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Do We Need Hadean Eras?

By April 19, 2014

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In geology, the rocks have a way of messing with our pretty schemes. One instance I'm thinking of involves the base of the geologic time scale. The Earth itself is about 4.5 billion years old--but the time scale starts at the base of the Archean Eon with a time unit called the Eoarchean Era, running from 4.0 billion years ago (4 Ga) to 3.6 Ga. Like most of the Precambrian time periods (and unlike the more familiar Phanerozoic time periods), the Eoarchean is based on arbitrary numbers rather than notable geologic events.

When this part of the time scale was put together, we figured, from long experience, that there weren't any rocks older than 4 Ga. So much for experience: now we have rocks in hand that are older than the official time scale, and some zircon crystals that are reliably dated at 4.4 Ga. Today, Hadean time is no longer a matter of conjecture. So I hope someone is thinking about setting up signposts in deep time for the Hadean Eon. I suggest the Paleohadean for 4.5 to 4.4 Ga, the Mesohadean for 4.4 to 4.2 Ga, and the Neohadean for 4.2 to 4.0 Ga.

Here's an oddity: in the Cenozoic Era, the Paleocene Epoch comes before the Eocene, but in the Archean the Eoarchean comes before the Paleoarchean. Why is that?


April 21, 2014 at 7:34 am
(1) Bill S. says:

Hi Andrew,

“some zircon crystals that are reliably dated at 4.4 Ga”

Could you tell us when this testing was done, and what method was used?

Thanks for a great site, and “lodes” of valuable info.

April 21, 2014 at 12:44 pm
(2) Andrew Alden says:

Bill, the Jack Hills of Australia have yielded zircon crystals of that age, as dated by the uranium-lead method. See a list of some of the relevant papers at http://geoscience.wisc.edu/geoscience/people/faculty/john-valley/the-earliest-piece-of-the-earth/

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