Pressure | Forces

[kingwinner]

1) "Cooler temperatures and lower pressure near the earth's surface cause rock to respond to stress by breaking, formaing fracture or fault."

I understand cooler temperatures cause rocks to break instead of bending (because the rocks get more rigid), but how does lower pressure on the rocks contribute to breaking instead of bending? (while higher pressure deep within the earth causes rocks to bend)

2) "There are 2 opposing forces: The force of gravity of the mountain (force of crust on mantle) and buoyant force (force of mantle on crust). If these two forces are balanced, the crust moves neither up nor down. However, when weight is added, it sinks until a balance of the forces is again reached..."

Are these 2 opposing forces an action-reaction force pair? What makes me think that it is, is that they are acting on opposing objects and what makes me doubt is that sometimes these 2 forces are not balanced! (some thing popping out of my head is that action and reaction forces are always of the same magnitude?!?! I honestly don't remember...I haven't studied physics for more than 1.5 year)

[UncleAl]

Brittle materials fail in tension by cracking. Under high pressures cracks cannot open.

[Qfwfq]

In a rigid solid, however, pressure isn't defined quite as in a liquid, where it is independent of direction and there can't be shear, flection or torsion strains. Pressure in the right direction may cancel out a tension that would otherwise be produced by the above types of strain.

Originally Posted by kingwinner

What makes me think that it is, is that they are acting on opposing objects and what makes me doubt is that sometimes these 2 forces are not balanced! (some thing popping out of my head is that action and reaction forces are always of the same magnitude?!?! I honestly don't remember...I haven't studied physics for more than 1.5 year)

Action and reaction, as in Newton's axiom, are certainly equal and opposite but in applying the principle one must count the ma term.