Plate Processes, Boundaries and Hotspots

Earth’s moving outer shell

Earth’s surface may seem fixed, but its rigid outer shell is divided into tectonic plates. These include the crust and the uppermost mantle and usually move only a few centimetres each year. Over millions of years, this movement creates features stretching for hundreds or thousands of kilometres.

There are two main types of crust. Oceanic crust lies beneath the oceans and is thinner and denser than continental crust, which forms the continents. This difference in density helps explain what happens when plates meet.

The mantle beneath the plates is rocky and mostly solid, but it can deform and flow extremely slowly. Heat within Earth produces convection currents: hotter, less dense material rises, while cooler, denser material sinks. This slow circulation helps move the plates. Their movement brings them apart, towards each other or past each other at plate boundaries, also called margins.

Why tectonic activity forms global belts

Earthquakes and volcanoes are not evenly scattered across the world. Most occur in long, narrow belts along plate boundaries, where moving plates interact.

Around the Pacific Ocean, many boundaries involve one plate descending beneath another. Frequent earthquakes and numerous active volcanoes form the Pacific Ring of Fire. Along the Mid-Atlantic Ridge, plates move apart, producing earthquakes and volcanic activity as new crust forms. The ridge is largely underwater, but it reaches the surface in Iceland.

Earthquakes also occur where plates collide, including the Alpine–Himalayan belt extending from Europe towards Asia. Where plates slide past each other at conservative margins, such as the San Andreas Fault in California, they can lock through friction and then slip suddenly, causing earthquakes. Crust is neither created nor destroyed there, and this movement does not normally generate volcanoes.

The distribution therefore reflects different types of plate movement. Earthquake belts and volcanic belts overlap in many places, but not every earthquake zone has volcanoes. Hotspots provide another important exception: volcanoes can form within plates, away from their edges.

Constructive margins: pulling apart and building new crust

At a constructive margin, plates move apart. This movement is called divergence. Magma—molten rock beneath Earth’s surface—rises into the opening. When it reaches the surface it is called lava; as it cools and solidifies, it creates new crust.

Beneath an ocean, repeated magma supply builds a mid-ocean ridge. Newly formed oceanic crust moves away on either side as the plates continue separating. The Mid-Atlantic Ridge is an example. ‘Constructive’ describes the creation of crust, not an absence of hazards: earthquakes and volcanic eruptions still occur.

Where continental crust is pulled apart, it stretches and fractures along faults. A block of land can sink between faults, creating a long rift valley. The East African Rift Valley is an example. The valley is formed by movement and sinking of the crust, rather than by a river eroding it.

Destructive margins: subduction and ocean trenches

At a destructive margin, plates move towards each other. Where an oceanic plate meets a continental plate, the denser oceanic plate is forced beneath the continental plate and descends into the mantle. This is subduction. Oceanic crust is removed from the surface, which explains the name ‘destructive’.

As the oceanic plate bends downwards, it creates a long, deep ocean trench beside the overriding plate. The trench marks the beginning of the subduction zone; it is not a gap created by plates pulling apart. The Mariana Trench is another example of a subduction trench, formed where one oceanic plate descends beneath another.

Plates can become locked by friction during subduction. Stress builds until sudden movement releases energy as an earthquake. Subduction also leads to magma formation, and this magma can rise through the overriding plate to supply volcanoes. Along western South America, the Nazca Plate subducts beneath the South American Plate, helping explain the earthquakes and volcanoes of the Andes.

The contrasting landforms follow from the movement: pulling continental crust apart can produce a rift valley, whereas bending a plate down into a subduction zone produces an ocean trench.

Hotspots: volcanoes away from plate boundaries

A volcanic hotspot is supplied by unusually hot mantle material rising towards the surface, often described as a mantle plume. Magma rises through the plate above it and can build a volcano without two plates meeting there.

Hawaii lies within the Pacific Plate rather than at its edge. The Pacific Plate moves north-west over a relatively stationary hotspot. A volcano forms above the hotspot, but plate movement gradually carries it away from its magma supply. Its activity eventually stops, while a new volcano develops above the hotspot.

Repeating this process creates a chain of volcanic islands. In the Hawaiian chain, islands generally become older towards the north-west, while the youngest volcanism is at the south-eastern end. The chain records the direction of plate movement: the plate moves, rather than the hotspot travelling from island to island.

Global pattern

• Most earthquakes and volcanoes occur along plate boundaries.
• Pacific Ring of Fire: widespread subduction, earthquakes and volcanoes.
• Mid-Atlantic Ridge: plates separate; new crust and volcanic activity.
• Alpine–Himalayan belt: major earthquake belt associated with convergence.
• Conservative margins: plates slide past; earthquakes, but no crust creation or destruction and normally no volcanoes.

Movement → process → landform

• Convection: hotter mantle material rises; cooler material sinks. Slow circulation helps move plates.
• Constructive margin: divergence → magma rises and cools → new oceanic crust and a .