Why can a small fire be put out by placing a beavy blanket over it

Fire is an exothermic reaction, called combustion, which is composed of four parts typically referred to as the Fire Tetrahedron.  This is advanced from the former description of the Fire Triangle, which consisted of heat, fuel and oxygen.  The fourth part is a sustaned chemical reaction, which was changed as the triangle describes better what is needed to begin combustion, but the fourth allows for combustion to continue.

This movement is also referred to as  glacial plucking