I think the sparks come from the fasteners that attach the skid block to the chassis. Ten of them can be flush with the surface of the skid block.
3.13.2 Fasteners used to attach the skid block to the car must :
a) have a total area no greater than 40000mm² when viewed from directly beneath the car ;
b) be no greater than 2000mm² in area individually when viewed from directly beneath the car ;
c) be fitted in order that their entire lower surfaces are visible from directly beneath the car.
When the skid block is new, ten of the fasteners may be flush with its lower surface but the remainder may be no more than 8mm below the reference plane.
I'd guess that the fasteners take most of the "scratching load": the skid block can't lose more than 1 mm in thickness during the whole race, or so I read at regulations, so I wouldn't be scratching it liberally. Hence the sparks that seem to happen (I concurr with mx_tifosi) every time a car bottoms out.