Constructive Plate Margins

Constructive plate margins happen where plates move apart. Most of these plate margins are under the oceans. As the plates move apart, magma rises from the mantle to the Earth’s surface. The rising magma forms shield volcanoes.

A constructive plate margin

Constructive (tensional) plate margins occur where plates move apart. Examples below include the South American Plate and African Plate and the Eurasian Plate and the North American Plate.

A map to show the Earth’s plates

Constructive boundaries are mainly located under the sea, e.g. 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 about 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 when mantle plumes appeared.

Map showing the tectonic plates that separate Iceland

Iceland has been 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 plate that form the constructive margin that runs through Iceland. To the left is the North American plate and to the right the Eurasian plate.