When a glacier meets a body of water, it becomes a “terminating glacier,” categorized as either a tidewater glacier (flowing into the sea) or a lake-terminating glacier (flowing into a freshwater lake). This transition marks a shift in how the glacier loses mass, moving toward dynamic processes at the ice-water interface. This interaction drives the mechanical breakup of the ice and the redistribution of water and sediment. The primary distinction between these environments is the density difference between the glacier’s freshwater melt and the surrounding water, which is significant in saltwater but minor in freshwater.
The Mechanics of Calving
The most visible result of a glacier meeting water is calving, the mechanical loss of ice mass where large blocks of ice fracture and break away to form icebergs. This process is driven by physical forces acting on the terminus. The two primary mechanisms involve bending forces created by water buoyancy and the propagation of cracks within the ice mass.
One significant driver is buoyant flexure, which occurs when the ice thins near the terminus and approaches flotation. The surrounding water pressure lifts the edges of the ice, creating tensile stress at the glacier’s base. This stress causes basal crevasses to open and propagate upward, sometimes leading to a “full-thickness” or “bottom-out” calving event where the entire ice column breaks off.
Crevasse formation due to longitudinal stretching is another factor, occurring as the glacier accelerates near the terminus. This acceleration creates stress that fractures the ice, forming deep cracks that act as zones of weakness. Calving occurs along these lines, often triggered by water filling the crevasses, which exerts a hydrostatic pressure that forces the cracks to deepen and widen. The rate of calving is sensitive to the water depth and the thickness of the ice at the terminus.
Water Dynamics and Meltwater Plumes
Beyond the mechanical breakup of ice, a hydrodynamic interaction occurs where subglacial discharge meets the water body. This discharge, which is water flowing through tunnels beneath the ice, is cold and fresh, leading to the formation of distinct meltwater plumes. The characteristics of the plume depend on whether the glacier is terminating in saltwater or freshwater.
In a marine environment, the cold, fresh meltwater is significantly less dense than the surrounding saltwater, causing it to rise rapidly in a buoyant plume along the glacier’s face. As the plume ascends, it draws in warmer, deeper ocean water toward the ice front in a process called entrainment. This circulation of warm water promotes accelerated submarine melting at the ice face, causing undercutting that contributes to further calving.
In freshwater lakes, the dynamics are different because the density contrast is much smaller and depends on temperature differences. If the meltwater is colder than the lake water, it is denser and tends to sink along the lake floor, potentially forming a bottom current. If the meltwater is heavily laden with fine sediment, it can create a hyperpycnal flow, a dense, sediment-rich current that flows along the bottom. Both scenarios contribute to mixing and stratification changes within the lake, though the circulation seen in tidewater plumes is less common.
Geological Legacy: Sediment Deposition
Glaciers act as conveyer belts for rock and sediment, known as till, which is released when the ice melts or calves. The point where the glacier lifts off the bedrock and begins to float is called the grounding line, which is a major site of sediment release. As the glacier ice meets the water, the till it carries drops out of suspension and accumulates on the seabed or lake floor.
This continuous dumping of material builds up characteristic geological features. Submarine or subaqueous moraines are formed as ridges of sediment piled up at or near the grounding line. These moraines can grow large enough to form a shoal, which can temporarily stabilize the glacier terminus by reducing water depth and limiting the access of warm water.
Further out from the ice face, meltwater runoff, often highly turbid, deposits finer sediment in fan-like structures. In the marine environment, these become submarine fans, while in lakes, they form proglacial deltas. The resulting features provide a long-term geological record of the glacier’s maximum extent and its recession history.