A glacier is a body of dense ice that forms on land and moves slowly under its own weight. This movement is driven by the continuous cycle of mass gain and mass loss. The dividing line that separates these two opposing forces on the glacier’s surface is known as the Equilibrium Line (EQL). At this altitude, the annual accumulation of mass exactly balances the annual mass lost. The EQL partitions the glacier into two distinct regions: the accumulation zone above and the ablation zone below.
The Accumulation Zone: Mass Gain Above the Line
The accumulation zone is the higher-elevation region of the glacier and the primary source area for the entire ice body. Mass gain is driven by solid precipitation, primarily snowfall, added to the glacier’s surface. This new snow must survive the summer melt season to contribute to the glacier’s growth and flow.
The fresh snow begins an irreversible transformation into dense glacial ice through a process called diagenesis. The weight of subsequent snow layers compacts the snowpack, squeezing out air and causing the snowflakes to recrystallize and become granular. This transitional material, which is denser than snow but not yet fully ice, is known as firn.
The buried firn continues to compress over a period ranging from a few years to several decades. Eventually, the density reaches a threshold (around 830 kilograms per cubic meter), sealing off trapped air bubbles, and the material is classified as solid glacial ice. Minor accumulation processes, such as wind-blown snow, avalanching, or the refreezing of meltwater, also contribute to the mass budget in this zone.
The Ablation Zone: Mass Loss Below the Line
The ablation zone is the lower-elevation area where the glacier experiences a net loss of mass annually. Ablation refers to all processes that remove snow or ice from the glacier system. Since this zone is at a lower altitude, warmer temperatures make mass loss processes more efficient.
The most common form of mass loss, especially in temperate mountain glaciers, is surface melting and subsequent runoff. Solar radiation and warm air temperatures cause the ice to turn to liquid water, which then flows off the glacier surface or through internal channels. This meltwater removes mass directly from the system and contributes significantly to river flow downstream.
A less common, but regionally important, loss mechanism is sublimation, where solid ice directly converts into water vapor. This occurs most significantly in very cold, high-altitude, and arid environments. For glaciers that terminate in a body of water, the breaking off of ice chunks, known as calving, is a third major loss process, accounting for substantial mass loss in tidewater glaciers and ice sheets.
Glacier Health: Determining Overall Mass Balance
Overall mass balance is the net difference between the total mass gained in the accumulation zone and the total mass lost in the ablation zone, typically measured annually. When mass gain exceeds mass loss, the glacier has a positive mass balance, causing it to thicken and advance downslope. Conversely, a negative mass balance, where ablation outpaces accumulation, leads to thinning and retreat.
The position of the Equilibrium Line is an indicator of the glacier’s mass balance and its response to climate. If the EQL moves higher up the glacier’s surface, it signifies a smaller area dedicated to mass gain and a larger area dedicated to mass loss, resulting in a negative mass balance. Sustained negative mass balance over decades is evidence that glaciers are out of equilibrium with the current climate.
For a glacier to maintain a steady state—neither advancing nor retreating—the EQL must remain at a stable altitude over time. This state is often approximated when the accumulation zone makes up about two-thirds of the glacier’s total area. The upward shift of the EQL observed globally reflects the impact of warming temperatures, which increase the rate of ablation across the ice surface.