The world’s ocean is a single, continuous body of saltwater covering approximately 71% of the Earth’s surface. This global water system is conventionally divided into distinct regions for geographical, navigational, and scientific purposes. These divisions allow for detailed study of the unique physical and biological characteristics of different marine areas. This article will identify and contextualize the third largest oceanic division, exploring its unique geography and the methods used to determine its size and boundaries.
The Order of the World’s Oceans by Size
The five recognized oceans are ranked based on their total surface area. The largest is the Pacific Ocean, which stretches across more than 165 million square kilometers and contains over half of the free water on Earth. Following the Pacific is the Atlantic Ocean, which occupies the second rank with a surface area of approximately 85 million square kilometers.
The third-largest ocean is the Indian Ocean, covering about 70.56 million square kilometers, or nearly 20% of the global ocean’s total surface area. It is significantly smaller than the Atlantic, but much larger than the remaining two divisions. The fourth-largest is the Southern Ocean, which encircles Antarctica, and the smallest is the Arctic Ocean, located in the northern polar region.
Defining Features of the Third Largest Ocean
The Indian Ocean is uniquely defined by its geographical containment, as it is the only major ocean largely landlocked in the Northern Hemisphere. It is bordered by Asia to the north, Africa to the west, Australia to the east, and the Southern Ocean to the south. This northern boundary prevents the Indian Ocean from extending into the Arctic, distinguishing it from the Atlantic and Pacific Oceans.
A characteristic of this ocean is its strong, seasonal monsoonal climate pattern, which dictates the direction of its surface currents. The currents reverse semi-annually, flowing in one direction during the summer and the opposite direction in the winter. This predictable, alternating pattern historically facilitated early long-distance sea trade between Africa and Asia.
Geologically, the Indian Ocean is home to an active, inverted Y-shaped system of mid-oceanic ridges where the African, Indian, and Antarctic tectonic plates converge. The deepest point is the Sunda Deep in the Java Trench, which plunges to approximately 7,258 meters off the coast of Indonesia. The ocean’s surface waters are generally warmer than the other oceans, contributing to its unique chemical properties, including a high negative water balance due to high evaporation rates.
How Ocean Size and Boundaries Are Determined
Defining ocean boundaries and quantifying their size is primarily overseen by the International Hydrographic Organization (IHO). This intergovernmental body publishes Limits of Oceans and Seas (S-23), which establishes the names and geographical limits for all major oceanic divisions and their adjacent seas. These limits are drawn using geographical reference points, such as coastlines, meridians of longitude, and parallels of latitude, to ensure a consistent standard for cartography and navigation.
Ocean size is quantified based on surface area, calculated by integrating the defined geographical limits. Volume and average depth are secondary measurements that provide additional context. The IHO’s most recent efforts included the formal recognition of the Southern Ocean, which necessitated revising the southern boundaries of the Indian, Pacific, and Atlantic Oceans. This formalization, based on the influence of the Antarctic Circumpolar Current, confirmed the Indian Ocean’s current third-place ranking.