The calvings away from the glacier front are in movement, much more than the glacier itself. Here is a detail where an iceberg looses a relative small chip, that topples over.
Note that this chip stays more or less in the same position, while the mother iceberg is thrown back quite a distance. Apparently(!) in violation of Newton's Third Law (the action=-reaction thing).
The two images are aligned to one particular iceberg on the left, all movements are relative to this iceberg.