The rift in the Atlantic ridge emerges in plain view in Iceland, where it forms a steep, canyon-like gap across the island.

All of Iceland’s recent volcanic eruptions have occurred along this gap, as have nearly all of its Earthquakes.

Deep cracks along the gap are slowly opening wider, and the gap as a whole is spreading by about two inches a year.

Volcanic islands near it are made of very young rocks. Farther away on both sides, the islands are older.

The distant islands as well as the near ones probably were born from the ridge and rift, and were carried away from it by the spreading of the Earth’s crust.

By comparing the ages of the islands and their distances fro the rift, scientists find that the average rate of movement has been an inch and a half a year.

What causes the spreading of the crust along the ridge-rift? One clue is found in the nature of its material.

Earthquake waves go through the ridge at high speeds, indicating heavy, dense rocks. Among samples dredges from the ridge are rocks even heavier and dense than basalt.

Such rocks probably make up the Earth’s mantle. This evidence suggests that dense material from the mantle has been rising under the ridge-rift.

This upward movement apparently breaks the crust and widens the rift. Another clue comes temperatures in the ridge-rift, which shows the rate at which heat from the interior flows up through it.

The rate of heat flow is several times greater than in the flat ocean floor and the continents.

From this we may conclude that hot material from the mantle is rising under the ridge-rift. A very different situation exists along the trenches that parallel the shores of the Pacific.

There the rate of heat flow is low, and so are gravity values. These facts suggest that cold, light material of the crust is being pulled down under the trenches.

Earthquakes are frequent there, but not at shallow levels. All occur several hundred miles down. Even at such depths, the rock is solid enough to break rather than flow. Why?

Presumably because it is cold, solid stuff sinking from upper levels. According to this picture of events, the Earth’s mantle is stirred by vast slow currents that rise under the rifts and sink under the trenches.

The currents are set in motion by temperature differences in the mantle. In a colder zone, material skins because it is denser and heavier.

In a hotter zone, material rises because it is lighter. Where a rising current approaches the top of the mantle, it spreads out on both sides and pulls the crust apart.

Splitting and Spreading Crust