Plates push, pull and slide, and the rock between them has to respond. Two words keep the cause and the effect apart. Stress is the force per area acting on rock, like pressure but possibly stronger in one direction. Strain is the change in shape or size that results.
- Compression squeezes rock together, shortening and thickening it: folds and reverse faults (colliding plates).
- Tension pulls rock apart, stretching and thinning it: normal faults (rifts).
- Shear pushes two sides past each other: strike-slip faults (transform boundaries).
Why it works
Whether rock bends or breaks depends on conditions. Near the surface, rock is cold and the confining pressure is low, so cracks can open and grow: rock is brittle and fractures. Deep down, heat lets atoms move and pressure keeps cracks closed, so crystals slowly slide and regrow: rock is ductile and flows, folding like warm wax. Speed matters too: a slow push for millions of years favors bending; a sudden one favors breaking. In quartz-rich crust the change from brittle to ductile happens at about $300^\circ\text{C}$, typically 10 to 15 km down.
Key idea
Stress (compression, tension or shear) causes strain. Shallow, cold rock under fast stress is brittle and breaks; deep, hot rock under slow stress is ductile and folds. Strain is measured as change in length ÷ original length.
Worked examples
Worked example: Shortening a belt
A belt of rock layers 80 km wide is squeezed to 60 km. What is the strain?
- Change in length: $60 - 80 = -20\,\text{km}$.
- Strain: $-20 \div 80 = -0.25$: shortened by $25\%$.
- The rock didn't vanish: the belt got thicker as it got narrower, which is how mountains grow.
Answer: $25\%$ shortening.
Worked example: Where earthquakes stop
With a surface of $15^\circ\text{C}$ and a gradient of $25^\circ\text{C/km}$, how deep is the $300^\circ\text{C}$ brittle–ductile change?
- $300 - 15 = 285^\circ\text{C}$ of warming.
- $285 \div 25 \approx 11.4\,\text{km}$.
- That is why most earthquakes in continents start in the top 10 to 20 km.
Answer: About $11\,\text{km}$.
Common mistake
It is natural to think solid rock can't bend. Given heat, pressure and time, it can: folded layers in mountain belts were bent while solid, never melted. Check a folded outcrop: the layers stay continuous around the bends, with no breaks, which only ductile flow can do.