The cracked eggshell
Earth's surface is like a cracked eggshell — rigid plates floating on the soft interior, slowly drifting. Where those plates meet, the ground shakes and mountains and volcanoes are born; plate tectonics is that slow-motion jigsaw.
A crust that drifts
Earth's rigid outer shell is broken into tectonic plates that float on the hotter, flowing mantle beneath. They move a few centimeters a year — about as fast as your fingernails grow. Where plates meet, geology gets dramatic.
- Divergent: plates pull apart, new crust forms (mid-ocean ridges).
- Convergent: plates collide, building mountains or subduction zones.
- Transform: plates grind sideways past each other (the San Andreas Fault).
Lab · Classify the boundary
- Two plates spread apart under the Atlantic — which type?
- The Indian plate crashes into Asia, raising the Himalayas — which type?
- Two plates slide past along a fault — which type?
What you should see: Spreading = divergent; colliding = convergent; sliding = transform. Most earthquakes and volcanoes cluster along these boundaries.