Imagine a Venice like city, but instead of water, it had lava/magma flowing through it. Would this be possible? Materials? Society changes?
In Venice, the streets are made of water; gondolas carry people from point to point, and walkways stretching over the water allow foot traffic. The buildings in Venice are set on a wooden foundation, sunk in sandy islands; without access to oxygen, the submerged wood is kept fresh, and eventually petrifies. The city is, quite literally, built on the water. In fact, much of the old city is often flooded when the tides rise too high.
Venice sounds like a quaint, beautiful place, doesn't it? And everyone knows that any quaint, beautiful place can be improved by adding something hot, dangerous, and toxic, right? Sure! We'll call it Venasty.
The supports for Venasty could not be wood, obviously; they would have to be rock or metal with a very high melting point, much higher than the lava next to it. Possible, but only in a setting with enough magic or technology. Even current technology would be hard pressed to create a long-term solution.
Speaking of heat, lava has a lot of heat. It's molten rock. If you've ever been near a campfire, you've felt the convection energy - it can get pretty hot, and the closer you get, or the bigger the source, the more heat it transfers to you. Lava is like that, only all the time, and Venasty is full of it. Since you want these "lava-ways" to surround each building, the buildings and walkways will have to be quite a ways up for anyone to be able to consistently live above it without burning up. Venice is built within a few feet of the water; Venasty will have to be a city of stilts and high walls, hundreds of feet above the lava.
Transportation would not be possible in the lava flow, meaning foot traffic would have to carry the weight of produce, products, and livestock. Again: possible, but not a good place to bring your wares. It would be hard to supply those in the inner city with goods, and any heavy machinery would have to be carried in and built piece-by-piece, rather than carting it in a big wagon or barge. Building big, fancy buildings out of stone would take forever. Venasty would take a long, long, long time to build - much longer than even Venice.
What else does lava do? It stinks. And not just a bad smell - the gasses released by lava will kill you pretty quickly. It contains a lot of things, including hydrogen fluoride (a super-corrosive chemical), hydrogen chloride (causes choking and suffocation), as well as lots of stuff that may not kill you, but will make it really hard to breathe, like CO2 and SO2. There will need to be a system to vent these dangerous gasses outside the city; gathering them in chimneys and venting them above the city won't work, since they will simply fall back down and suffocate everyone.
Keeping it in
Water is easy to keep in, since the damage it does to its container is minimal and slow. Lava, on the other hand, will not only try to melt the contain it is in, but will also cool rapidly, hardening into various volcanic rocks. Which means the city will need a deep pit of either super-hot lava, or a moving lava flow that sweeps away any cooled chunks. The former would make for a very difficult location to build; the latter would require constant maintenance to keep flows hot and flowing.
Worse, the sides aren't the only part you have to worry about. Dropping water, or water-filled objects (like wineskins, barrels, animals, people, etc.) into lava will cause the water to very quickly boil. Depending on how the object landed and how quickly the water boils, the result may be quite violent, flinging steam and lava high into the air. If the city is high enough to be safe from the heat, it should be fine. Well, safe from lava being flung up by boiling water, anyway.
The resulting city will be situated hundreds of feet in the air, with natural or man-made caverns directing the heat and gasses from the lava away from the city. The supports and foundations of the city will require constant maintenance, costly in both materials and lives. The city would constantly be covered in ash and smoke. Life in Venasty would be hot, dangerous, and difficult. No one would want to visit. One wrong move, and you slip to your death; one bad engineer, and entire housing developments could collapse, burning everything inside.
In short: welcome to Hell.
Lava is not boiling coolaid.
You have to deal with convection, the fact that lava is really god damn dense so it would be hard to get anything to sink into it (or stand up straight if the flow pushed against it). And oh yeah, it expels toxic fumes.
You will not go lava surfing today.
First of all I don't believe that our current level of material science knowledge is sufficient to overcome this obstacle. In other words there's nothing we have that we could build those structures out of.
Incidentally this is also an issue when imagining a craft which might float on lava.
Last but not least, imagine living over a boiling river of lava - day in, and day out. The amount of heat generated would be staggering. Would people even be able to walk about without protective suits?
Has anyone watched the film, "Core"? 5 scientists and 2 pilots take a craft down into Earth's core to restore it's rotation. Only 1 scientist and 1 pilot returns to the surface. So my 2 cents here:
It would be hot and poisonous. We have fume hoods and natural gas and geothermal power plants, massively upscaling them, we could potentially harness the energy of the lava flow for either power generation or massive air conditioning.
Our knowledge of materials is enough to let us know that a suitable high melting ceramic sheath which is cooled internally by say, thermoelectric cooling or by refrigerants, could in fact be stable on lava. Also, lava is very dense, so it's easy enough to float on it even if your ship is very heavy (i.e., dense).
Our best bet would in fact have to be a kind of Venice where walls of buildings are internally cooled and are made of heat-resistant ceramics, such as those used for space shuttles. Also, the lava flow should be covered by some means, possibly by obsidian or quartz skin close to the buildings, which would prevent the air near these buildings from being contaminated.
Massive scale of air purification would be required. Chemical, physical, whatnot.
As for the foundations, the city could be a floating one, or it could built on stilts like @ArmanX's proposition. Maintenance would be a huge concern. The protective heat shielding would need to be continually replaced from the inside, so buildings would have to built with that in mind.
To fend off radiant heat, infrared-reflective coatings could be incorporated into the surface of the heat shielding.
Transport is feasible (by our current ideas of economy) only via aerial paths high above the lava to be reasonably safe, uncovered, from the lava's convective heat and poisonous gases.
Honestly, the main problem would be the populating it. I suspect it would be a ghost city, what with hair getting singed every time one went out on the balcony.
If humans ever had to colonize Io, I would tell them to look elsewhere.
Lava? Lava! Get the Lava out already!
That comic is true.
Since lava nearly destroys anything that comes into its path, it's pretty dangerous. When a volcanic eruption threatened to destroy the main harbour, people began spraying water all over it in an attempt to cool it.
Even if you kept lava in town, people wouldn't be able to survive. The ash from the lava, as well as the fumes it generates (such as Hydrogen Fluoride - that's an acid that can eat away at glass) would make for a deadly, toxic environment.
The trick is just controlling it.
Of course, even if you cool the lava, the heat would still stay: The rocks don't conduct heat very well - which is why Iceland is able to grow bananas :D
Comics source: Scandinavia and the World
You may have fun reading this comic as well.
OK, in an attempt to make this cool idea workable let's start with the answer from @Tamoghna Chowdhury that proposed floating buildings on the lava using similar thermal protection ceramics as the space shuttle used (note: modern variants, not those exact shields). Google confirms that the shuttle surface could handle 1,260 °C and Wikipedia quotes lava as ejecting at 700 to 1200 °C so yes, the city could float on the lava. In fact, given other answers about the density of lava this makes much more sense than trying to sink foundations.
Noxious gases are, I think, manageable - there's no denying that it would be difficult to pump fresh air in from miles away and redirect all of the fumes coming off the lava but it seems achievable in the sense that it's just scaling up what metal foundries and deep mines do already.
Which left me with radiant heat as the nail in the coffin for this coolest of ideas.
An interesting comparison is How much radiant heat does molten steel give off which provides us with some information about radiant heat in environments where the base material is at 1500 °C rather than the cooler 1200 °C of molten lava at ejection.
That page has anecdotal comments from a foundry worker about standing next to near-molten steel. The answer as to why this was possible is that the radiant heat is a factor of the "the temperature to the fourth power and the angular area (a.k.a. solid angle) to the first power". So it's less about how far away you are and more about how much lava is visible (feel free to step in and correct me if I'm hand-waving too much physics away). So yes, you can stand next to a small almost molten steel bar but there's no way you'd walk into the bowl of a volcano filled with lava that's 300 degrees cooler. It's the amount of lava that matters.
So. If we want to have our guests (what's the point in having a home floating on lava if you're not going to invite people back there?) to be able to roast marshmallows off the balcony the key is to keep the visible lava flows strictly minimised. What we're talking about here is essentially a set of large barges with narrow streams of lava just visible between them. Most of the lava is below the heat-radiant thermal shielding that has a very high exterior temperature to prevent lava build-up but doesn't transmit that heat.
What I'm not sure about is the life-times of those heat shielding materials. You're almost certainly going to need to 'drydock' barges from time to time to maintain their shielding. That said, if this works at all it'll work over a large area so you could have dry-dock facilities for individual barges as part of the city.
So, will it work? What am I missing?