19.5: What Is Glacial Advance and Retreat?

What Is Glacial Advance and Retreat?

Glacial advance and retreat are dictated by the amount of material entering the zone of accumulation Links to an external site. and exiting the zone of ablation Links to an external site.. The glacier will advance (i.e. extend its terminus Links to an external site. down the length of the valley) when accumulation exceeds ablation. For this to occur, more snow and ice must fall and accumulate than is melted or evaporated away. The glacier will recede (i.e. the terminus retreats up the length of the valley) when ablation exceeds accumulation. More snow and ice is melted or evaporated from the zone of ablation than is added to the accumulation zone. (Figure 19.7). It is important to remember that even in retreat, glaciers are always moving forward. You can liken their movement to the conveyor belts we put our groceries on at the store.

Cross-section of a valley glacier. Left, zone of accumulation where snowfall collects. Middle, the equilibrium-line altitude (ELA). Right, zone of ablation where evaporation and melting occur. Far right, terminus of the glacier. — **Figure 19.7:** The zones of accumulation and ablation. (CC-BY 4.0, Chloe Branciforte, own work)

Over the last century, we have witnessed many glaciers retreating worldwide. This has become an important marker for global climate change. Here in the US, many of our National Parks contain glaciers which are heavily monitored, including at Yosemite National Park here in California, but also Glacier National Park in Montana and the many National Parks and glaciated areas in Alaska.

Box 1, two stick figures talking, stick figure A says“Glaciers are so neat. You Can’t see it, but this ice is slowly advancing”. Box 2, stick figure B contemplates the glacier. Box 3, stick figure B walks away. Box 4, stick figure B returns with two swords. Box 5, stick figure B drills into the glacier. Box 6, stick figure B places a sword into the drilled hole. Box 7, stick figure lifts sword to fight glacier. — **Figure 19.8:** Comic from xkcd. (CC-BY-NC 2.5 xkcd Links to an external site.)