dakota, 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 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, each spoken by more than 10,000 people. The Indian government recognizes 22 of these as official languages. People who speak these languages are generally split into different geographic regions. The 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 United States–high-income country (HIC)–and the country of Mexico–a low- and middle-income country (LMIC)–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, Manhanttan's Upper West Side, in New York, New York, U.S., is a wealthy neighborhood with internationally recognized universities and hospitals. Melrose, also in New York, 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. Until 2019, in Saudi Arabia all women needed to have a male guardian to do common tasks. This guardian's approval was required for women to travel, seek healthcare, 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, many of the country's Southeast Asian migrant population live apart from 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 the people of southern Sudan voted to secede from Sudan as a separate nation, called South Sudan, 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.