Its a terrifying idea, clay that sinks underneath your house and suddenly becomes liquid. This geological phenomenon appears to be the cause of yesterdays landslide in Ask, Norway, north of Oslo. Ten people are still missing, at least a thousand people have been evacuated and dozens of homes have been destroyed.
Kvikkleire is called it in Norwegian, fast clay. They are well acquainted with the phenomenon in Scandinavia, because it happens there more often. In 1702 a medieval town was wiped out in southern Norway, in 1893 116 deaths were somewhat north by a landslide leaving a crater of several kilometers, in 1978 two amateur filmmakers managed to capture the phenomenon on camera for the first time at the town of Rissa. This year there was another landslide in northern Alta where eight houses washed away. Also in Canada, the phenomenon occurs with some regularity.
Landslides can be caused by various causes, but according to a spokesman for the Norwegian Water and Energy Directorate, Ask was about such a fast clay shift. This clay has the special property of being able to collapse and become liquid through desalination and under high pressure. In this video you can see how that works.
The landslide at Ask created a large crater. A resident tells how he got into it with his car:
Rainfall can play a role in the collapse of the clay, says geologist Peter van der Gaag. “Landslides often occur after heavy rainfall, sometimes the rainwater can seep through the soft topsoil to the hard rock mass. The soft topsoil is then soaked, becomes heavier, and the heavy mud starts moving.”
Under the clay in Norway is often a hard rock bottom that does not allow water to pass through, which speeds up the process even more. “There can even be a running layer of water between the hard impermeable rock bottom and the overlying heavy muddy soft soil. In the end, the soft mass with the water layer becomes unstable and the soil moves”
According to the Norwegian broadcaster NRK, it has rained heavily in the area and the ground may indeed have become extra unstable. There is a fuss in Norway about why it was allowed to build on this unstable ground. In 2005, geological research had already shown that there was a risk of landslides, reported another Norwegian channel, TV2. Despite that report, houses were built three years later.
Geologist Tim van Hattum of Wageningen University & Research knows that heavy rain can cause more leaching of the clay. In the Netherlands we also have clay soils, but we dont have to be afraid of these kinds of landslides, he expects. “In our country there is much less height difference and no rock bottom. Here we have other problems, such as drought and salinization.”