A prismatic sphere doesn't destroy the surface
The Effect of Color column (as per the table appearing in the description of the spell prismatic wall) has the layer's first sentence's effect occur against attacks, objects, and effects that attempt to pass from beyond the prismatic sphere into the prismatic sphere. The layer's second sentence's effect occurs to any creature that attempts to push its way through the prismatic sphere.
The seventh violet layer stops any other kind of object or effect not covered by the previous layers that are launched from beyond the sphere from entering the sphere. Casting prismatic sphere while standing on a surface, because of that seventh violet layer, does not, for instance, obliterate the surface on which the caster's standing, causing the typical caster to fall 10 ft. so that the caster's at the bottom of the sphere.1
The prismatic wall table is underwritten. Pathfinder creative director James Jacobs in this 2010 Paizo messageboard post says
Objects, in this [prismatic sphere] case, refers to nonmagical non-living things that are used to try to breach the wall. Like thrown rocks, thrown tables, arrows, catapult boulders, and so on. Any objects or items or whatever that are "attended" (as in, carried or worn by a creature) are NOT destroyed, but travel with the person carrying/holding them off to whatever other plane that person ends up going to. If the person makes their Will save to avoid being sent to another plane, he can stroll right on through the wall with all his stuff intact.
Allowing prismatic wall to automatically destroy every object that passes through it, in other words, IS a bit excessive. The intent of the spell is to prevent anyone from making ranged attacks with weapons or spells or abilities against those on he other side, basically, not to provide a static disintegration wall. The limitations of the table format forced us to be a bit more brief than we should have been in describing it, alas.
Artifacts can't be destroyed by a prismatic wall unless the Destruction line of the artifact says otherwise.
Thus the prismatic sphere's layers have "their effects on creatures trying to attack you [with effects from beyond the prismatic sphere] or [on creatures that] pass through the sphere[, respectively]." However, this reader had to add all that bracketed information because, as Jacobs says, "The limitations of the table format forced us to be a bit more brief than we should have been in describing it, alas." (Note that Jacobs' alas actually dates back at least to the 2003 D&D 3.5e's description of the spell prismatic wall from which Pathfinder draws many spells.)
To be clear, the first red layer doesn't destroy mundane ranged weapons it catches within its area when the spell's cast nor does the third yellow layer prevent creatures caught within the area when the spell's cast from ever again using their poison or petrification special abilities. (And, instead, those two layers can deal up to 100 points of damage to a creature attempting to penetrate the sphere.) Similarly, the seventh violet layer doesn't destroy anything it touches when the spell's cast but, instead, destroys objects and effects launched from outside the sphere at the sphere. (And that layer, instead, can plane shift to random plane a creature attempting to penetrate the sphere.)
The spell isn't prismatic hemisphere, but it may seem that way
The spell prismatic sphere is still a spell, and a spell needs line of effect, and that line of effect is typically blocked by a surface on which the caster stands. That's why the description of prismatic sphere says, "Typically, only the upper hemisphere of the [prismatic sphere] globe exists, since you are at the center of the sphere, so the lower half is usually occluded by the floor surface you are standing on."
The word occluded here is a synonym for stopped. The sphere doesn't typically have line of effect to below the surface on which the caster stands, so usually "only the upper hemisphere of the globe exists" because the game assumes no one's casting prismatic sphere while flying.2 That is, if the caster's standing on a surface when the spell's cast, the spell's entire area—a 10-ft.-radius sphere centered on the caster—doesn't have line of effect past the surface. Instead, only the unobstructed area is filled with the prismatic sphere, and that means, despite the spell's name, the usual effect everyone sees is, in fact, a prismatic hemisphere.
So a prismatic sphere created in an endlessly long 5-ft.-wide-and-10-ft.-tall corridor, for instance, will appear to be a 5-ft. wide multicolored band starting 10 ft. down the corridor on one side the caster, arcing over the caster near the ceiling, and ending 10 ft. down the corridor on the other side of the caster. However, were the corridor somehow widened by 10 ft. in each direction, a multicolored hemisphere would appear around the caster. Then, were the surface on which the caster stood disintegrated, the multicolored sphere would appear around the caster! (And, then, of course, the typical nonflying caster would fall…) This is because, like antilife shell or globe of invulnerability, the prismatic sphere spell's effect is partially blocked by obstructions, yet the spell otherwise has its normal effect for its duration.
(A spell of the abjuration school (like prismatic sphere) doesn't need the same kind of clear space for its effect to occur that, for example, a spell of the conjuration school needs. However, an abjuration spell's effect is blocked by obstructions like any other, and its effect doesn't usually, for example, conform to the obstructions. That is, casting prismatic sphere while in room that's a 5-ft cube won't have any effect until line of effect can be traced to 10 ft. beyond the caster as the spell's entire area is occluded by the stupid room's walls!)
By extension this means if a caster creates a prismatic sphere while the caster stands on a surface, it's possible some creatures can bypass that prismatic sphere by traveling underneath the sphere. For example, a burrowing creature can safely reach a foe that's standing on a surface and that's within a prismatic sphere because there's never line of effect from the exterior of the sphere (that's 10 ft. from the caster) to the creature!3,4
Of course, a creature that does enter a sphere this way is now inside a prismatic sphere with a creature capable of creating a prismatic sphere, and that's a whole 'nother problem to solve.5
1 That'd be so embarrassing.
2 A level 17+ wizard that can fly? Poppycock!
3 How about instead casting prismatic sphere while flying?
4 Burrowing actually solves a shocking number of problems.
5 Burrowing causes this problem.
