Marine deposits and landslides

Quick clay (and other types of sensitive clay) can be found in Norway in areas with marine deposits, and is therefore classified as a marine clay. This is clay that was originally deposited in salt water, but after the post-glacial uplift can be found near sea level or above. Groundwater discharge has gradually reduced the ion concentration of the sediment's pore water; that is, the electrically charged particles have been washed out. These particles serve to stabilize the weak clay structure, and without them the clay becomes unstable.

The quick clay deposits found in Norway are the result of its glacial past. The land mass was weighed down by thick ice sheets, allowing sea water to flow over the land as the glaciers withdrew. The highest former sea level in a given area after disappearance of the ice is called the marine limit (ML).  The level varies throughout Norway. The highest ML can be found in the Oslo region, where it reaches 220 metres above today's sea level, while in inner-Trøndelag the ML reaches 200 metres above sea level. In general, the ML decreases as one moves from the inland areas to the coastline. 

Picture: From the landslide at Byneset in Trondheim 2012.
From the landslide at Byneset in Trondheim 2012.

Large amounts of melt water brought mud that formed thick deposits in the fjords and sea. After the ice age, landmasses that had been under the pressure of huge ice sheets rose, and the layers of clay deposits were lifted above sea level. Occasionally, other types of material were deposited over the clay. Long term flows of freshwater caused quick clay to form in some areas. River erosion is a common trigger for quick clay landslides in geologic time . Evidence of prehistoric quick clay slides is usually found under the Marine Limit, and is a natural part of the long-term landscape evolution. Today, human interference is the most common cause of quick clay slides.

Distribution and occurrence

In Norway, marine clay is most often found in Trøndelag and in Eastern Norway, but can also be found throughout the country. The prevalence of marine clay, including quick clay, is restricted to the areas under the marine limit. It is well-known that there are similar deposits of quick clays in parts of Canada and Sweden. Some of the most notable quick clay slides in Norway include the Verdalsrasset, in 1893, when 116 people died and Rissa slide, in 1973, which resulted in one fatality.

Video: The amateur film from the landslide in Rissa 1978.

 

Landskapsutvikling som følge av landheving
Landscape formation as a result of uplift