Processes at Plate Boundaries and Hotspots

How plate movement changes the Earth's surface

Tectonic plates are rigid slabs made of the Earth's crust and uppermost mantle. They move slowly, typically a few centimetres each year. A plate boundary, or margin, is where two plates meet. Over long periods, movement at these boundaries creates new crust, recycles oceanic crust and builds mountains. It also produces earthquakes and volcanic activity.

The important distinction is the plates' relative movement: they can move apart, move together or slide past each other. Where plates move together, the type of crust affects what happens next.

Constructive boundaries: plates move apart

At a constructive boundary, two plates move away from each other. Hot magma rises from the mantle into the opening. At the surface it becomes lava, which cools and solidifies to form new crust. The boundary is called constructive because crust is being created.

Under the ocean, repeated formation of new crust builds a long underwater mountain ridge. The Mid-Atlantic Ridge is an example. Parts of this ridge rise above sea level in Iceland, where volcanic activity can be seen on land.

As the plates separate, rocks fracture and move along faults, producing earthquakes. Constructive boundaries therefore have both volcanic activity and earthquakes.

Destructive boundaries: an oceanic plate subducts

At a destructive boundary, plates move towards one another. Where an oceanic plate meets a continental plate, the denser oceanic plate sinks beneath the less dense continental plate. This process is subduction.

A deep ocean trench forms where the oceanic plate bends downwards. The sinking plate carries oceanic crust into the mantle, so this boundary is called destructive: oceanic crust is being removed from the surface and recycled.

The Nazca plate subducts beneath the South American plate, forming the Peru–Chile trench. Earthquakes occur within the descending plate and along the contact between the plates. Volcanic activity also occurs on the overriding plate, where magma reaches the surface.

Collision boundaries: continents crumple together

A collision boundary forms where two continental plates move towards each other. Continental crust is relatively buoyant, so it does not sink into the mantle in the same way as dense oceanic crust. Instead, the crust is compressed, folded and pushed upwards, with one plate able to be forced beneath the other without the deep oceanic subduction described above.

Over millions of years, this produces fold mountains. The Himalayas formed through the collision of the Indian and Eurasian plates.

Movement and deformation of the rocks produce earthquakes, but collision boundaries do not normally produce volcanoes. This distinguishes them from destructive boundaries involving oceanic subduction.

Conservative boundaries: plates slide past

At a conservative boundary, plates slide horizontally past one another. They may move in opposite directions, or in the same direction at different speeds. Neither new crust nor a subduction zone is formed: crust is neither created nor destroyed.

The plates do not slide smoothly all the time. Friction can hold sections together while movement continues elsewhere, building stress. When the stuck section suddenly slips, an earthquake occurs. This boundary movement does not generate volcanic activity.

The San Andreas fault in the USA is an example of a conservative boundary.

Hotspots: volcanic activity away from boundaries

Not all volcanoes occur where plates meet. A hotspot is an area supplied by rising hot material from the mantle. Magma rises through the overlying crust and can erupt at the surface, forming a volcano within a plate.

The key contrast is the location and process: constructive and destructive volcanism is associated with interactions between plates, whereas hotspot volcanism can occur away from a plate boundary. A volcano's presence alone therefore does not prove that two plates meet beneath it.