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.
In the northern Baltic Sea and especially near its coasts, the sea rests on ancient bedrock composed of granites and other hard rocks. This crystalline bedrock is characterised by elongated features visible in the terrain, known as tectonic lineaments. In some places, there are fracture zones formed in ancient times. These zones of weakness divide the bedrock into smaller sections or blocks. This is evident in the fragmented and diverse landscape of the Archipelago Sea and the coast of the Gulf of Finland.
The situation is different in the southern parts of the Baltic Sea, as well as in the central parts of the Bothnian Sea and the Bothnian Bay, and the southern coast of the Gulf of Finland. There, the crystalline bedrock is covered by younger sedimentary rocks, such as limestone and sandstone, which often smooth out the depth variations of the seabed.
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 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 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 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.
The Baltic Sea bottom also features other, smaller-scale seabed formations, such as submarine valleys and rocky outcrops, sea holes and narrow canyons. These are particularly found in the areas of crystalline bedrock along the coasts of Finland and Sweden.
Paskamonttu
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.
A bathymetric map of the seabed from the Neugrund meteorite impact crater site off the coast of Estonia. The structure was formed circa 535 million years ago and is considered one of the best-preserved seabed asteroid collision structures in the world.
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.
The traces of the ice age are visible
The Baltic Sea region has experienced several ice ages over the past millions of years. Each of these ice ages has, in turn, erased nearly all traces of the warm periods between them, known as interglacials. As a result, there are hardly any seabed deposits from before the last ice age found in the marine areas.
There are abundant signs of the last ice age. The coastal rocks of Finland show the wear marks of the continental ice sheet. The coastal cliffs slope into the water and continue similarly below the sea surface. Elsewhere, the continental ice has accumulated moraine. As the land rises, the moraine formations slowly ascend and eventually emerge from the sea. Especially in the Kvarken region, there are many elongated moraine formations rising from the sea.
Sand ridges are found in those areas of the Baltic Sea where there are 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.
