Magnetic Fossils Shed Light on Earth’s Early Development

11 04 2007

MagnetosphereGeophysicist John Tarduno at the University of Rochester in New York, US, and colleagues, have made the earliest direct measurement of Earth’s magnetic field. They discovered that the magnetosphere was already in existence 3.2 billion years ago, 500 million years earlier than scientists previously believed.

Magnetic Fossils
Tarduno’s team studied the early magnetosphere by digging up what are essentially magnetic fossils. “When an igneous rock cools,” says Tarduno, “the magnetic minerals contained within it lock in a record of Earth’s magnetic field direction and strength.”

The researchers extracted minimally disturbed bits of feldspar and quartz from 3.2 billion-year-old rocks in the Archaean Kaapvaal craton in South Africa. The difficulty has been that over billions of years such rocks undergo changes in their magnetisation due to geologic temperature changes and chemical interactions, rendering them unreliable.

Tarduno’s team found a way around the problem by isolating individual crystals from within the rocks and examining the nanometre-sized traces of magnetite they contain. At that scale, magnetic information is much harder to destroy, and the armour of silicate protects the magnetite from external forces. These tiny magnetic particles encode information about Earth’s Archaean-era magnetic field far more accurately than bulky rocks do.

Read the whole article here.

From NewScientist.com news service, by Amanda Gefter, 05 April 2007

More:
NASA’s “What is the Magnetosphere?
Magnetosphere in Wikipedia

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