One of the big answers to this question is, By the weathering of rock into small particles.
In nature, this happens slowly. In our laboratory demos, it happens fast--through the violent shaking of jars that quickly produces rock dust, for instance.
The violent shaking was provided by eager third and fourth graders. Whole lot of shaking going on.
Chicago's lakefront trail offers a peerless setting for exercise, and winter weather only improves it--rendering it quieter and starker.
A small but dauntless group of Ancona students is running along the lake every Friday afternoon this winter. The students choose to run; it's an elective class.
As the second photo shows, they're not afraid to do a little work on their core and upper bodies, either.