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Earth’s prehistory: In the days before life 
Deep Earth: Headline News Stories 

Earth’s prehistory: In the days before life

Long before the rise of free oxygen in Earth’s atmosphere allowed for complex oxygen-breathing life forms to evolve, the as-yet-uninhabitable planet was still forming, separating into buoyant crust, churning mantle and dense core, expelling gases and heat, and setting the stage for later life. This year has seen both leaps forward and some setbacks in understanding how and why some of these key events in Earth’s earliest history occurred.

How the continental crust formed — whether the melted rocks that formed the crust were extracted from the mantle in fits and starts, or gradually — has been a long-standing question. Reporting Sept. 13 in Nature, Graham Pearson of Durham University in the United Kingdom and colleagues looked at the isotopic signatures and dates of certain erosion-resistant minerals in ancient mantle rocks on Earth’s surface, and found similar clusters of dates that closely match the crustal dates, supporting the idea that the crust emerged from the mantle in pulses, billions of years ago (see Geotimes, November 2007).

Scientists may also be narrowing in on the age of Earth’s magnetosphere, the magnetic field that protects the planet from harmful solar radiation particles and keeps the atmosphere intact — and is therefore a key ingredient for life to form. The field was previously thought to have developed by 2.7 billion years ago, but geophysicist John Tarduno of the University of Rochester in New York and colleagues found that magnetic signatures in ancient crystals pushed that date back by another half-billion years to 3.2 billion years ago, they reported April 5 in Nature (see Geotimes, June 2007). And the field may be even older still: Theoretical models have predicted that the field may first have formed as early as 3.5 billion years ago. It probably isn’t much older than that, though — 3.9-billion-year-old dust grains from the moon containing trapped atmospheric gases from ancient Earth suggest that the protective field may not have existed yet.

According to another study, one hypothesis about when Earth’s atmosphere actually formed and how quickly it expelled gases from its cooling interior out into the atmosphere may require some revision. For decades, scientists have used isotopes of inert noble gases such as argon as a kind of clock to tell how long it takes for a non-reactive gas to travel to the surface of the planet, first within melt rising from deep within the mantle, and then by diffusion in the upper mantle. However, new research by geochemist Bruce Watson of Rensselaer Polytechnic Institute in Troy, N.Y., and colleagues, suggests that argon isotopes and possibly other inert gas isotopes may not be quite as reactive as once thought. Instead, they may fit cozily into the framework of some minerals, and lingering rather than escaping as quickly as possible (see story, this issue). If true, models of degassing based on inert gases may require an overhaul.

Carolyn Gramling

Links:
"How does your continent grow?," Geotimes, November 2007 
"So, when did Earth become attractive?" Geotimes, June 2007 
"Argon’s gassy ride to the atmosphere," Geotimes, December 2007 

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Deep Earth: Headline News Stories

Missing anchor found Beneath Tibet
Tibet became the roof of the world 50 million years ago, when the Indian subcontinent crashed into the Eurasian Plate. But it’s higher than it should be based on crustal thickening and compression alone. Seismic monitoring of the land beneath Tibet has revealed that a piece of lithosphere may be buried deep in the mantle. That “anchor,” which once helped weigh the crust down, may have peeled off and sunk into the mantle about 15 million years ago, allowing the land to rise even further.
Geotimes, April 2007; Sol et al., Geology, June 2007.

An ocean in the deep
Earth’s lower mantle may contain enough water to rival its surface oceans. Three-dimensional seismic maps of the mantle revealed anomalies of “dampened” signals that one study suggests indicate the presence of water. The water may have been dragged into the mantle with cold slabs of subducting crust — suggesting that water-bearing minerals may remain stable at much greater depths than thought.
Geotimes, May 2007; Lawrence and Wysession, AGU Monograph, 251-261, 2006.

It’s a smaller world
Researchers at the University of Bonn in Germany reported in July that Earth is about five millimeters smaller across than indicated by measurements made five years ago. To get the new diameter, the scientists used Very Long Baseline Interferometry, in which thousands of radio telescopes around the world make simultaneous observations of a signal from space, and the time lags between the observations gives the precise distance between the stations. Though small, the diameter difference matters for positioning satellites that measure, for example, sea-level changes.
Agence France-Presse, July 5, 2007.

What happened under Iran
Ten million to 20 million years ago, the Arabian Plate collided powerfully with Eurasia, forming a mountain chain stretching from the Alps to the Himalayas. Little was known of the geologic history before that collision, but researchers investigating the geology of what is today Iran have found evidence of crustal extension from 45 million to 40 million years ago, which formed deep faults at the surface and metamorphosed rocks deep belowground. The extension could be due to subduction of crust during a collision event — and could help explain a wave of volcanism in the region around that time.
Geotimes, September 2007; Verdel et al., GSA Bulletin, July/August 2007.

Heating up the crust
Parts of North America would be underwater if the crust weren’t so hot. The temperature, like the density and thickness, of the crust above the mantle affects land elevation — and in North America, heat alone is keeping some cities, such as Los Angeles and New York, above sea level, one study says. Residents shouldn’t be alarmed, however: It takes a billion years for continents to lose their heat.
Geotimes, September 2007; Chapman and Hasterok, Journal of Geophysical Research, June 23, 2007.

Borehole across the San Andreas reveals secrets
In September, scientists with the San Andreas Fault Observatory at Depth (SAFOD) recovered a core of rock from a 3.2-kilometer-deep borehole that passes through a part of the San Andreas Fault. The first core retrieved from an active fault zone, scientists hope the rocks will answer long-standing questions about why and how earthquakes occur. Data from the borehole have already suggested the answer to one question: The presence of soft talc in the fault zone could explain why some parts of the fault move slowly and steadily and don’t produce catastrophic earthquakes. This month, SAFOD will “auction” off portions of the core to scientists eager to study the fault.
Geotimes, October 2007; Geotimes online, Web Extra, Oct. 5, 2007; Moore and Rymer, Nature, Aug. 16, 2007.

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