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 scalessome 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
the two images for evidence of brittle and ductile behavior.
Move your cursor over the images to reveal outlines
of the geologic structures, then answer question 1 below.