Final answer:
When glaciers suddenly move forward, they are said to 'surge', a phenomenon that occurs when the glacier accelerates its flow, often due to lubrication from meltwater beneath it.
Step-by-step explanation:
When glaciers suddenly move forward they are said to “surge”. This movement is a relatively quick and dramatic shift in a glacier's pace and position, differing from the typical gradual flow. Factors influencing this can include the build-up of water pressure beneath the glacier that acts as a lubricant, enabling the glacier to glide over bedrock more easily. In Glacier National Park in Montana, glaciers have significantly retreated over time due to climate change. However, they can still experience surges when conditions allow. The glaciers in the park consist of bodies of snow and ice which move under their own weight as long as they exceed 0.1 km2 in size, which is around 25 acres. Glacier formation occurs when winter snowfall is greater than the summer melt, and they retreat when this trend is reversed.