I’m fascinated by this. A flow of lava will be traveling along, and stuff (particularly trees and other readily flammable objects) will burst into flame before the lava makes physical contact with it. What exactly is going on?
Almost everything has a ‘flash point’ – a temperature at which it will burst into flame in air. The temperature of lava is in the hundreds of degrees. Things in proximity to it will be rapidly heated by it, in much the same way as holding your hand near an open flame is generally an unwise degree.
If you keep in mind that lava can emerge from a vocano at well over 2000 degrees (F), then it’s not so surprising that it can ignite a nearby tree. Lava, after all, is rock that has been heated so high that it melts. Dude, that’s very hot. Melted rock. Yeah, that will set some wood on fire just by being near it. Remember that paper ignites at Fahrenheit 451.
Ever burn something in the oven? Now imagine a 2000-degree oven with a tree inside.
Thats thousands of degrees…
The term you are looking for is radiant heat.
Radiant heat. It isn’t just lava that can do this either. A wildfire spreads by radiant heat when it really gets going. Whilst we get taught that the radiant energy (i.e. light) falls off with the square of the distance, this is only true of point sources, or sources that can be treated as a point. One you get into the near field of the source it gets nasty. If the source is extended (i.e. actually has length or area) and your distance to the source is about the same as the source’s size, or closer, the rate of energy drop off with distance is much lower. In this near field, a radiating surface’s intensity remains constant with distance. So if you are ten feet away from a lava flow that is ten feet high, the radiant energy intensity is pretty close to the same as if you were one inch away. In a wildfire, if the trees are alight, and they are 30 feet high, at thirty feet away, you are already dead. This is why wildfires can travel faster than you can run. One of the common ways houses ignite in such a fire is that flammable material inside the house starts to burn from the radiant heat coming in through the windows. Even when the house itself isn’t alight.
To be precise, it’s intense radiation (at infrared and some visible wavelengths) raising the temperature of combustible objects (e.g. trees) above their auto-ignition temperatures.
Neato! So whats stopping us from making 1920’s style HEAT RAYS ? That pesky inverse-square rule making such a device ineffective at long distances?
/Was a pyro as a kid
//Likes the idea of a gun that makes things burst into flame 
Like a flamethrower?
I opened this thread in the hopes that the answer to the question in the thread title was actually more complex than “by being very, very gorram hot.”
Kinda disappointed now, actually.
Maybe, then, you could clarify your question. The answers appear to connect directly to your post. Very hot source and a high degree of radiant energy emanating from it. Try again.
Um, dude, I’m not the OP. 
Many of the lasers used for industrial cutting (or for potential weapons) are infra-red lasers. So we do have literal heat rays in use today. The main problem has been providing enough power to the system and making it portable enough.
The other problem with an infra-red laser is that it’s so focused that it tends to totally disintegrate the point of contact rather than causing it to burst into flame. But from an industrial and warfare point of view, you’d rather have it work that way - no point in burning the whole outside of the target when you can cut a precise hole.
I’d blame a total lack of awareness of the Rule of Cool in the military. Really, it’s pathetic. Like, they come up with heatless rays that make people feel like they’re on fire, except they’re not. This is completely, absurdly, embarrassingly backwards.
If you’re going to pour R&D money into that sort of thing, it’s obvious to *anyone *that the end product you want is a device that sets things on fire without any pain or discomfort. Then, you can light the Marine Corps on fire and ask them (politely, you know how hung up on decorum Marines can be) to calmly walk at crowd, while on fire, with a look of grim determination. There’s your perfect, non-lethal form of riot control. Who wouldn’t scamper instantly at the sight of pissed off Marines ON FIRE ?
Next up : why the Striker Brigades project was stoopid when for the same grants they could have made bear cavalry happen. Bear cavalry with heavy machine guns. And flamethrowers. Think about it, man.
That’s a coincidence, there’s a book with that title!
Don’t be. Radiant heat travels at the speed of light and requires no medium. Far more interesting than conduction or convection, the other types of heat transfer. (God, I’m such a dork.)
There’s a book called “Wood on Fire”? Cool! 
No, “Dude, That’s Very Hot”.
But it’s porn.
Regards,
Shodan
Isn’t that a Sarah McLachlan song?