Sometimes it’s the littlest things…

I enjoy winter hiking—I enjoy the crisp air, the opportunity to hike without sweating too much, the snow on the peaks. 

One of my favourite winter phenomena is frost heave. This is when moisture in the soil freezes. Since water expands when it freezes, the ice crystals push soil and rocks upward. We get frost heave at home, but in the mountains, where there is both more water and colder temperatures, the phenomenon can be spectacular.

On a cold Matariki morning a few weeks ago, I snapped a photo of five-centimetre-long ice needles near Foggy Peak. Each needle was topped by gravel—the whole top centimetre or more of the sloping surface lifted. As the sun rose and melted the ice, every rock fell a few centimetres downhill from where it started. I imagine this process happening daily all through winter—a slow-motion conveyor belt shifting the mountain downhill. 

Meanwhile, higher up on the mountain, water seeping into the cracks in rocks and then freezing shatters them day by day into smaller fragments to be added to the icy conveyor belt.

It is such a small thing, frost heave. But its slow action has a big effect. 

The Southern Alps are rising at a rate of 10 to 20 millimetres per year—some of the fastest rising mountains in the world. If no erosion had ever occurred, the mountains would currently stand over 20 kilometres tall. Our tallest mountain, Aoraki Mount Cook, is 3754 metres tall. 

Of course, when we think of erosion, we think of the big events like landslides and rock avalanches. These events can be spectacular. 

On 14 December 1991, a rock avalanche on Aoraki lowered the summit by 10 metres over the course of a few hours. Fourteen million cubic metres of rock and ice tumbled down the mountain at speeds of up to 300 kilometres per hour. The shock waves from the landslide were recorded on seismographs as far as 58 kilometres away.

But without frost heave, the 1991 Aoraki rock avalanche might never have happened. Frost heave slowly weakened the rocks, slowly snapped them into smaller and smaller pieces, slowly shifted their weight. Centimetre by centimetre, those little ice needles brought the mountainside down.

I like to think of frost heave as a metaphor. Each of those tiny ice crystals, by itself, can move a pebble, and together they bring down mountains.