Poster Convergence and Subduction

Quick Answer: The Poster Convergence and Subduction is an earth-science wall poster that explains what happens where two tectonic plates move towards each other and one sinks beneath the other, showing the trench, the descending slab, earthquakes and the volcanoes that result. It is used in secondary and early university geology lessons.

Where Plates Meet and One Sinks

At a convergent boundary two plates close on each other, and when at least one of them carries dense oceanic lithosphere, it bends down and sinks into the mantle: this is subduction. A convergence and subduction poster typically shows the process in cross-section, with the deep ocean trench at the surface, the cold slab descending at an angle, a wedge of scraped-off sediment at the edge of the overriding plate and a line of volcanoes set back from the trench.

Several observations students can check on a map follow from this one diagram. Earthquakes become deeper the further they are from the trench, tracing the sinking slab in a pattern known as the Wadati-Benioff zone. The volcanoes form because water released from the slab lowers the melting point of the mantle above it, producing magma that is richer in silica and more explosive than the basalt erupted at ocean ridges. The Andes, Japan and the Aleutian Islands are classic examples.

Convergence does not always end in subduction. When two continents meet, neither sinks far because continental crust is too buoyant, and the result is crustal thickening and high mountains, as in the Himalaya and the Alps.

Specifications

Applications

• Explaining the pattern of deep earthquakes beneath South America and Japan
• Contrasting explosive arc volcanoes with the quieter eruptions of mid-ocean ridges
• Teaching how continental collision builds mountain belts
• Paired lessons with the divergence poster to cover the whole life of an ocean

Care & Handling

• Hang the poster so the deep part of the cross-section, where slab and magma are drawn, sits at student eye level.
• Keep a world plate map close by so locations on the section can be found on the map.
• Store it rolled in the same tube as the divergence poster so the pair stays together.

Why Choose LabEquip

Geology teachers usually buy this poster with the Divergence and Oceanic Opening poster, since between them the two explain how oceans open and close. Both are part of Educational Charts, and the LabEquip contact page is the route for ordering them together.

Frequently Asked Questions

Why does oceanic crust subduct rather than continental crust?

Old oceanic lithosphere is cold and dense, so it sinks into the mantle fairly easily. Continental crust is thicker and made of lighter rock, so it resists sinking and piles up into mountains instead.

What is an ocean trench?

It is the long, deep depression in the sea floor where the subducting plate bends down. The deepest point on earth, the Challenger Deep in the Mariana Trench, lies at such a boundary.

Why are volcanoes found some distance from the trench?

Magma forms only once the slab has sunk deep enough, roughly 100 km, for water released from it to trigger melting in the mantle above. That depth is reached some way inland of the trench, so the volcanic arc runs parallel to it.

What is the Wadati-Benioff zone?

It is the inclined band of earthquakes that follows the subducting slab down into the mantle. Plotting earthquake depth against distance from the trench reveals the angle at which the slab is sinking.

What happens when two continents collide?

Neither sinks far, so the crust crumples, thrusts over itself and thickens, raising mountain belts such as the Himalaya. Collision usually follows the subduction of the ocean that once separated the two continents.

How can students model subduction in class?

Slide two sheets of card towards each other on a table, letting one bend down beneath the other, and mark where the trench and volcanoes would form. Layers of modelling clay pushed together can show crustal thickening in a collision.

Last Updated: September 2026