Geography
How Glacier Surges Differ from Normal Glacial Flow
Quick fact
During a surge, a glacier can advance up to 100 times faster than its normal speed—some have surged at 30–50 metres per day, compared to a few centimetres to a metre per day normally.
Why this is interesting
You’ve probably seen a glacier inching forward, but did you know some glaciers suddenly sprint, moving dozens of metres a day, before freezing in place again? What makes these icy giants break into a run?
Read the full explanation
Understanding How Glacier Surges Differ from Normal Glacial Flow
Imagine you’re watching a glacier like a slow river of ice. Normally it creeps along at a pace you’d only notice over weeks or months. But occasionally, some glaciers decide to sprint—this is a surge. Surging is an episodic, dramatic speed-up that can last months to a few years, after which the glacier returns to its slow state. The key difference is not just the speed—it’s the pattern: normal flow is steady, driven by gravity pulling the ice downhill, while surging is a violent, cyclical release of built-up stress. During a surge, a glacier can advance its terminus forward by kilometres, often riding over its own debris. Think of normal flow as a marathon and a surge as a sprint. But why do they sprint? The answer lies at the glacier’s base.
A deeper explanation
Glaciers flow because of two processes: internal deformation (the ice crystals slowly re-aligning under pressure) and basal sliding (gliding over the ground on a layer of meltwater). In most glaciers, the rate of sliding is controlled by how much water lubricates the bed. In normal conditions, water pressure is relatively uniform, so flow is steady. But in surging glaciers, the subglacial drainage system can change abruptly. Ice normally compresses sediment at its base, squeezing out water, but if water from surface melting or rain gets trapped, pressure builds. This high-pressure water can lift the glacier off its bed, dramatically reducing friction. The ice then surges forward, like a car on a hydoplaning layer. Additionally, some surging glaciers sit on deformable sediment (like clay or silt). When enough water builds up, this sediment becomes weak and fails, allowing the whole ice mass to slide over it. This process is not continuous—it follows a cycle. During the quiescent phase (which can last decades to a century), the glacier accumulates ice in its upper reaches but its terminus may stagnate. Then, for reasons often still not fully understood, something triggers a rapid transfer of mass to the terminus. The surge ends when the ice thins or the drainage system reorganizes, releasing the water pressure and restoring friction. This mechanism matters because it shows that glacial behaviour is not purely climate-driven; internal dynamics can cause rapid ice movement that doesn't correspond to temperature changes. Understanding surges also helps interpret ancient glacial landforms and predict potential hazards like sudden crevassing and ice avalanches.