Source of methane

methane is very readily to be burnt. Even some are produced from the combustion of long hydrocarbon/fossil fuel, the amount of methane is very small.

In cracking, methane is not the side product.
The purpose of carcking is to produce lighter hydrocarbons.
Unless you say the purpose of cracking is to produce alkene/hydrogen only ( quite non-sense)

other source of methane
i)decomposition of organinc matter(eg there is really a lot of methan in landfill sites)
[2006 marking:
increase in race paddies/ cattle. the remain/manure decays to form methane.]
ii)due to grobal warming, the solid methane in deep ocean is released to the atmosphere. The solid methane is formed by high hydrostatic pressure. When temperature increases slighly, the pressure is reduced, Thus the solid meathane escapes.[I don'e suggest you wirte this in your exam paper. It is just for reading. Markers themselves may not even know it.]

cracking may not be occur
the methane maybe come from the fossil fuel ( the mixture of hydrocarbon )