I have a campaign coming up in which the adventurers will be traversing expanses on lava and I was wondering if a spellcaster could use a spell such as Sleet storm to create a bridge of ice on top of the lava. Or use another spell with a cold descriptor to cool the top layer of lava and form a bridge. Perhaps a spell like ray of frost. Sleet storm states that it "...causes the ground in the area to be icy. A creature can walk within or through the area of sleet at half normal speed with a DC 10 Balance check." I think something like ray of frost would be a bit of a stretch but wanted to see what kind of rules were out there in regards to spells effecting the environment based on descriptor.
Spells only do what they say they do, but the DM can improvise. That is, I am unaware of any spell that directly transforms lava into a surface that any creature can fearlessly traverse. Also, alternative ways magic can overcome this hazard are far too numerous to list.
In short, the game doesn't say that a spell like ray of frost or sleet storm freezes the surface of lava into a bridge, but the DM can say that such spells do. The problem, however, is that spells that can solve the same problem without DM intervention exist at equally low levels, so the DM's made the most powerful force in the game—magic!—more powerful, versatile, and unpredictable.
For example, the lowest-level spell that allows traversing the lava is the 0-level Sor/Wiz spell prestidigitation [univ] (PH 264). This article allows the spell to be used to grant an object for 1 hour fire resistance 2, and the rules on Lava Effects say that "resistance to fire serves as an immunity to lava or magma" (DMG 304). Simply casting the spell prestidigitation on a plank and a paddle means for 1 hour being able to traverse lava!
More reasonably (and not relying on what's probably an oversight), for example, the 2nd-level Sor/Wiz spell alter self [trans] (PH 197)—the same spell level as the spell sleet storm—can allow even a humanoid to fly over the lava if the caster knows of a winged humanoid.
This DM is always hesitant to expand spells' mandates, but if it's necessary for your plot, go for it. Just keep in mind that the smallest change can send ripples throughout the campaign. Six months from now when the town is threatened by a lava flow, do you really want the PCs stopping the disaster with a wand of ray of frost?
There are no rules for this.
How much "cold" a spell will generate and if that is enough to cool down the upper layer of a lava stream is an interesting thought experiment in applied physics.
I'd personally say use the Rule of Cool. If it makes your game better and your players don't abuse it, a wizard should definitely be able to freeze a bridge over a lava stream. Including the fact that the hot lava stream will eat that bridge sooner or later when the spell expires. Probably when the epic fight to hold said bridge is at it's peak. Epic story material. Go for it!
The Sandstorm and Frostburn books may be your best bet for a lot of this stuff. Sandstorm contains a lot of nifty information on high heat environments. Frostburn actually has more things that might be useful for cheesing your way through, in particular I saw a couple spells which you might be able to use for this -
Flash Freeze (Dr 2) - Drains heat from earth, stone + water turning it into everfrost/ice. Affects one 10 ft square to a depth of 1 ft per level. I suppose it's your call if lava is still earth/stone or if it is a new substance.
Leomund's Tiny Igloo (Sorc/Wiz 2) - Creates an igloo that holds 1 large, 3 medium or 12 small creatures. The igloo does not melt and maintains a constant temperature of 50 degrees F inside. Lasts 2 hrs/level. Cast the spell, flip over the igloo, break the roof and you now have an awkwardly shaped, temperature controlled raft that you can sail across the lava fields. Spell level is low so multiple castings could be accommodated for a larger party. The hp of the igloo is terrible (and 0 hardness) so not really an option if lava combat is going to be a thing. This also comes as a magic item with twice the size that is usable for 16 hours a day. Cost is 11k gp.
Wall of xxxx - There are some of these that don't need to be anchored and could potentially be used to create permanent boat objects as needed. Wall of Salt in Sandstorm might be a good candidate.
Blue Ice (also from Frostburn, p 80) is an ice with a melting point comparable to that of iron, which (in the real world) is much higher than the temperature of lava. Wikipedia tells me lava is usually 1,300-2,200 degrees F, while iron melts at around 2,800 degrees F. It also weighs 1/2 of what iron does, so you could probably make a boat out of it. Not sure what the material cost would be but probably fairly high. You might be better off making a raft? Alternately you could make some kind of skin canoe or boat out of a fire resistant material and then line it with this to make a better atmosphere on the boat/raft/turtleshell.
The Planar Handbook has clothing + paper that can survive on a primarily Fire plane. I would think these would survive in lava as the plane does 3d10 damage per round while lava is only 2d10. Also, there's not really any mention of whether or not these insulate so they may not light on fire but they may cause your skin to burn anyway.
Also in the planar handbook is a celestial cloak that gives resistance 1 to acid, cold and electricity for 1k gold. You could probably make an infernal cloak for a similar amount that would provide resistance to fire 1.
There's also a plain old Ring of Minor Energy Resistance for 12k gold that provides resistance 10 to fire, at which point the party could just swim through the lava.
I believe that by its very definition magic can defy or alter reality. So a spell with the "cold" descriptor could be used (with careful application) to make a bridge to cross magma. But magic sometimes (or at least it should and it does in my home brew) have consequences; usually dire consequences. Example: the ice bridge will only last as long as the spell. Once it wears off, the super heated magma has a violent reaction with the ice causing an explosion as the two opposing forces interact. The concussive force could strike the PC's. Steam envelopes the area briefly and could burn or temporarily blind the PC's. And so on.
Also consider what other posters have said: The environment in which you'd find rivers of magma are highly dangerous to humans (or elves, dwarves, etc). Not only is the heat an issue but the deadly gases and the low oxygen levels and high barometric pressure would pose a hindrance to put it mildly. Thus these would also need to be overcome as well.