physical science

Geological Time Spiral

Evidence of our planet’s antiquity is revealed by the once-molten rocks that form the Earth’s crust. These rocks contain radioactive elements whose isotopes decay at known rates. Study of geo-strata, paleontology and atomic dating of certain rocks is a reliable method for determining the age of the Earth. According to the US Geological Survey publication 2008-58, Earth is 4.5 billion years of age.

Geological epochs are like onionskin leaves in a folio

Due to my enthusiastic use of CiteULike citation and (somewhat) social bookmarking for scholarly journal and research publications, I realized that the U.S. Geological Survey calculation of the Earth’s age might not be correct.


I was taking CiteULike’s beta release through its paces, testing whether USGS DOIs (digital object identifiers) were supported, using USGS 2008-58 (call it PAGE1) as my test case. They weren’t, so I returned to the USGS website to investigate. Instead, after some cursory digging, I found a second geological time explanation. Let us refer to it as “PAGE2”. PAGE2 featured the same spiraling graphic as the one above, from PAGE1, except the accompanying text was different:

So far, scientists have not found a way to determine the exact age of the Earth directly from Earth rocks because Earth’s oldest rocks have been recycled and destroyed by plate tectonics. If there are any primordial rocks left in their original state, they have not yet been found.

NB: I am NOT suggesting a massive deception about the Earth’s age, perpetrated by the United States Geological Survey upon an unsuspecting public! Nor will I conclude that the creationists are correct, not about this.

Both pages described the geophysical method, based on radiometric dating of Uranium-235 and Uranium-238 isotope decay, identically. PAGE2 cited the many elderly rocks strewn about our planet, in Swaziland, Greenland, Michigan and Australia. Most are 3.4 to 3.7 billion years of age. The oldest rocks discovered to-date are zircon crystals hailing from Western Australia, at 4.3 billion years of age. They are also sedimentary, rather than being fresh from the crust. Plate tectonics is the villain. In fact, tectonic activity makes it extremely unlikely to find any rocks that are directly from the primordial crust associated with Earth’s formation. Even the oldest known rock samples do not actually tell us the Earth’s age. So, based on PAGE2, 4.3 billion years is the oldest verifiable age of the Earth, and even that is only an estimated value.

Recall the PAGE1 estimate, of 4.5 billion years. That’s a mere 0.2 billion year discrepancy. Well, “merely” isn’t the best word choice, as 200 million years is a 4.5 percent difference between PAGE1 and PAGE2.


The moon is actually more useful for determining the Earth’s age. The moon wasn’t subject to plate tectonic activity. Unfortunately, we have a limited supply of moon rocks available for study. Our ability to gather new samples for testing is also limited!  Our moon rocks are 4.4 to 4.5 billion years old. This further extends the probable age of the Earth. According to PAGE1,

the best age for the Earth comes not from dating individual rocks but by considering the Earth and meteorites as part of the same evolving system…determining an age for the Earth and meteorites, and hence the Solar System, of 4.54 billion years with an uncertainty of less than 1 percent.

I see no need to alert the authorities. Consider this as a take-away: The first answer one finds, even from an official site, is not necessarily the most accurate. Context is everything, though. I suspect that expedience would make PAGE2 adequate for most readers’ purposes.

My next post will not involve ad hoc auditing of the U.S. Geological Survey website.

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