How hot does it have to get to melt a rock?
Some as low as 1200 degrees but Lava can go up around 2300. Air cools at a very quick rate so this video while being pretty amazing is probably very real. Like said above, You can get really close to this stuff depending on wind direction. Its hot down there but the size of this thing is whats fooling you guys. They're nowhere near as close as what it looks like. They're probably 300 yards away from the lava.
These guys had probably gone down in it a few times taking pictures and just one day decided to walk in front of their cameras.