natural action,
natural process,
activity,
action a process existing in or produced by nature (rather than by the intent of human beings); "the action of natural forces"; "volcanic activity"
freezing,
freeze fixing (of prices or wages etc) at a particular level; "a freeze on hiring"
liquefaction the conversion of a solid or a gas into a liquid
melt,
melting,
thawing,
thaw a relaxation or slackening of tensions or reserve; becoming less hostile; "the thaw between the United States and Russia has led to increased cooperation in world affairs"