Ocean plates sink; continents don't. When subduction swallows the ocean between two continents, they collide. And when a continent is pulled apart, it floats lower and splits. Buoyancy explains both.
Why it works
Continental crust is about $2.7\,\text{g/cm}^3$, far lighter than the mantle ($3.3$). It can be dragged down a short way, but it bobs back up, so a collision crumples and stacks the crust instead. Shortening a belt of crust thickens it (in cross-section, width × thickness stays the same), and thick crust floats high, with a deep root: the Himalayas and the Tibetan Plateau rest on crust about 70 km thick, double the normal. Stretch a continent instead and the crust thins; thin crust sinks, faults drop long blocks, and hot mantle rises and melts, building a rift valley. Stretch far enough and the continent splits and the sea floods in.
Key idea
Continental crust is too buoyant to subduct, so collisions thicken it into high mountains with deep roots (only about 18% of the extra thickness shows as height). Stretching thins it into rift valleys and, eventually, new oceans.
Worked examples
Worked example: Doubling the crust
A belt of crust 400 km wide and 35 km thick is squeezed to 200 km wide. How thick does it become?
- Width × thickness stays the same: $400 \times 35 = 200 \times H$.
- $H = 70\,\text{km}$, as under Tibet.
Answer: $70\,\text{km}$.
Worked example: How high does it float?
The crust thickens by 35 km. With crust $2.7$ and mantle $3.3$, how much higher does the surface stand?
- The extra thickness splits into height $h$ and root $4.5h$: $h + 4.5h = 35$.
- $5.5h = 35$, so $h \approx 6.4\,\text{km}$; in reality erosion and weaker crust keep Tibet at about 5 km.
Answer: About $6.4\,\text{km}$ in this simple model.
Common mistake
It is tempting to expect continents to subduct like ocean floor. They are too light to sink far, which is why collisions build the highest mountains instead of trenches. Check with fossils: marine limestone at the top of Everest is old seafloor scraped up between India and Asia, not continent that sank and came back.