What are the three types of plate boundary?

Divergent boundaries, where plates move apart and new crust is created (mid-ocean ridges and continental rifts); convergent boundaries, where plates move together and crust is destroyed by subduction or thickened by collision (trenches, volcanic arcs, mountain ranges); and transform boundaries, where plates slide past each other and crust is conserved, producing strong shallow earthquakes but no volcanoes.

What happens at a convergent plate boundary?

Two plates move together. Usually the denser plate subducts — bends and sinks into the mantle to be recycled — forming a deep trench, a band of deepening earthquakes (the Benioff zone) and a volcanic arc, as where the Nazca plate builds the Andes. Where two continents meet, neither subducts; they crumple into mountain ranges such as the Himalayas.

Why are there no volcanoes at transform boundaries?

At a transform boundary plates slide horizontally past each other. Nothing pulls apart to let mantle rise and melt, and nothing subducts to generate magma, so no volcanism occurs. Instead the locked fault stores strain and releases it as powerful, shallow earthquakes, as along the San Andreas Fault.

How do you tell plate boundary types apart?

By their earthquakes and volcanoes. Divergent boundaries have shallow earthquakes and gentle basaltic volcanism along ocean ridges. Convergent boundaries have deep trenches, earthquakes that deepen inland (the Benioff zone) and explosive volcanoes. Transform boundaries have strong shallow earthquakes but essentially no volcanoes.

What happens at plate boundaries

The Earth never grows or shrinks — so wherever new crust is made, an equal amount must be destroyed, and in between, crust just slides past. That single rule leaves only three things two plates can do where they meet. This powerbook works through all three, and the question that unlocks every boundary: what happens to the crust here?

boundary types

short sections

rule that ties them together

What happens at plate boundariesSection 1 of 5

0 of 5 locked in

This book has 5 short sections. Read each one, answer the check, then use Next to move on — the bar above tracks how far you've come.

The seams you can only see when the Earth moves

The Earth’s rigid outer shell is not one piece. It is cracked into about a dozen great slabs — plates — that drift slowly over the hotter, weaker rock beneath. You cannot see the cracks between them directly. But plot every earthquake and most volcanoes on a world map and a pattern jumps out: they line up in narrow belts. Those belts are the plate boundaries. The middles of plates are quiet; almost all the action happens at the edges.

There is one rule that makes the whole topic snap into place: the Earth is not growing or shrinking. Its surface area stays the same. So if new crust is being created somewhere, exactly that much old crust must be destroyed somewhere else — and in between, crust can simply slide past without either. That bookkeeping leaves only three things two plates can do where they meet.

They can move apart, move together, or slide past. Those are the three boundary types — divergent, convergent and transform — and each is really an answer to one question: what happens to the crust here? Created, destroyed, or just conserved. Hold that question and every boundary makes sense.

New words

plate

A large rigid slab of the Earth’s outer shell that moves as one piece.

plate boundary

The edge where two plates meet — traced by lines of earthquakes and volcanoes.

🔒 Lock it in

Why does "the Earth stays the same size" mean there must be more than one kind of plate boundary?

Answer the check to continue

Quick reference & sources

The interactive book above is the lesson. What follows is a plain recap you can revise from, plus the sources and how this maps to the syllabus.

The three boundary types, by what happens to crust

Divergent — plates move apart, mantle rises and melts (decompression melting), and new crust is created: mid-ocean ridges and continental rifts. Convergent — plates move together; the denser one subducts and crust is destroyed, or two continents collide and crust is thickened. Transform — plates slide past and crust is conserved, with strong shallow earthquakes but no volcanoes.

Convergent boundaries depend on what collides

Ocean–continent: dense ocean plate subducts under the continent — trench plus volcanic arc (the Nazca plate building the Andes). Ocean–ocean: one ocean plate subducts under another — trench plus volcanic islands (the deepest trenches, like the Mariana). Continent–continent: neither subducts; the crust crumples into mountains (the Himalayas), with great shallow earthquakes but little volcanism.

Reading boundaries from earthquakes and volcanoes

Earthquake depth and volcanic style identify a boundary. Shallow quakes + gentle basaltic volcanism along an ocean ridge → divergent. A trench + a Benioff zone deepening inland + explosive volcanoes → convergent subduction. Strong shallow quakes + no volcanoes → transform. The Ring of Fire is this signature wrapped around the Pacific.

For HSC students

This powerbook covers the Plate Boundaries thread of Module 2 (Plate Tectonics), framed by the inquiry question what occurs at plate boundaries? Working through it, you should be able to:

locate divergent, convergent and transform boundaries and model the processes that form them;
explain what happens to crust at each boundary type — created, destroyed or conserved;
distinguish ocean–ocean, ocean–continent and continent–continent convergence;
relate earthquake distribution and depth, and volcanic activity, to boundary type.

The detailed landforms each boundary builds — rift valleys, ridges, trenches, mountain ranges, faults and folds — are the focus of the next chapter, the landscapes plates build. Explanations here are original and are a study aid, not a copy of the syllabus.

Sources

US Geological Survey, This Dynamic Earth and Understanding Plate Motions — boundary types, subduction, and the global distribution of earthquakes and volcanoes.
Geoscience Australia — plate tectonics, earthquakes and the Australian plate setting.
Smithsonian Global Volcanism Program — distribution of volcanism and the Ring of Fire.
NSW NESA Earth and Environmental Science Stage 6 Syllabus (2017) — used only to scope the Module 2 content; explanations above are original.

Keep going

Three things two plates can do, one rule behind all of them: somewhere crust is born, somewhere it dies, and somewhere it just slides by — and the planet shows you which by where it shakes and erupts.