Snow is a material with unusual properties. On the one hand, it is easily malleable – or 'ductile', to use the scientific jargon – which comes in handy when forming a snowball. On the other hand, it is also very brittle: if you throw the snowball against a wall, it smashes into smaller pieces. Whether snow behaves in a ductile or brittle way depends on the speed at which a force acts on it.
However, there is also a third state, midway between these two, which researchers at the WSL Institute for Snow and Avalanche Research (SLF) have now investigated more closely. If you compress a snowball at a certain speed (known as the 'strain rate'), it triggers something which the researchers call 'microquakes'. The ball is deformed, and tiny cracks form in the snow as bonds between ice crystals break.
And yet the snowball does not break apart. This is due to a kind of repair process that takes place within fractions of a second: new bonds are formed between the snow crystals, which 'heal' the crack. This process of crack formation and healing is repeated periodically. The researchers used a force sensor to make this process visible: every time an (invisible) crack occurs, the strength in the snowball diminishes but then immediately builds up again. Eventually the pressure becomes too great and the snowball breaks apart.
But what has this got to do with earthquakes? Earthquakes are characterised by similar recurring cycles of strengthening and release of tension, known as the stick-slip effect: when two plates of the earth's mantle rub against each other and catch ('stick'), it generates a huge tension which deforms the rock. At some point the plates then jerk apart ('slip'), the tension subsides and the process begins again.
Based on this description of earthquakes, the SLF researchers developed a mathematical stick-slip model for snow microquakes. This model helps to better understand the ductile-brittle transition in failure behaviour, which among other things plays a role in avalanche formation.