How fast can rocks weaken?

Okay, currently in my story, the characters are trapped in a chamber with a big rock wall blocking the exit. Swiss-Army Weapon girl wasn't able to smash it with a hammer. So their strategy is to use Elemental Powers to heat and freeze the rock rapidly for it to weaken then use the hammer to smash it again which I'm planning to work.

But would this be practical? Let's just ask if you have a rock and put it in fire to heat it up then the freezer to.... well..... freeze it. Repeat the above. Then how long would it take to just half the rocks toughness?

edited 3rd Mar '11 4:46:20 PM by PsychoFreaX

Important: What is the rock's composition?

Depends on the rock I guess. /non-helpful answer

I would think this is one of the situations were speed of plot is perfectly acceptable.

Also, if water is available as an elemental power, once you have cracks in the rock, filling them with water and freezing it would make things go faster.

edited 3rd Mar '11 6:04:53 PM by Luthen

Yep, the big question is details about the rocks. Different type will react differently, and at different speeds. Something porous like sandstone or shale will break and shatter, not simply weaken. Granite will probably only be affected on the surface, so it would be more like "Treat the surface, then chip that part off, then do it again, and so on."

^ Exactly. Though when it comes to granite it depends heavily on the crystal make up and structure and how large the crystals are. (The larger the crystals of any mineral in granite, the harder the granite is to break.)

Beware however, some forms of rock are much harder than they let on. Aggregates and conglomerates are called "nature's concrete" for a damn good reason.

True that. Aggregates and conglomerates might well be weakened faster by direct application of water at high pressure in a narrow jet to the "concrete" part.

How powerful is the freezing power? Almost everything becomes incredibly brittle if you can freeze it below -173C or so.