Geology And Geophysics
The Baltic Sea

The Geology and Formation of the Baltic Sea

The Baltic Sea
Jose Juan Gutierrez's image for:
"The Geology and Formation of the Baltic Sea"
Caption: The Baltic Sea
Image by: Chino
© CC-BY-SA-3.0 via Wikimedia Commons

The Baltic Sea is a brackish body of inland water situated above the northernmost part of Europe. The Baltic Sea is bordered by a number of European countries and connects to the Atlantic Ocean through the Skagerrak Strait via the Kattegat Bay. More than 250 freshwater streams drain into the Baltic Sea, with additional freshwater coming from precipitation, which add to the low salinity levels of the sea. Due to its location in the northern hemisphere, the Baltic Sea is usually ice-covered for around 45 percent of its surface area during a typical winter season. The Baltic Sea is believed to have been formed by glacial melt during the last ice ages.


The Baltic Sea is located between the mainlands of Northern Europe to the south and Sweden, in the Scandinavian Peninsula, to the west. Finland, Russia, Estonia, Latvia and Lithuania form the east boundary of the Baltic Sea. Poland and Germany form the southern border, and Denmark and its various islands form the most southwestern end of the Baltic Sea, and connect the Baltic Sea to the North Sea through the Skagerrak Strait. The Gulf of Bothnia is on the northern region of the Baltic Sea. The Gulf of Finland connects the Baltic Sea to St. Petersburg, in the east. The gulfs of Riga and Gdansk are located in the southeast and south, respectively.

Sea ice

During a typical winter, the Baltic Sea is covered by ice on approximately 45 percent of its surface area. The area that is covered by ice includes the gulfs of Bothnia, Finland, Riga and the archipelagos in between. The rest of the Baltic Sea does not freeze during a regular winter. The ice cover reaches its maximum extent during the months of February and March, with an ice thickness of about 70 cm (27.5 inches) in the northernmost regions of the Gulf of Bothnia. The ice extent depends on the severity of winter, but there have been times when the ice cover has permeated as far as the Danish Straits.


The Baltic Sea has many tributary freshwater rivers. It is estimated that an approximate number of 200 rivers flows into the Baltic Sea. Freshwater, along with the salty water entering through the Danish Straits, create a gradient of salinity in the Baltic Sea. The salinity of the water is greatest at the Danish Straits, although not totally salty because of the outflow of freshwater. The salinity of the water in the Baltic Sea decreases toward the north and east direction, although toward the Gulf of Bothnia, the water ceases to be salty. The salinity gradient, along with a temperature gradient, limits many animal and plant species in the Baltic Sea. In the northernmost part of the Baltic Sea, where the water is fresh, many freshwater species thrive.

Post-glacial emergence

The Baltic Sea lies on a region formed by glacial erosion during the last ice ages. In the past, much of modern Finland was part of the Baltic Sea. In the present, the land where the Baltic Sea is located is emerging in a process known as post-glacial rebound. This phenomenon causes the surface area and the depth of the sea to decrease. The uplift in the northernmost part of the Bothnia Bay is of about 8 mm every year. At this rate, it is estimated that in a very short period of time, large land areas will emerge from within the Baltic Sea. Geological studies have demonstrated that in the past, the region where the Baltic Sea is situated was a wide plain crossed by a river.

It’s estimated that an approximate number of 85 million people live within 10-50 km (6.2-31 miles) off the coast of the Baltic Sea, and about 90 percent of these numbers live along the coastline. The geological formation of the Baltic Sea resembles that of an estuary with lots of freshwater rivers flowing into it. According to, the Kiel Canal in the northernmost part of Germany is one of the world’s busiest man-made waterways. The Kiel Canal connects the Baltic Sea to the North Sea, saving hundreds of miles in travel time and reducing costs in marine transportation.

More about this author: Jose Juan Gutierrez

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