Deep down on the ocean floor, some of the tectonic plates of the Earth’s crust are slowly pushing apart. New molten rock wells up from the mantle into the gap between them and freezes onto their edges. As plates are destroyed at subduction zones, so new plate spreads the ocean floor wider.
The spreading of the ocean floor centers on ridges down the middle of some oceans, mid-ocean ridges. Some of these ridges link up to make the world’s longest mountain range, winding over 60,000 km beneath the oceans.
The Mid-Atlantic Ridge stretches through the Atlantic from North Pole to South Pole. The East Pacific Rise winds under the Pacific Ocean from Mexico to Antarctica.
Along the middle of a mid-ocean ridge is a deep canyon. This is where molten rock from the mantle wells up through the sea-bed.
Mid-ocean ridges are broken by the curve of the Earth’s surface into short stepped sections. Each section is marked by a long sideways crack called a transform fault. As the sea floor spreads out from a ridge, the sides of the fault rub together starting earthquakes.
Unlike subduction zones, which create explosive volcanoes, diverging plates create volcanoes that ooze lava gently. For this to happen above the ocean surface is rare. Mantle Transform fault Central canyon Ocean plate Ridges are lower and older away from the centre
As molten rock wells up from a ridge and freezes, its magnetic material sets in a certain way to line up with the Earth’s magnetic field. Because the field reverses every now and then, bands of material set in alternate directions. This means that scientists can see how the sea floor has spread in the past.
Rates of sea floor spreading vary from 1 cm to 20 cm a year. Slow-spreading ridges such as the Mid-Atlantic Ridge are much higher, with seamounts often topping the ridge. Fast-spreading ridges such as the East Pacific Rise are lower, and magma oozes from these just like surface fissure volcanoes.
Magma bubbling up through a mid-ocean ridge emerges as hot lava. As it comes into contact with seawater it freezes into blobs, pillow lava.
Mid-ocean ridges may begin where a mantle plume (see hot-spot volcanoes) rises through the mantle and melts through the sea-bed.
This is a cross-section of the top 50 km or so of the Earth’s surface. It shows where the sea floor is spreading away from the mid-ocean ridge. Mid-ocean ridge Magma erupts through the gap as lava solidifies into new sea floor .
About 10 cubic km of new crust is created at the mid-ocean ridges every year.