On the Age of the Earth: more than just meteorites!

If you inquire of your favorite search engine as to the 'age of the Earth', one number should dominate your results: 4.54 billion years. During the recent debate, Ken Ham was quick to point out, however, that this age is obtained through analysis of meteorites, and not the Earth itself. To conclude that the Earth is 4.54 billion years, therefore, we must argue that the principal, planetary components of our solar system formed at approximately the same time. In other words, this age depends on the fundamental accuracy of nebular theories in depicting our planet's earliest history.

A recent commenter on this blog also raised this point, so I'll reiterate my response:

"[Ken Ham] was correct that the accepted age of the Earth (~4.5 billion years) derives mainly from dating of meteorites. There's nothing wrong with this, so long as the meteorites are indeed the same age as the earth and derived from the same material from which our solar system form...

Although no rock on Earth is dated exactly at 4.5 billion years (why would we ever expect that?), we do have samples of zircon minerals that date to 4.2–4.4 billion years old... The fact that Earth materials approach 4.5 billion years, but never actually reach it, confirms the meteorite-based age of 4.5 billion years for the solar system and for the Earth itself."

Yesterday's issue of Nature Geoscience featured the latest research (Valley et al., 2014) on the very zircons I cited (see the BBC's report here). These tiny minerals had previously been dated through a very precise, U-Pb concordia method. Given their long and likely unpleasant history (it was a hot young Earth!), however, researchers sought to confirm the validity of the age through the latest in hi-tech microanalysis. Questions remained as to whether factors like radiation damage or trace-element diffusion had compromised the models used to calculate the zircons' ages. Radiation damage would have occurred during the radioactive decay of Uranium, when high-energy particles damaged the crystal structure on their way out. Also, little is known about the mobility of lead (Pb), the radiogenic daughter product of this decay chain, within the crystal itself. If the diffusion of lead were higher than expected, then the assumption of a closed system is no longer valid, and the calculated ages of 4.2–4.4 billion years would represent slight overestimates.

Using a combination of instruments—Scanning Electron Microscope (SEM), Electron Backscatter Diffraction (EBSD), Secondary Ion Mass Spectrometer (SIMS), and a Local Electrode Atom Probe (LEAP)—the research team constrained the distribution of trace elements, the integrity of the mineral structure, and the measured isotopic ratios to confirm that the previously calculated ages were indeed valid. These results are consistent with a 4.54-billion-year-old Earth and solar system, an early magma ocean, and the presence of solid crustal rocks shortly after Earth's formation.

So the next time anyone takes Ham's lead in raising doubts about the currently accepted age of the Earth by directing you to meteorite analyses, be sure to inform them that the oldest Earth materials date so close, that the difference is inconsequential. One added lesson to this tale is that while geochronology demands that assumptions are made, these assumptions can be—and frequently are—tested thoroughly through independent techniques. Simply pointing out that assumptions exist in the historical sciences is not an argument against their conclusions. But since Ken Ham's paradigm lacks the predictive capabilities and scientific rigor of conventional geology, these red herrings are perhaps the best he can offer.