This is more of a terminological question, but I think it fits here. I've been reading a book by Rachel Carson, from the early 50s, and she uses the phrase "Archaeozic period". I haven't seen this used elsewhere - is it just an archaic (pardon the pun) form of "Archean", or does it describe a different period? Etymologically, it seems like it's more oriented around "life", but that may just be a semantic distinction.

  • $\begingroup$ Hi, if you think I answered your question, you should check the check mark next to the answer. $\endgroup$
    – Spencer
    Commented Jul 7, 2022 at 15:41

1 Answer 1


The Archaen was named by American geologist James Dwight Dana in 1872, to refer to the entire span of time before the Cambrian Period. A synonym was "Azoic" as this period was considered lifeless. Later, it was found that the carbon isotope ratios changed in rocks about 3.8 billion years old between ones that indicated no life present to ones that indicated life. So "Azoic" began to refer to the "lifeless" period before 3.8 billion years and "Archaeozoic" for the "earliest life" after that point.

That's the way it stood when Rachel Carson wrote her book. The 1969 World Book Encyclopedia which sat on my parents' bookshelf divided the Precambrian into the Proterozoic, Archaeozioc, and Azoic eras.

By 1972 this wasn't working, so Preston Cloud suggested the term "Hadean" to refer to the time between the formation of the Earth/Solar System and 4 billion years ago. At the same time, the boundary adjustment caused "Archean" to be preferred to "Archaeozoic".

But "Archaeozoic" matches up enough to be considered a synonym for "Archean" these days.

Here's a rabbit hole (PDF) to dive down.

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    $\begingroup$ Thank you, that's a very interesting answer. The history of our understanding of the planet is almost as interesting as the history of the planet itself. $\endgroup$
    – T.S
    Commented Jul 4, 2022 at 21:49

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