Glaciers are huge masses of ice that gradually flow from mountains down into valleys.
A mass of land ice that, due to gravity, flows downhill (via internal deformation and/or sliding over the bedrock) and, due to internal pressure and friction, is bounded from below and on both sides. At higher elevations, snow accumulation causes glaciers to grow; at lower elevations, this is balanced out by melting or their crumbling into the sea. Glaciers are formed when, over an extended timeframe, more snow is deposited in a suitable land form during the winter half of the year than can melt in the subsequent summer half. Currently, this is only the case in the polar regions and the high mountains. Here, complex transformation chains are set off. Through melting and freezing, snow is compacted into old snow. When the pore volume subsequently decreases further and the density rises, firn is formed. Once the firn is air-free, it becomes ice. In the context of glaciers, a distinction is drawn between the accumulation or collecting zone and the ablation or dissipation zone. The dividing line between the accumulation and ablation zones is referred to as the Equilibrium Line Altitude.
As their name implies, glaciers gain mass through snowfall, firn and ice formation in the accumulation zone, while the ablation zone is characterised by melting processes and loss of mass. Beyond melting, mass can be lost through wind drift, sublimation, and snowslides.