I did an experiment based on stuff I saw at Pinterest about killing weeds, and while two of the three methods I tried were complete failures, one worked so well I'm happy to share it.
Pour straight vinegar (white vinegar is what I used) on the weeds. Broadleaf weeds turn brown and die in two hours. Thin blades and grass took a day. Completely dead. Once they're dead, they break off from the ground easily. So far, the roots in the ground have not produced new weeds.
This wouldn't be a good method in a garden or lawn, but if you've got bare rocks and weeds, buy the big bottle and have at it. Here,a gallon costs $2.50.