Rocks along active plate boundaries are subjected to physical stress. They can experience squeezing (compression), stretching (tension), or pushing in different directions (shear stress). How rocks respond depends on the type of stress, the rate at which it is applied, and the environmental conditions of the rocks, such as their temperature and depth.

Generally, rocks respond to stress in one of two ways: they break, or they bend.

• When a rock breaks, it is called brittle deformation. Any material that breaks into pieces exhibits brittle behavior.
• When rocks bend or flow, like clay, it is called ductile deformation. What conditions do you think would be necessary for rocks to respond like clay?

Rock features that form in response to stress are called geologic structures. Structures can be recognized at a variety of scales—some structures can be identified within a single hand sample, and some stretch over entire mountain belts. Structures can reveal the type of stress and environmental conditions to which rocks have been exposed.

from：http://www.classzone.com/books/earth_science/terc/content/investigations/es1102/es1102page01.cfm