You have glaciers with surrounding ice sheets, all of which move down to the ice shelf on the coast and into the freezing ocean. But sometimes the ice doesn't agree on how fast to move. Most of it is really slow, perhaps 100 meters per year, but sometimes parts decide they want to go faster. These are ice streams, flowing from the sheet to the shelf at blazingly fast speeds up to 1000 meters per year.
It seems that people sometimes conflate the terms glacier and ice stream, but ice streams are usually on glaciers. Sure glaciers move, but as I already explained, ice streams move especially fast. They could even outrun a cheetah, if said cheetah had frozen to death earlier that week.
Not only are ice streams faster than dead cheetahs, they're bigger, too. 'Bigger', as in several kilometers
deep thick and 25 times that across.
One peculiarity of ice streams is that they seem to change speed without apparent cause, sometimes giving up all together. Why do ice streams slow, accelerate, and die? We don't entirely understand just yet. Theories exist, and we can account for some of the changes, but no conclusive answer has been found.
One reason ice streams are so fast is due to underground rivers (of water) help to keep the ice moving. The type of bedrock also makes a difference. But bedrock doesn't suddenly change from sedimentary to igneous. Tides play a role. But geology is complex. Lots of factors. Lots of mysterious activity.
So some ice is faster than the rest. Big deal.
It is a big deal! Ice streams make up around 10% of the ice sheets, yet around 90% of the ice that leaves the ice sheet for the ice shelf and eventually into the sea flow from these streams. Without this, the gulf stream will shut down, right?
Additionally, the differing speeds of the ice streams v. the surrounding area result in softer ice along the 'bank'. Crevasses form. Ever fallen into a crevasse? I didn't think so. It's a big deal when you do.