Monumental Shift

The discovery of 165-million-year-old crystals Easter Island has upended the longheld notion of how the Earth's "conveyor belt" moves

- IAN RANDALL

THE EARTH'S MANTLE MIGHT NOT ALWAYS move along in lockstep with the overlying tectonic crust as set out in science textbooks for decades-but may instead behave differently.

This is the conclusion of an international team of geologists who made a mysterious find on Easter Island, the Chilean special territory in the Pacific famous for its giant statues.

The idea that the Earth's crust and mantle might move together like a "conveyor belt" as a result of convection currents in the latter was first proposed in 1919 by the British geologist Sir Arthur Holmes.

His suggestion provided a mechanism by which large continents might drift across the surface of the Earth-a then contentious theory based on evidence including how landmasses like Africa and South America seem to fit together and had matching rocks and fossils despite being an ocean apart.

Now, however, researchers have found crystal "time capsules" on Easter Island that appear to have stayed in the same place in the mantle for This some 162.5 million years is incompatible with the conveyor belt theory, suggesting that the mantle's behavior may be far more complex than previously thought.

Geologically speaking, Easter Island, which lies some 2,200 miles off the coast of Chile, is something of a spring chicken. The oldest parts of the island were formed by volcanic eruptions that took place 2.5 million years ago, atop an ocean plate not that much older.

In their research, geologist Yamirka RojasAgramonte of Kiel University, Germany, and her colleagues initially set out to precisely calculate the island's age. To do this, they turned to tiny, uranium-bearing crystals called zircons, which are like natural little time capsules, preserved in lava.

The more time that passes after the crystals form from cooling magma, the more of the uranium decays into lead. By measuring the ratio of the two, geologists can work out how old the crystals are.