USGS CMG InfoBank: Rocks Fold

Source: Annenberg/CPB Resources - Earth Revealed - 8. Earth's Structures

Keywords: structure, "James Sadd", rock, heat, pressure, deformation, fold

Our transcription: We usually think of rocks as being very hard and brittle.

They break when a force, such as the blow of a hammer, exceeds the strength of the rock itself.

But if rocks are very hot or under great pressure, or if they're exposed to stress very gradually over a long period of time, a rather surprising type of deformation takes place.

The rocks can actually bend or flow forming a geologic structure called a "fold."