Minefields decay slowly. See MineDecayRate, WebMineDecayRate, MinefieldDecay stage.
When two hostile minefields overlap, mine units from both fields explode until they do no longer overlap. Normal mines usually don't destroy web mines and vice versa. See AllowMinesDestroyMines, AllowMinesDestroyWebs, AlternativeMinesDestroyMines, MinesDestroyMines stage.
