If the glacier mass is accumulating faster than it is melting, in other words if the glacier mass balance is positive over a number of years, it will normally result in the glacier snout advancing. Similarly, if the glacier mass balance is negative over a number of years, it will normally result in the glacier snout retreating.
The position of the snout is also influenced by other factors, particularly the bottom topography under the glacier ice, and whether the glacier is covered by moraine or not. For example, a glacier with a negative mass balance can lose ice through decrease of ice thickness without the position of the snout changing.
Some glaciers show a so-called surging behaviour. Such glaciers do not have a straight relationship between the mass balance and the snout position. They may have a stagnant snout position over many years, followed by a very extensive and rapid advance, which in turn is replaced by a (rapid) retreat.
Some glaciers may advance simply because more of the ice mass moves downslope, without any change, or even with a reduction, in the total ice mass. The change is thought to be due to increased elasticity resulting from an increased internal temperature of the ice (similar to heating thick sugar syrup). The glacier then becomes longer but thinner.