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.
When crustal plates meet, and one plate is denser than the other, the denser plate is forced under the other plate (at the subduction zone) and melts to form magma. If both plates are of equal density they collide and crumple up against each other forming mountains.
Fractured rock at Thingvellir, Iceland. Rock weaknesses may be caused, as here, by tectonic movement. In this location, the North American plate moves away from the European plate. Iceland is growing wider by an average of 2 cm/0.8 in per year.
At Thingvellir, the North American plate is moving away from the European plate at an annual rate of 2 cm/0.8 in. Thingvellir marks the position of the Mid-Atlantic Ridge, a constructive plate boundary where new material is formed by the cooling of lava. There are steep-sided rift valleys, and a broad flat plain in the centre of the rift valley.
A steep rift-valley wall at Thingvellir, Iceland. Rift valleys are created when tectonic plates pull apart, and part of the Earth's crust subsides. The Mid-Atlantic Ridge can be seen in action here at Thingvellir, where the North American plate is pulling away from the European plate.
Rocks at Thingvellir, in Iceland, are disrupted by earth movements as two plates of the Earth's crust move apart, as well as broken down by repeated freeze-thaw action.
Some of the lava from the 1973 eruption of Helgafell, a volcano on the Icelandic island of Heimaey, was distorted as it flowed from the cone. Pressure from movements of the Earth caused it to bend; as the lava cooled and solidified, the curves and folds were preserved.
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.