Ancient rock points to life's origin

Chromite: A mineral from beneath an ancient ocean

By Dr David Whitehouse BBC News Online science editor

The continents were moving across the face of the Earth much sooner than had been thought, according to new evidence from China.

The new data come from a huge chunk of the rock that lay beneath the sea floor 2.5 billion years ago.

Tim Kusky, of St Louis University, US, says it is the first large intact piece of oceanic mantle ever found from our planet's earliest period, the Archean.

Located not far from the Great Wall of China, the ancient mantle rocks are preserved in a highly faulted belt 100 kilometres (62 miles) long.

It may also contain clues as to when life developed on Earth.

Origin of life

Working with researchers from Peking University, Kusky found the rock section where last year the same team discovered the Earth's oldest complete section of oceanic crust.

The newly found rock was formed tens of kilometres below the ancient sea floor. Scientists say it preserves 2.5-billion-year-old minerals that hold clues to the origin of how continents move across the globe - plate tectonics.

The minerals, including an unusual type of chromite (iron chromium oxide) deposit previously only known from deep ocean floor rocks, appear to have been deformed at extremely high temperatures before they were completely crystallized by volcanic heat.

This indicates that the rocks were moving away from mid ocean ridges, say scientists. This suggests that the continents were moving more than 500 million years earlier than was previously believed.

Thermal vents

The discovery that ancient tectonic plates were shifting could throw some light on the origin of life on Earth.

Earth's mantle

The region of the Earth's interior that lies between the crust and the core

The mantle accounts for approximately 80% of the Earth's volume

Consists largely of peridotite, an igneous rock composed mainly of the minerals olivine and pyroxene

Hot volcanic vents on the ocean floor may have provided the nutrients and conditions required for life to begin.

Because such volcanic vents are associated with tectonic movements Kusky says that it is possible that life developed and diversified around these vents as the plates started spreading.

The research is published by the Geological Society of America.