| The title suggests this is unusual, but it's not really. The simplest way to look at a glacier's changes over time is mass balance. A glacier accumulates mass, principally from snowfall in winter, which compacts into ice and flows downhill. It losses mass through what's called ablation - for a typical mountain glacier this is principally surface melt in summer, or for a glacier in polar regions it may be ice calving into the sea. For obvious reasons, the accumulation zone for a mountain glacier will be at higher altitudes, and the ablation zone will be at lower altitudes. The equilibrium line altitude is the altitude at which mass lost equals mass gained. As Mount St. Helens' glaciers disappeared following the eruption, the glacier(s) will start to regrow as snowfall causes ice accumulation above the equilibrium line altitude. Eventually, as the articles hints, the glacier will come back* into mass balance. I've simplified this all somewhat, as there can be lots of other factors that influence mass balance (as the article mentions, rock cover will reduce surface melting). *I say come back, as because of the natural variations in climate from year to year, mass balance is never steady-state. |