Inside a Glacier: The Layers That Make Ice Move

The Perito Moreno Glacier in Argentina is over 19 miles long and moves forward almost 6 feet every single day. That's fast enough that if you camped next to it for a week, you'd need to move your tent. But glaciers don't move like a sled sliding on top of ice. They flow slowly from the inside, layer by layer, like thick honey being poured. To understand how a 200-foot-thick wall of ice can move at all, we need to look inside it.

- The four main layers found inside a glacier, from surface to bedrock - Why snow at the top slowly turns into solid glacier ice - How pressure and gravity make ice flow instead of just sitting still - What crevasses are and why they only form near the surface

A glacier starts as ordinary snowflakes. Each winter, new snow piles on top of old snow. The fluffy top layer is full of air pockets, just like the snow you'd make a snowball from. But under the weight of more snow piling on year after year, the flakes get squeezed. After about a year, that snow turns into a grainy, packed material called firn (pronounced 'fern'). Firn looks like sugary, compacted snow, and it's the halfway point between snowflake and solid ice.

Keep piling snow on top of firn for 20 to 100 years, and the air gets squeezed out almost completely. What's left is dense, blue-tinted glacier ice. It looks blue because the tightly packed ice absorbs red light and reflects blue light back at your eyes -- the same reason the sky is blue, but happening inside solid ice. This layer can be hundreds or even thousands of feet thick in places like Antarctica.

Deep inside the glacier, the ice is under enormous pressure from everything stacked above it. This pressure lets the ice behave almost like a very slow-moving liquid. The ice crystals actually slide past each other and stretch, flowing downhill under gravity's pull. Scientists call this internal deformation. It's why glaciers are sometimes described as 'rivers of ice' -- they genuinely flow, just thousands of times slower than water.

At the very bottom, the glacier meets the rock beneath it. Friction and pressure can melt a thin film of water right at this boundary, even when the air above is freezing. That meltwater acts like a lubricant, letting the entire glacier slide over the bedrock -- a process called basal sliding. As it slides, the glacier grinds and scrapes the rock below, carrying away boulders, gouging out valleys, and leaving scratch marks called striations that geologists can read thousands of years later.