Area dispel and area spells/SLAs centered on the caster

[Shin Okada]

From SRD "Dispel Magic",

For each ongoing spell whose area overlaps that of the dispel magic spell, you can make a dispel check to end the effect, but only within the overlapping area.

Assuming a spell/SLA's area is centered on a caster/user (like Magic Circle), and that area overlaps the area of dispel magic spell and the dispel check is successful. What will happen? Will the effect's area become something like, say, a crescent? What will happen if the caster/user moves away after that dispel? Does the area keep that shape?

 

[Rabulias]

The various magic circle against X spells are emanations, so the magic continuously emanates from the point of origin. I would say that the circle would be restored almost immediately. Whether that would give a trapped creature enough time to escape the circle is an interesting question, and would depend on the creature I think.

As an area dispel, you would need to include the point of origin in the area of effect and hope for the best.

Otherwise, you would want to use a targeted dispel magic on the point of origin to bring down a magic circle effect, which is the best way.

 

[Celebrim]

I concur largely with Rabulias - it depends on the sort of area of effect spell in question.

Spells like your example circle of protection are emanations, and I concur with his analysis that the point of origin must be included in the spell if only for the sake of simplicity and coherence.

Spells that have no point of origin I'd probably rule are in the area of effect if any part of them overlaps the dispel zone - for example, a wall of fire. Again, the goal here is simplicity.

The one area where I don't like that all or nothing interpretation is shapeable spells, which in 3.X are generally composed of some number of 10' or 20' cubes that can be arranged according to the caster's preference. For those, I'd tend to rule that dispel destroyed only those cubes which overlapped the area dispel. This is slightly more complex, but simple to adjudicate because the spell area breaks into clear components anyway, and for me feels 'right'. It's similar to how I'd rule an area dispel against a spell with multiple individual targets - only those targets within the area of effect are effected.

I don't know what the official rules are, if any, but I suggest the above because I know it would work in pretty much all cases. And it feels fair.