There can be many probable causes of fire, however, they can be classified into 3 main categories, excluding arson: natural fires, accidental fires and accidental incendiary fires
One of the most well known causes of wildfires, a lightning strike has the potential to melt soil and sand into glassy lumps called fulgurites. It can also be devastating to the objects it strikes, splitting or exploding trees, peel bark from trees and even split rock.
Lightning can also simply strike the ground, causing nearby fuels to conflagrate. Lightning caused fires can also smolder for a long time before finally causing a fire, as will be mentioned in the next section about spontaneous heating.
Internal heating may occur due to biological and chemical reaction. This would mostly be caused by decomposing piles of organic material underground of inside old trees or stumps which a smoldering fire had been previously induced. These are Sleepers that may be induced by microorganisms or lightning and even have the probability of persisting for long periods of up to even a year.