Postcards from the Field

Close to the earth’s surface, rocks undergo brittle deformation by breaking and sliding along faults when subjected to external forces like when continents collide or break apart. However, just ~15 kilometers beneath the surface, higher temperature and pressure cause rocks to flow in response to these same forces and undergo ductile deformation. At these depths, the mineral grains and clasts that make up rocks accumulate penetrative strain and change shape, allowing the entire rock to also change its shape! The depth where the change in deformation style takes place is called the brittle-ductile transition.