One of many classic simple traps, this one is triggered by being in the wrong spot, likely by stepping on a rigged tile. Once triggered, a big rock rolls out of somewhere and smacks the target. Bigger rocks do more damage, and make a scene like the one early in Raiders of the Lost Ark.
Detect: Per-based Traps at -2.
Disarm: DX-based Traps at -3.
Circumvent: Automatic (don't step on the trigger).
Evade: Dodge at -1.
Effects: 1d crushing.
Steal: No, though you could keep the rock. It's likely about 5 lbs.