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Andrew Alden

AGU Day One: The Early Earth

By December 5, 2011

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The giant meeting known as AGU has begun, turning downtown San Francisco into a sea of geoscientists. I'll be here all week (really!), and for my first morning's scientific session I picked a set of invited talks on the early Earth—namely, the Hadean and Archean eons of geologic time.

Linda Elkins-Tanton showed us that all of the Earth-type planets, everything from Mercury to Mars, should have been born with water oceans. New models allow water-bearing material to have been part of the cosmic dough that became these planets, with the water well mixed. Even after the violent phase of planet formation, when planetoids were splatting together like sumo wrestlers (if you can imagine sumo wrestlers that coagulate together), there should have been enough water to swiftly build up oceans several hundred meters deep. And that's before any later veneering with water by comet impacts.

Aaron Cavosie discussed the oldest Earthly objects, the tiny zircon crystals found in extremely old rocks that date from Hadean time, more than 4 billion years ago (4 Ga). He showed from isotopic evidence that these probably formed in continental granitoid rocks, meaning that at least some continents were around from the beginning, around 4.2 Ga. He said they show no evidence of subduction, plate tectonics, or other modern geological processes.

The next two speakers addressed the origin of the geomagnetic field, which is very important in Earth history because it created the magnetic shield in space that has preserved the atmosphere from being eaten away by the sun. We know that Moon dust preserves atoms sputtered away from Earth's atmosphere before 3.9 Ga (imagine that), meaning that there was no geomagnetic field then. Calculations point to a geodynamo starting by around 3.5 Ga, and hard evidence from rocks in the Barberton Belt document a strong field as of 3.45 Ga.

The talk that boggled me most was by Patrice Rey, who argued that the earliest continents were so soft, and the early seas so shallow, that mountains higher than about 3 kilometers didn't exist during the Archean. He described a sea world in which dry land covered less than 5 percent of the globe. This state of affairs kept rocks from weathering, the atmosphere from changing, and life from adapting for a very long time.

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