The terms “sea” and “ocean” are often used interchangeably, but they represent distinct geographical and geological features of the planet’s saltwater system. The classification relies on specific scientific criteria concerning size, depth, location, and the nature of the boundaries that define the water body.
How Seas Differ from Oceans
Oceans are the five vast, continuous bodies of saltwater—the Pacific, Atlantic, Indian, Southern, and Arctic—that collectively cover approximately 71% of the Earth’s surface. These represent the deepest reservoirs of the planet’s water, with an average depth across all oceans nearing 3,800 meters. Seas, in contrast, are smaller, more confined divisions of the global ocean system. They are typically found where the open ocean meets the land, resulting in a significantly shallower profile. For instance, the Pacific Ocean’s average depth is approximately 4,028 meters, while the Mediterranean Sea has an average depth of only about 1,500 meters.
The Defining Geographic Relationship
The defining characteristic of a sea is its relationship with the continental landmass. The majority of seas are situated above the continental shelf. This location means that most seas have depths ranging from 50 to 200 meters, making them considerably shallower than the ocean basins. The proximity to land and the restricted exchange with the open ocean create unique physical conditions, leading to greater variations in both temperature and salinity compared to the stable conditions of the deep ocean. Furthermore, land proximity contributes significant sediment runoff from rivers and coastal erosion, resulting in a higher sediment load and turbidity compared to the open ocean.
Categorizing Seas by Enclosure
Seas are classified based on their degree of enclosure by land and their connection to the open ocean.
Marginal Seas
The most common type is the marginal sea, which is partially bordered by island chains, peninsulas, or archipelagos and lies directly adjacent to an open ocean. The Caribbean Sea and the Arabian Sea are examples that maintain a wide connection to a larger ocean basin.
Intercontinental Seas
A second classification is the intercontinental or Mediterranean sea, which is a large basin significantly separated from the main ocean by landmasses. These seas are connected to the ocean only by a narrow strait, which severely restricts water circulation and exchange. The Mediterranean Sea, connected to the Atlantic only by the Strait of Gibraltar, is the namesake for this category.
Inland Seas
The third type is often termed an inland sea, which is a body of saltwater completely landlocked without any natural outlet to the world ocean. While technically large saline lakes, they retain the historical “sea” designation due to their immense size or high salinity. The Caspian Sea, the world’s largest enclosed body of water, illustrates how historical naming conventions contribute to the “sea” designation even when lacking an oceanic connection.