It is amazing how long that is. My only experience with a belt was working at Walgreens on warehouse truck days. Even our 60 foot belt continually had product falling off of it. I wonder what kind of oversight is done to make sure the items stay on it.
So they probably put the rock in boxes and put the boxes on the conveyor belt. The tiny guardrails on either side should be enough to keep the boxes on the belt. Small pieces might spill over if the boxes aren't covered, but most of the cargo will arrive intact.
Check out this video of a 10.5km limestone conveyor belt in China [0]. Notice the rollers have a tilt to them? I found a better photo of the belt itself on PBS. [1] if you look on the left hand side of the photo you can make out a cylindrical object beneath the belt. That is the roller, it has the a tilt to it, just like the Chinese belt. Any vibration will result in the load gathering toward the bottom the the belt. Most of what we see on the ground is small particles that are swept off of the belt by wind (Used to live near a port and a concrete factory. Lovely air quality, S&W Ready Mix.)
[0] http://youtube.com/watch?v=Iq5TN_H_C4o
[1] http://www-tc.pbs.org/prod-media/newshour/photos/2012/12/21/...
Then you will have the TWO longest conveyor belts in the world.
Ka-ching!
[Edit: Typo]