The Mediterranean Sea is not a lake; it is formally classified as a sea, an arm of the global ocean. The confusion stems from its nearly landlocked geography, as it is almost entirely surrounded by continental masses. Despite this confinement, the Mediterranean maintains a direct and active exchange of water with the Atlantic Ocean, a connection that defines its oceanic status. This unique arrangement makes it one of the most distinctive marginal seas on Earth. The characteristics that distinguish it as a sea are rooted in formal scientific definitions concerning its connection, size, and composition.
How Seas Differ From Lakes
The primary factor separating a sea from a lake is the connection to the world’s oceans. A sea is an expanse of saline water that is either partially enclosed by land or is a section of the ocean, but it must maintain a pathway to the global saltwater body. In contrast, a lake is a body of water completely surrounded by land, existing as an isolated, inland basin. Even the largest lakes, such as the Caspian Sea, are classified as lakes because they lack this direct, open connection to the ocean.
Salinity provides a second major distinction, as the continuous connection to the ocean ensures a sea remains substantially saline. Most lakes are composed of fresh water, though some, like the Great Salt Lake, are hypersaline due to high evaporation and no outlet. Furthermore, seas are typically much larger and deeper than lakes. The Mediterranean Sea, for example, is far deeper than most large lakes, with an average depth of approximately 1,500 meters.
The Mediterranean’s Link to the Global Ocean
The Mediterranean’s status as a sea is definitively established by the Strait of Gibraltar, the narrow channel that links it to the Atlantic Ocean. This strait, which is only about 14 kilometers wide at its narrowest point, allows for a continuous, two-way exchange of water. This exchange is a density-driven phenomenon resulting from the Mediterranean’s unique climate.
The high rate of evaporation across the Mediterranean’s surface exceeds the inflow of fresh water from rivers and rainfall. This constant surface water loss increases the salinity and temperature of the remaining water, making it denser than the water in the Atlantic. This density difference creates a two-layered flow through the Strait of Gibraltar.
Less dense, less saline Atlantic water flows eastward into the Mediterranean as a surface current, typically reaching depths of around 125 meters. Simultaneously, the denser, saltier Mediterranean water flows westward beneath this surface layer, forming a deep outflow into the Atlantic. This deep, westward flow, known as the Mediterranean Outflow, is a substantial current.
The presence of this sustained, two-layer hydrological exchange is the definitive mechanism that prevents the Mediterranean from drying up and becoming a hypersaline lake. The volume of water flowing in and out is approximately 20 times greater than the net annual water loss from surface evaporation. This continuous oceanic interaction confirms the Mediterranean’s classification as a partially enclosed sea.
A History of Being Landlocked
The reason the question of the Mediterranean being a lake persists is due to an event in its geological past. Approximately 5.96 million years ago, during the late Miocene epoch, the Mediterranean basin was temporarily cut off from the Atlantic Ocean. This geological isolation was caused by tectonic movements that closed the precursor to the Strait of Gibraltar.
With the Atlantic connection severed, the Mediterranean Sea, subjected to its high evaporation rate, began to dry out over a period of a few thousand years. This event is known as the Messinian Salinity Crisis (MSC) and lasted until about 5.33 million years ago. The basin experienced cycles of near-total desiccation, leaving behind a deep, dry basin with scattered, hypersaline lakes.
Evidence of the MSC is found in the immense layers of salt and gypsum, called evaporites, which lie beneath the current seafloor, reaching thicknesses of up to five kilometers in some areas. The crisis ended abruptly with the massive Zanclean flood, when the Strait of Gibraltar reopened, allowing the Atlantic Ocean to rapidly refill the basin.