A gulf is a portion of the ocean that penetrates land. Gulfs vary greatly in size, shape, and depth. They are generally larger and more deeply indented than bays. Like bays, they often make excellent harbors. Many important trading centers are located on gulfs.

Gulfs may be formed by movements in Earth's crust. The planet's tectonic plates may rift, or break apart, creating a gulf. Or one plate may fold under another, a process called subduction. Subduction may create a gulf by making downfolds, or troughs, in the rock under the ocean.

Gulfs are sometimes connected to the ocean by narrow passages of water called straits. Gulfs can also have wide openings and are sometimes indistinguishable from larger bodies of water.

Major Gulfs

The Gulf of Mexico, bordered by the United States, Mexico, and the island nation of Cuba, is the world's largest gulf. It has a coastline of about 5,000 kilometers (3,100 miles). The Gulf of Mexico is connected to the Atlantic Ocean by the Straits of Florida, between Cuba and the U.S. state of Florida. It is connected to the Caribbean Sea by the Yucatán Channel, between Cuba and the Mexican peninsula of Yucatán.

The Gulf of Mexico is an important economic site for all three countries. The process of upwelling occurs near the Florida coast of the gulf, creating a rich variety of sea life. Upwelling is the process in which cold, nutrient-rich water from the bottom of the gulf is brought to the surface.

Fish and other organisms thrive in areas of upwelling. Commercial, sport, and recreational fishing thrive in the Gulf of Mexico. Oil deposits sit beneath the western Gulf of Mexico. Both Mexico (in the Bay of Campeche) and the U.S. (mainly around the coasts of Texas and Louisiana) have oil wells in the Gulf of Mexico.

The Gulf Stream, one of the most powerful ocean currents in the world, originates in the Gulf of Mexico. Gulf ports, including Houston, Texas; New Orleans, Louisiana; Veracruz, Mexico; and Havana, Cuba, continue to be important cities where goods are imported and exported by sea.

The Gulf of Mexico is also the site of strong storms. Hurricanes and other storms need warm water to develop. The Gulf of Mexico is a very warm body of water, so storms can often increase their strength. Cuba and Florida are regularly hit by hurricanes on their Atlantic and Gulf coasts.

Pollution also threatens life in the Gulf of Mexico. Oil shipping and drilling can spill tons of petroleum into the ecosystem. Two huge rivers, the Mississippi in the U.S. and the Grijalva in Mexico, empty into the gulf. Chemicals used for agriculture and industry have seeped into the water, helping to create one of the largest dead zones in the world. (A dead zone is a region where there is little oxygen or life beneath the surface of the ocean.)

River management has redirected the flow of the Mississippi River. Canals, dams, and drainage systems for agriculture and industry have provided power and irrigated land. They have also reduced the wetlands at the rivers mouth and delta. The Gulfs wetlands slow storms as they move toward land. The loss of these wetlands may have contributed to the destruction brought by Hurricane Katrina to the Gulf Coast between central Florida and Texas in 2005.

The Gulf of Carpentaria, on Australia's northeast coast, is an inlet of the Arafura Sea. Because the sea and the gulf are shallow, the exchange of water between the two is reduced. Sediment collects at the mouth of the gulf, forming underwater barriers. The low shore is bordered in some areas by wetlands and swamps.

This shallow gulf with a wide mouth creates the conditions for a yearly spectacle called the Morning Glory Cloud. In September and October, sea breezes from the Gulf of Carpentaria and the Arafura Sea meet and create an enormous, fast-moving cloud over the gulf. The Morning Glory Cloud can be 1,000 kilometers (621 miles) long and move at a rate of 60 kilometers per hour (37 miles per hour).

The Persian Gulf is an arm of the Arabian Sea bordered by Iran, Iraq, Kuwait, Saudi Arabia, Qatar, Bahrain, the United Arab Emirates, and Oman. Vast deposits of petroleum in this region make the Persian Gulf strategically important. Middle Eastern countries depend on the gulf for trade and for access to the Indian Ocean. All countries that consume oil from the region, including the U.S., have a vital interest in keeping the gulf open to shipping.