Rock Heaves

Why do rocks work their way up out of the ground? Ten years ago the spot in this picture was level without any visible signs of rocks or dips in the grass. Now there is this huge 200 kg (440 lb) rock emerging a little more each year as shown here. I can't get my riding mower over it anymore as I have in the past.

It turns out that this rock is lifted by ice buildup referred to as an ice lens below the rock. This is typical of a frost heave and apparently these don't occur due to the expansion of water due to freezing as I first thought, but due to the capillary action of water in fine soil that creates these ice lenses and it is brought on by alternating freezing and thawing near the frost edge. This happens in fine porous soil like our glacial till, but does not happen in sand or clay since neither allow capillary action of water. The soil also exerts pressure on the sides of the rock which by friction will hold up the rock. As the ice underneath melts and drains away it leaves an empty void and soil likely erodes and trickles in to partially fill this void from the sides before the rock settles back down although a little higher than the original position. 

What is interesting in this picture is the sink hole to the right of the rock that is about the same size in volume as the amount of rock protruding above the soil, so this soil is evidently creeping down under the rock over time. 


Extracting such a large rock out of the ground is a lot of work. Instead, one might let nature do the work over a longer time. One can fill the depression and build up earth around the rock to keep it lifting and just mow around it. At some point the rock will be lifted so high that all one has to do is remove the surrounding earth and take the rock away more easily. In the mean time it can be a conversation piece in the lull of a party and squirrels can use it as perch while nibbling on pine cones.  This is living with nature I figure.