I put the disable gameoption in so it wouldn't effect scenarios.
The thaw function (if its enabled) runs through the map when the game starts. If the plot can be "frozen" it saves the origional terrain type, applies a random 10-100 timer on the plot, then converts the plot to a frozen terrain type. Each turn it counts down one on the timer and when the timer reaches 0 it converts back to its true type.
If loaded with a scenario it would freeze the map for that scenario but after 100 turns all the plots would be back to their origional map values.
We can also use this mechanic to allow spells to temorarily modify the terrain (like maybe an illian spell that refreezes the world, or a malakim spell that spreads deserts) or have a unit transform the terrain he passes over to another terrain type for a few turns.
