Explore the Deep Sea

Volcanoes & Vents

Diverging plates: mid-ocean ridges & rifts

What happens when tectonic plates move apart.

Earth birth

Diagram of tectonic plate spreading.

Some of the tectonic plates that make up the Earth's crust are slowly but surely moving apart as a result of forces deep in the mantle below. Molten rock (magma) from the Earth's interior feeds volcanoes spread along the boundary between the two plates. At these spreading centers, new crust is formed.

Mid-ocean ridges, rifts and cliffs

In different parts of the world, plates spread apart at different rates (from less than 10 mm to more than 90 mm per year). The shape of the new seafloor depends on the spreading rate and on the composition of the rock rising from beneath. The rising rock generally creates a ridge (hence, deep-sea spreading centers at plate boundaries are sometimes called mid-ocean ridges). All ridge regions contain cracks and fissures through which lava and super-hot fluids rise. At slow spreading centers, a deep rift valley usually forms, bounded by great cliffs. When the rate of spreading is fast, we see a narrow, shallower crack instead.

Lava: lakes, pillows and bathtub rings

Lava erupting out of the ocean floor cools when it hits the overlying water. The lava can solidify into all sorts of shapes—depending on the rate and temperature of the eruption, the lava's chemical make-up and on the shape of the seafloor. On shallow slopes, "pillow lavas" form when lava erupts slowly—so-called because the lava solidifies in mounds that look like heaped pillows. Lava can flow into dips in the seafloor to form "lava lakes." Sometimes these lakes drain away through other cracks in the seafloor, leaving "bathtub rings" around the edges—where lava cooled before it could drain away.

Mountains in the ocean

When two plates move apart, lava and super hot fluid typically erupt in a very narrow zone along the plate boundary: a few hundred to a few thousand yards across. But the movement of magma up from the mantle raises the surrounding crust for miles around. So most zones where plates move apart are marked by mountain ranges. These ranges run for thousands of miles, mostly in the depths of the ocean. But the mid-Atlantic island of Iceland is one place on dry land where two plates are moving apart: the North American Plate is moving west, while the Eurasian Plate moves east. Iceland thus straddles the Mid-Atlantic mid-ocean ridge system, most of which is deep in the Atlantic Ocean.

Next: Plates colliding: subduction, trenches and mountain building