Constructive Plate Margins

AQA GCSE Geography > The Challenge of Natural Hazards > Constructive plate margins

Constructive Plate Margins

Constructive (divergent) plate margins happen when plates move apart.

Oceanic-Oceanic Crust Constructive Plate Margins

Most constructive plate margins are under the oceans, where two sections of oceanic crust move apart. At a constructive plate margin, where two sections of oceanic crust are separating, the lithosphere becomes thinner, leading to the upwelling of mantle rock. The reduced pressure exerted by the crust (because it is thinner) onto the mantle leads to mantle material melting (through a process called decompression melting), leading to diapirs, which feed magma chambers along the ridge. Magma can then rise to the surface in dykes, leading to surface eruptions and the formation of new crust, which drives the process of ridge push. The rising magma can form shield volcanoes.

An oceanic-oceanic constructive plate margin

Examples of conservative margins include the South American Plate, the African Plate, the Eurasian Plate, and the North American Plate.

A map to show the Earth’s plates

Constructive boundaries are primarily located under the sea, for example, the Mid-Atlantic Ridge. Here, chains of underwater volcanoes have formed along the plate boundary. One of these volcanoes may become so large that it erupts out of the sea to create a volcanic island, e.g. Surtsey and the Westman Islands near Iceland.

Iceland is approximately 18-25 million years old and is thus one of the youngest countries in the world. The formation of Iceland started about 60 million years ago when the mid-Atlantic ridge (the boundary between the North American tectonic plate and the Eurasian tectonic plate) started to give way, and mantle plumes appeared.

Map showing the tectonic plates that separate Iceland

Iceland has formed as the result of a constructive plate margin.

The image below shows Brú milli heimsálfa, or the ‘Bridge between continents’, which links the North American and Eurasian plates that form the constructive margin that runs through Iceland. To the left is the North American plate, and to the right is the Eurasian plate.

Brú milli heimsálfa, or the ‘Bridge between continents’

Find out about conservative and destructive plate margins.

Summary

Plate Movement
At constructive plate margins, tectonic plates move apart, allowing magma to rise and form new crust.
Formation of Mid-Ocean Ridges
As plates separate under the ocean, magma rises and cools to create underwater mountain chains called mid-ocean ridges.
Decompression Melting
A drop in pressure in the mantle allows rock to melt into magma, which rises through the crust at constructive margins.
Volcanic Activity
Volcanic eruptions can occur at constructive boundaries, often producing wide, low-profile shield volcanoes.
Example: Iceland
Iceland is located on the Mid-Atlantic Ridge and was formed by volcanic activity at a constructive plate boundary.
Ridge Push
New crust formed at ridges is pushed outward by gravity, helping drive the movement of tectonic plates.