The lithosphere is broken into about a dozen large plates and many small ones. They move a few centimeters a year, and almost all the action happens where they meet: the plate boundaries. There are three kinds.
| Boundary | Motion | Examples |
|---|---|---|
| Divergent | Plates pull apart; new lithosphere forms | Mid-Atlantic Ridge, Iceland, East African Rift |
| Convergent | Plates push together; one sinks under the other (subduction) or they crumple | Andes, Japan, Himalayas |
| Transform | Plates slide past each other sideways | San Andreas Fault in California |
Key idea
Plates pull apart at divergent boundaries, push together at convergent boundaries and slide past each other at transform boundaries. New lithosphere made at one kind of boundary is balanced by old lithosphere destroyed at another.
Why it works
Earth is not getting bigger. Every year seafloor spreading makes new lithosphere at divergent boundaries. The same amount must disappear somewhere: at convergent boundaries, dense oceanic lithosphere bends down and sinks back into the mantle. At transform boundaries lithosphere is neither made nor destroyed, only scraped past.
Worked example: Name the boundary
Along the San Andreas Fault, the Pacific Plate slides northwest past the North American Plate. What kind of boundary is it?
- The plates move sideways past each other, neither apart nor together.
- That is a transform boundary.
Answer: Transform
Worked example: Where does the old floor go?
The Atlantic floor is getting wider. Why isn't Earth getting bigger?
- New lithosphere is made at the Mid-Atlantic Ridge (divergent).
- An equal amount sinks into the mantle at convergent boundaries, such as the trenches around the Pacific.
Answer: Lithosphere is destroyed at convergent boundaries
Common mistake
Thinking plates moving apart leave an open gap. Hot rock rises and fills a divergent boundary as fast as the plates separate, so no gap opens. Meanwhile old lithosphere sinks at convergent boundaries, so Earth stays the same size. Check: the ocean floor at a ridge is solid new rock, not a hole.