I found this crumbly rock embedded in the channel walls of a small creek in Everett, WA, USA. (North Creek that flows through McCollum Park.) If possible, I'd like to know how the rock formed... what process leads to its formation? It's the crumbly part that I'm most curious about.
The entire thing is made of different types of 1-3 millimeter (or so) sized crystals.
I would have called it granite or some other relative; to my non-expert eye, I see feldspars, some quartz, and mica. Except that it's very crumbly. Here's a video that shows better the texture and how loosely-bound the crystals are to each other. You can flake them all off with your hand.
- https://youtu.be/FAMZ0do98yU <-- Very useful video
How is this formed? Are the minerals dissolved in the creek and they happen to have found a nice place to precipitate out? (If so, what would happen as time goes on? Would the rock grow larger with same crystal size, or larger crystal size but same overall rock size?)