[thedoc (OP)]

"Smith, Justine (TWP)"  asked the Naked Scientists:
   
Can I freeze a rock lump of say 400mm with liquid nitrogen and smash it into small pieces?
 
Regards,
Justine Smith

What do you think?

[chris]

Making something very cold can alter the internal stresses, making it more brittle. Bubbles of gas can shrink, composite materials (made of a mixture of different chemicals or minerals) can also be stressed as some substances alter their volume more than others introducing weaknesses, so I'd say it's plausible that dramatic changes in temperature can make a rock more likely to fail.

[yor_on]

Don't think so? Metal gets brittle but a rock? How about rocks in space?

[Matildasmith]

No i don't think that happens..rocks have a heavier mass and the molecules are closely packed with each other so there is a less chance of becoming brittle after heating and than instantly cooling it.

[yor_on]

Chris, I'm not sure at all. You have a real good point in that freezing something change the material. And heating by fire, and then cooling rocks by water was the first way, I think, people mined. But I'm not sure if that is the same though? It's steam they used, but they also used sudden temperature change. If you are right there then rocks in space should be comparatively easy to split into fragments due to their internal temperature though? And then there is transitions, isn't it so that depending on temperature a material can present several transitions, maybe a better word for it is 'emergences' I think, giving us different reactions in a material under stress?

"I’m a geologist who studies how rocks deform. So what am I doing in my kitchen? I’m in my kitchen because rocks deform in the same way a lot of other things deform.”

“When something changes its shape in response to stress, it’s been deformed. Deformation can be brittle or ductile.”

“Take a breadstick. Bend it. It breaks. That’s brittle deformation.”

“Take a caramel bar. Bend it. It changes shape, but it stays in once piece. That’s ductile deformation.”

“What controls whether something is brittle or ductile? Three things: temperature, composition and something called strain rate.”

“Let’s consider how temperature effects whether something is brittle or ductile.”

“Take a frozen stick of butter. Bend it. It breaks. That’s brittle deformation. Now take butter at room temperature. It flows like the caramel bar. That’s ductile deformation.”

“Now if we take a candle at room temperature. Bend it. It breaks. That’s brittle. But what happens if we warm it up a bit? If the candle’s warmed up a bit, it flows, like the caramel bar. Butter at room temperature and wax at room temperature deform differently because they have different compositions. But at lower temperature, both are brittle and at higher temperature, both are ductile.”

“So what do butter and wax have to do with rocks? Rocks are cold near the earth’s surface. And they get hotter and hotter with depth. Cold rocks near the surface tend to break if they’re stressed. They’re brittle. At about 15 kilometers within the earth, most rocks are hot enough to flow if they’re stressed. They’re ductile.”

“Let’s talk about strain rate, which is how fast deformation occurs. Different strain rates can cause rocks to be either ductile or brittle at the same temperature. Here’s how.”

“Take Silly Putty. If I pull it slowly it flows in a ductile fashion. Pull it fast. It breaks. Brittle.”

“Layers of sedimentary rock are just like these layers of Play-Doh. If I push on them slowly, they fold. Some mountains made of folded rocks look just like this.” "

So, maybe you're right?

[lightarrow]

Can I freeze a rock lump of say 400mm with liquid nitrogen and smash it into small pieces?

Yes, you only have to shoot it a cannon ball. But I suspect in that way you could achieve it the same at room temperature  []