A ‘cool’ new picture of early Earth
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The first 700 million years of Earth’s 4.5-billion-year existence are known as the Hadean period, after Hades, or, to shed the ancient Greek
name, Hell.
That name seemed to fit with the common perception that the young Earth was a hot, dry, desolate landscape interspersed with seas of magma and inhospitable for life. Even if some organism had somehow popped into existence, the old story went, surely it would soon have been extinguished in the firestorm of one of the giant meteorites that slammed into the Earth when the young solar system was still crowded with debris.
Over the last decade, the mineralogical analysis of small hardy crystals known as zircons embedded in old Australian rocks has painted a picture of the Hadean period “completely inconsistent with this myth we made up,” said T Mark Harrison, a professor of geochemistry
at the University of California, Los Angeles.
Geologists now almost universally agree that by 4.2 billion years ago, the Earth was a pretty placid place, with both land and oceans. Instead of hellishly hot, it may have frozen over. Because the young Sun put out 30% less energy than it does today, temperatures on Earth might have been cold enough for parts of the surface to have been covered by expanses of ice.
In a new analysis, the zircons, the only bits of earth older than 4 billion years definitively known to have survived, provide another hint about the Hadean period. Harrison and two UCLA colleagues, report that minerals trapped inside zircons offer evidence that the processes of plate tectonics — the forces that push around the planet’s outer crust, forming and shaping the continents and oceans — had already begun.
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http://timesofindia.indiatimes.com/H...ow/3785782.cms |
