Rocks are made up of different compounds. Gases like nitrogen, oxygen and water vapour and some non-metallic oxides react with these compounds to form new compounds and then break down into fine particles, due to breaking of bonds between the atoms, caused by the reaction of gases.This leads to break down of rocks into fine particles
Some rocks are sedimental. They are soft. Rocks are made out of gases etc. So some of the gas in the air makes a reaction with the rock which helps break the rock down.
