Bedrock is the foundation of the Baltic Sea

The relatively flat seabed features of the southern Baltic Sea differ generally from the northern Baltic, due to the patchy/complex and fragmented seafloor of its coastline and archipelago areas. The differences in the seabed feature and structure between these areas are mainly due to their different bedrocks.

Areas with crystalline basement rock are diverse 

In the northern parts of the Baltic Sea, such as the coastal and archipelago areas of Finland and Sweden, the seafloor consists primarily of ancient crystalline basement rock. Tectonic lineaments and fracture zones divide this bedrock into smaller sections or blocks. This can be seen, for example, in the fragmented and complex landscapes of the Archipelago Sea and the Gulf of Finland.

In the southern Baltic Sea and the southern coast of the Gulf of Finland, as well as in the central Bothnian Sea and Bothnian Bay, the crystalline basement bedrock is covered by younger sedimentary rocks. These rocks often level out the depth differences on the seabed. In areas of crystalline basement, the seabed often forms more diverse and fragmented habitats than those covered by sedimentary rocks.

The main surface features of the seafloor were formed even before the last Ice Age

Several factors have all combined to influence the composition and the structure of the seabed of the Baltic Sea basin. These include the surface features of both crystalline basement rock and sedimentary rocks, glacial erosion and sedimentation processes, as well as modern erosion and sedimentation conditions. However, the primary features of the seabed were formed in the early stages of bedrock development, well before the most recent Ice Age.

 Distribution of bedrock in the Baltic Sea basin.
The bedrock of the Baltic Sea basin. In the northern Baltic Sea and particularly along the coast the crystalline bedrock is exposed, while in the southern and central parts of the basin it is covered with sedimentary rocks such as sand- and limestone. Kaskela & Kotilainen, 2017.

The Baltic Sea is one of the world’s shallowest seas

The Baltic Sea is an inland sea situated on the Eurasian continental plate and connected to the ocean through the narrow Danish straits. The Baltic Sea sits on the continental shelf and is very shallow overall, with an average depth of only 54 metres, making it one of the shallowest seas in the world.

Due to the shallowness and shape of its basin, the water volume of the Baltic Sea is small and water exchange is slow. Although shallow, it is still the second largest brackish water basin in the world, after the Black Sea.

 Depth variations in the Baltic Sea.
Bathymetric map of the Baltic Sea. EMODnet Bathymetry.

The bottom of the Baltic Sea is mostly flat 

In general, the seafloor of the Baltic Sea is quite flat. Flat areas i.e. plains, and basins cover two-thirds of the seafloor area. However, in some deep areas, e.g. the Landsort Deep (459 m), the Åland Sea trench (301 m) and the Bothnian Sea basin (293 m), the water depth can be hundreds of metres.

Other seafloor features, such as submarine valleys, sea holes and narrow canyons are typically found in certain areas. 

The seafloor areas of the Baltic Sea can be separated from one another based on seabed features and landscapes. If the  distribution of the subdivisions of the Baltic Sea were to be based solely on seabed features, then these new sections would look quite different and would also differ in size, compared to the existing and recognised subdivisions of the Baltic Sea.

Depth variation in a sea hole known as Paskamonttu, in the Gulf of Finland.
A sea hole known as Paskamonttu, situated in the Gulf of Finland, south of Haapasaari. The image is created using depth information produced by multibeam echosounder.

Canyons and valleys direct currents and transport water

Seafloor canyons, which are often formed in bedrock fault lines and thrust zones, are specific features of both the Åland and Archipelago Seas. The largest valleys and sea holes of the Bothnian Bay and the western Baltic Sea are commonly found in ancient bedrock fault zones.

In the northern Baltic Sea, submarine valleys and canyons correspond to submerged extensions of terrestrial river valleys. Many of these direct the near-bottom water currents and can transport both oxygen- and nutrient-rich water from one area to another. For example, the canyons cutting across the Archipelago Sea ensure water exchange between the main Baltic Sea basin and the Gulf of Bothnia.

The submarine valley systems mapped in the Danish straits and the western Baltic Sea run in the same direction as ancient river beds and glacial rivers.

Eskers, reefs, sills, and elevations shape the seabed

Seabed features also include elevations of many sizes, such as eskers, reefs, and sills. These elevations are often covered with clay, particularly in offshore areas. They are less eroded than those in shallower areas closer to the coast, where wave erosion is more pronounced. Locally, clay elevations on the seabed such as those found in the Bothnian Sea, provide an indication of moraines below.

These submerged moraines, deposited by the last continental ice sheet, have been exposed in the northern Baltic Sea, especially in the Kvarken area. Here, land uplift slowly but continuously reveals the deeper seabed deposits, exposing them to erosion by currents, waves, and ice.

Sandbanks occur in areas of the Baltic Sea with large sand deposits. For example, in the Bothnian Sea and Bothnian Bay, large glaciofluvial features, i.e. eskers, rise above the seabed as sandy ridges and large sandbanks belonging to Natura 2000 habitats.

Conversely, bedrock elevations are characteristic of archipelago areas, where the crystalline basement bedrock has been exposed to glacial erosion during the Ice Age.