I am with Deadmanwalking (and Iago) on this one.
Spells have two natures in this game, Rule and Manifestation. The rules by which the spell was created define its direct and intended effect. The way in which the spell manifests tells us its secondary effects, and in most cases these secondary effects have no specific rules associated with them and so common sense and player/GM agreement must be used. For instance a water, fire, force, air, or earth attack spell are all exactly the same in this game in terms of how you cast them and the damage they do (Rules). However, they all manifest in completely different manners and so have different side effects, or act differently based on the enemies resistances.
In this case the rules say that the spell creates a zone border that then makes movement between zones more difficult. That is the RULES portion, and the RULES portion only. The way in which it manifests varies based on element. (One idea for each, there are many ways to represent each element though).
Fire = a (spell power*3') wide patch of fire that one must jump over to get safely to the other side. A failed roll lands you in the fire and you take damage and back out, stuck still on the wrong side. This may give the scene an "On fire" aspect, and the player may invoke this aspect to use the smoke as a chance to increase their defense roll against ranged attacks.
Force = a thin force barrier that pretty much uses the same rules wards use for bouncing people who aren't strong enough to get through, except it uses a movement roll, not an attack roll, to see if the person gets through. People trying to pass it casually will always fail their first attempt, bumping their nose most likely. No matter what the power level this probably also acts as a Block: 1 against ranged attacks, as such attacks will pass through, but are slowed and possibly deflected a little so their accuracy is effectively decreased.
Air = A bunch of miniature tornadoes that move around randomly, forcing the person passing through to dodge them or be knocked back and onto the ground. The dust and debris kicked up could be invoked to help avoid being shot.
Water = A series of waves that crash down towards the enemy, each (spell power*1.5') tall. Pushes back on those crossing the border if they fail their roll to do so. Also acts like force, providing a Block:1 most likely, as it will affect targeting and the attacks trajectory.
Earth = A wall of height equal to (spell power*1.5'), so a 4 power wall will be 6' tall. The wall has all the effects of a normal wall of that height and so can make targeting, and therefore ranged attacks, impossible. However, if the wall can be shot over or around then the player must get up close to it for a "Behind Cover" aspect to use it as a defense. There is no need for Block or armor unless the attacker tries to shoot directly through the wall. Then I would say the wall would act as Armor:1. If the PC wants it to be stronger, just suggest it is not that thick of a wall so the armor protection is minimal.
However, if the PC wanted a guaranteed block he would just have to build it into the spell by putting some points to each affect. Then perhaps it would be a small wall of earth that shifts and grows to intercept anything crossing its designated line of protection, including ranged attacks.
