Land uplift

Figure 1. Land uplift in Northern Ostrobothnia between 5000 BCE - 2000. Topographic data: (c) National Land Survey of Finland, 2012; Map design and animation: (c) Juha Oksanen, 2012 .

The Fennoscandian land uplift has been known for centuries. As early as 1491, residents of Östhammar city complained that the shoreline had moved so far away from the city that the old harbour was unusable. The phenomenon was known all over the coasts of the Gulf of Bothnia, where new land was appearing from the sea and the old harbours became unusable. The most famous of the early uplift researchers was Anders Celsius, who in the early 1740s determined an uplift value of 13 mm/yr near the city of Gävle. Although the value in Celsius is quite close to the current value, he did not understand the cause of the uplift. Rather, it was thought that the phenomenon was due to a lowering of the sea, not by the land uplift.

Using the sea level gauges, i.e., tide gauges (mareographs) or simpler water height scales, it is possible to monitor the sea level changes relative to the coast. The sea level observations at Stockholm began already in 1770's, first with a water scale. A permanently recording mareograph was installed in 1889, two years after the oldest mareograph in Finland, at Hanko. Only after Gerard de Geer published the Fennoscandian land uplift map in late 1800s, and when at the same time the phenomena caused by the Ice Age were studied, the mechanism and cause of land uplift were also revealed.

The height changes far from the sea can be monitored by means of repeated precise levelling. The first national levelling was carried out in 1892-1910; the second one in 1935-1975, and third in 1978-2006. Due to the repeated levelling, the land uplift is known very well in Finland. Similarly, the other Nordic countries had their own levelling campaigns, and the uplift is well known in the whole Fennoscandia. Today, crustal movements can be monitored using continuously observing GNSS stations. With GNSS, one can also see the horizontal movements due to the uplift.

The land uplift is fastest in the Quark area, nearly 1 cm per year. The smallest uplift in the territory of Finland is in the Southeast corner, where the speed is less than 3 mm per year. When one goes to St Petersburg, no uplift is visible anymore.

Figure 2. Fennoscandian land uplift (mm/yr) relative to the centre of the Earth.

When the ice retreated from Fennoscandia more than 10,000 years ago, the Earth's crust was depressed half a kilometre due to the weight of the ice which was 2 kilometres thick. When the ice melted, the crust began to rise, and the recovery still continues. Near the Gulf of Bothnia, where the ice was thickest, the current uplift rate is about 2 centimetre per year, and there is still some 100 meters to rise.

The land uplift is just one consequence of the phenomena caused by global glaciation. The Glacial Isostatic Adjustment (GIA) affects the crust and upper mantle of the Earth, but also the sea level height, the glaciation cycle and gravity changes. The shape of the Earth changes, the mass distribution changes, and at the same time, there is a slow mass flow in the mantle of the Earth. The mass transfer is huge, even on the scale of the whole Earth: the glaciation-deglaciation cycle of 100,000 years causes more than a hundred-meter sea level change. The total mass transfer is up to 5×1019 kg (this is nearly a ten-thousandth of the Earth’s mass!). Massive glaciers grow and melt, depressing the Earth's crust and releasing it again. This has continued for at least the last two million years.

Figure 3. Glaciation cycle and its effect on sea level and the Earth's mantle. (a) The situation prior to the glaciation.(b) Beginning of glaciation. Water moves from the sea to the glacier, sea level sinks, except near the glaciated area where the increasing gravity changes the shape of the geoid (sea level). The Earth's crust is compressed elastically. (c) Glaciation continues. The mantle flow start slowly, the mass of mantle is flowing out below the glacier, around the glaciated area there is land uplift. Sea level will continue to sink, except near the glacier. (d) The glacier melts, the water moves back into the sea, sea level is rising except near the melting glacier where the change is very small. In the Earth's crust, the elastic deformation is restored immediately. The mantle mass flow causes a slow land uplift; the situation is similar to the current uplift in Fennoscandia.

The last Ice Age was the strongest in the northern hemisphere, Fennoscandia, and northwest Russia, as well as in North America. In these areas, the land uplift shows a history of glaciation. In Greenland and Antarctica, the ice age continues.

Melting glaciers raise the sea level. The current global sea level rise is more than 1.5 mm per year, but in recent years, the rate has approximately doubled. A sea level rise in coastal areas reduces the observed land uplift rate. So far, the sea level rise has been smaller than the uplift rate at the Finnish coast, so new land will still appear from the sea. In particular, at the coast of the Gulf of Bothnia, the phenomenon is clearly visible.

The land uplift rate in Finland is precisely known due to repeated precise levelling and GNSS observations. However, we still do not fully understand the uplift mechanism and its effect on gravity change and uplift details. The Finnish Geospatial Research Institute conducts research of the land uplift and sea level rise at our coasts.

Figure 4. Land uplift of Finland relative to the centre of the Earth. The sea level rise is about 1.5 mm/yr, and therefore uplift relative to the sea level is smaller by that amount.