Explore the Deep Sea

Volcanoes & Vents

Plates Stretching: basins

What happens when tectonic plates are stretched.

Plate stretching causes sea-floor basins & volcanoes

Diagram of tectonic plate stretching.

In areas where two plates collide, one plate typically rides over the other. The sinking plate is denser: its weight pulls on the plate it is moving under, causing the overlying plate to stretch and thin. In some regions, such as the western Pacific Ocean, this stretching causes huge basins to form in the seafloor of the overlying plate. Sometimes, the Earth's crust in these basins stretches so much it cracks, allowing magma through from the mantle beneath. Hence, basins often contain active volcanoes and hydrothermal vents.

The Lau Basin: an example

The Lau Basin is found between the islands of Fiji and Tonga.

Ridges in basins differ from mid-ocean ridges

Volcanoes in ocean basins typically form and go extinct on the timescale of a few million years. By contrast, volcanoes on mid-ocean ridges, where the Earth's plates are moving apart, can exist for hundreds of millions of years. Mid-ocean ridge rocks tend to be fairly uniform in chemical composition along the ridge. However, the volcanic rocks in a basin can vary somewhat in mineral composition from place to place because the magma from which they form can be fairly variable in its makeup—containing more or less of certain compounds which have entered the mantle from the nearby subducting plate.

Next: hot rocks