How rocks deform

Part of Hall of Planet Earth.

In the Museum's Hall of Planet Earth, a specimen and an explanation panel about how rocks deform under heat and pressure. AMNH/R.Mickens

Rocks seem to be rigid and unyielding, but given enough time, they can change shape, and even flow. Unlike cold, brittle rocks, hot rocks under pressure are ductile — they flow and deform like putty. The dramatic folds exposed on mountainsides and in the samples in this exhibit were created when rocks were deformed in this way. Similar folds are forming today, deep in the roots of mountains.