Why do some rocks bend while other rocks break?

Answer 1

This depends on the confining pressure, the temperature and the strain rate applied to the rock mass.

In general, the lower the rate of strain, the more likely ductile or plastic deformation leading to bending or folding will occur. The higher the strain rate, the more likely brittle deformation is to occur, leading to the rock "breaking".

As the confining pressure increases, a materials shear strength will increase (this usually coincides with a greater depth of burial) and due to the Earth's thermal gradient an increase in temperature.

As the shear strength of the rock increases, the occurrence of brittle failure is less likely while the higher temperature means that plastic deformations are more likely to occur.