There's a little bit of controversy about this one, so we're just going to take it at face value, explain what you're seeing, and let you judge for yourself. This photo, taken by Darren Pearson, allegedly shows the path of a lightning bolt striking a tree and dissipating through the roots as the tree absorbs the shock. Pearson claims that he took the image on a long exposure, and therefore was able to chart the flow of the electricity through the tree.
If he's to be believed, then what we're seeing is a purple-white hot glow in the center of the tree as the bolt first strikes it, which then fades to blue as the charge passes down into the roots, before fading out completely. It seems incredible that a tree could survive such a blow, but they do it routinely. Many critics on the internet have suggested that the image is fake, and the details of the strike have been added in using Photoshop or other photo manipulation.