Each kind of plate boundary builds its own landforms. Knowing the boundary tells you what to expect, and the landforms tell you what kind of boundary lies beneath.
| Boundary | Landforms | Example |
|---|---|---|
| Divergent, in an ocean | Mid-ocean ridge with a rift down its center | Mid-Atlantic Ridge |
| Divergent, on a continent | Rift valley, volcanoes | East African Rift |
| Convergent: ocean meets ocean | Deep trench and an arc of volcanic islands | Japan, Mariana Trench |
| Convergent: ocean meets continent | Trench offshore and a chain of volcanic mountains | Andes, Cascade Range |
| Convergent: continent meets continent | High folded mountains, no volcanoes | Himalayas, Alps |
| Transform | Long faults, offset streams; earthquakes but few volcanoes | San Andreas Fault |
Key idea
Ridges and rift valleys mark divergent boundaries; trenches with volcanic arcs mark subduction; high folded mountains mark continents colliding; long, straight faults mark transform boundaries.
Why it works
Where an ocean plate meets another plate, the denser ocean plate sinks into a trench. As it goes down, water squeezed out of it lowers the melting point of the mantle above, so magma rises and builds volcanoes in a line parallel to the trench. Continents are too light (buoyant) to sink, so when two collide, neither subducts: the crust crumples and stacks up into the tallest mountains on Earth.
Worked example: From landforms to boundary
A deep ocean trench lies just off the coast of a continent, with a chain of volcanoes on land. What kind of boundary is this?
- A trench means one plate is sinking: convergent.
- Volcanoes on a continent next to it: an ocean plate is sinking under a continental plate.
Answer: Convergent, ocean meets continent
Worked example: Why no volcanoes?
The Himalayas have no active volcanoes, even though India is still pushing into Asia. Why?
- Both plates carry continents, which are too buoyant to sink.
- Without a sinking ocean plate, the mantle doesn't melt, so no magma forms; the crust just crumples and thickens.
Answer: No plate is subducting, so no magma forms
Common mistake
Thinking all mountains are volcanoes. The Himalayas and the Alps are crust crumpled and stacked where two continents collide; they have no volcanoes. Check: volcanic mountains are cone-shaped and stand alone or in chains near trenches and rifts, while folded ranges are long belts of bent rock layers.