constructive margin

A rift in the crustal plates where new material is being formed causes the plates to be pushed apart. This usually occurs as a result of volcanic action.

A river runs through a steep-sided rift valley at Thingvellir, in Iceland. In this area, the North American plate is pulling away from the European plate at an annual rate of 2 cm/0.8 in. It is part of the Mid-Atlantic Ridge, a constructive plate boundary where new material is formed by the cooling of lava.

The steep inclines on these massive slabs of rock at Thingvellir, Iceland, testify to the tectonic forces at work here. This is the site of part of the Mid-Atlantic Ridge, where the North American plate is pulling away from the European plate. This is a constructive plate boundary, where new material is formed by the cooling of lava, but it also leads to the subsidence of some of the Earth's crust.

In plate tectonics, a boundary between two lithospheric plates, along which new crust is being created. The term usually refers to divergent margins, where two oceanic plates are moving away from each other. As they diverge, magma (molten rock) wells up to fill the open space, and solidifies, forming new oceanic crust. Similar processes occur where a continent plate is beginning to split apart.

Subduction zones could also be classified as constructive. As the oceanic plate subducts, it releases water to the overlying mantle. This decreases the melting point of the mantle rocks, turning them into magma. This magma then migrates upwards and erupts, forming chains of volcanoes and adding to the crust above the subduction zone.