Glaciers break down rock through freeze-thaw action.
This follows the same process as freeze-thaw weathering:
- Small cracks in the rock surface fill with water from precipitation or melted ice
- As temperatures drop, the ice on the surface is the first to freeze over
- When the rest of the ice freezes, it expands, putting pressure on the rock and causing the crack to become bigger
- This process is repeated as the larger cracks fill with water and freeze

Plucking often accompanies this process when glaciers are involved.
As a glacier move through a valley, pressure is put on the valley sides and bottom.
The friction from this creates heat, causing the edge of the glacier to melt.
Meltwater fills cracks in the rock surface and freezes.
As the glacier continues to move, it drags this ice with it, pulling away some of the rock.

As the rock is carried by the glacier, it can scrape away at the side of the valley and erode it.
This process is called abrasion.
Very large boulders being carried by the glacier can do more damage than smaller ones.