Divergent boundaries

Between 1975 and 1984 the ground around Krafla volcano in Iceland split open again and again, until the cracks had widened by about 7 m.

Two conveyor belts

USGS describes a spreading centre with a picture of two conveyor belts facing each other and moving slowly in opposite directions, each carrying freshly made ocean crust away from the crest. That crest is a mid-ocean ridge. The best known is the Mid-Atlantic Ridge, a submerged mountain range that runs from the Arctic Ocean to beyond the southern tip of Africa. It is only one part of a ridge system that wraps around the whole globe like the seam on a baseball.

Why is there a ridge at all? The new plate at the crest is hot, and hot rock expands, so it stands high. As it moves away it cools, shrinks and sinks, so the sea floor deepens with distance from the ridge. At the very top, the plates are stretched so much that a block drops down between faults, making a rift valley. Magma from the mantle rises into the crack, some of it erupts on the sea floor and some freezes in the crack below as new crust. Earthquakes here are frequent but small and shallow, because the plate at the crest is thin and hot.

Stripes on the sea floor

Lava that cools contains magnetic minerals that line up with the Earth's magnetic field of the moment, like tiny compass needles frozen in place. The field has flipped many times. So as the plates move apart, the ocean floor on each side of the ridge carries a pattern of stripes: rock magnetised in the normal direction, then reversed, then normal again. The pattern on one side of the ridge is the mirror image of the other side, exactly what you would expect if new floor is made in the middle and carried outwards in both directions. In the model the dark and light bands are these stripes.

The same stripes work as a clock. Because the ages of the reversals are known, the distance from the ridge to a given stripe tells how fast the plates have separated. This is one of the ways the 2.5 cm per year of the Mid-Atlantic Ridge was measured. It also explains a puzzle about the Earth's size. If new crust is made at ridges every year and the Earth is not growing, crust must be destroyed somewhere else at the same rate. Harry Hess, an American geologist, reached that conclusion, and the next unit shows where it happens.

Where you can stand on a ridge

Iceland sits on top of the Mid-Atlantic Ridge, so the spreading that is normally hidden under the sea happens on dry land. The island is splitting between the North American and Eurasian Plates. Around Krafla volcano, cracks widen and new ones open every few months; during the rifting episodes from 1975 to 1984 the ground would rise 1 to 2 m and then drop suddenly before an eruption, and the total displacement was about 7 m. In East Africa a continent is being pulled apart. Spreading has already torn Arabia away from Africa to form the Red Sea, and USGS suggests East Africa may be the site of the Earth's next major ocean.