A Spiraling, Slightly Squiggly Path to the Past
The series of geological epochs are like pages in a long and complicated book, recording events of the past
The Earth is 4.5 billion years of age. Evidence of our planet’s antiquity is
primarily contained in the rocks that form the Earth’s crust. These once-molten rocks contain radioactive elements whose isotopes decay at known rates. Thus, the results of studies of rock layers (stratigraphy), and of fossils (paleontology), coupled with the ages of certain rocks as measured by atomic clocks (geo-chronology) offer the means for determining the age of the Earth.
UPDATE: As a result of my enthusiasm for the CiteULike electronic citation and somewhat social bookmarking service for academic, research and peer-reviewed publications, I realized that this calculation of the Earth’s age was not quite correct. I was taking CiteULike’s beta release through its paces, testing the service for Geological Survey citation functionality, particularly images. I returned to the U.S.G.S. site. After some very cursory digging about, I found another U.S.G.S. geological time page. It featured the same spiralling image as the one above. Except that the accompanying textual content was different. Let us refer to it as “Page 2″:
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 the process of plate tectonics. If there are any of Earth’s primordial rocks left in their original state, they have not yet been found.
Please be aware: I am NOT implying a massive deception about the Earth’s age perpetrated by the U.S.G.S. on an unsuspecting public. Nor will I surmise that our planet is actually only 6000 years old. The creationists were right all along! No. Not about this.
The geophysical method, based on radiometric dating of Uranium-235 and 238 isotope decay was identical on both U.S.G.S. pages. Page 2 cited the many elderly rocks strewn about our planet, in Swaziland, Greenland, Michigan and Australia. Most were 3.4 -3.7 billion years of age. Let’s use this as the boundary value for the youngest possible Earth.
The oldest rocks discovered to-date are zircon crystals hailing from Western Australia, at 4.3 billion years of age. However, these too have sedimentary origins. Plate tectonics is the villain, making it extremely unlikely to find rocks that were part of the “primordial crust” associated with Earth’s formation. Thus even these rock samples do not actually tell us the Earth’s age. They can only offer a data point of 4.3 billion years or older. This would be our estimate for the Earth’s age based on the approach described by the first U.S.G.S. source (“Page 1″). Page 1′s estimate was 4.5 billion years. That’s a mere 0.2 billion year discrepancy.
Well, “merely” isn’t the best word choice, as 0.2 billion years seems longer in different units. There is a 200 million year difference between the estimate from Page 1 versus Page 2. In relative terms, that’s a 4.5 percent difference.
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! Yet many of our moon rocks are 4.4 – 4.5 billion years old. This further extends the probable age of the Earth. According to Page 2,
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 Page 1 adequate for most readers’ purposes.
My next post will not involve ad hoc auditing of the U.S. Geological Survey website. Instead, I prepared a Security Update. I’ll include a new, free magazine from Y-Combinator (sort of), a few doo-dads, maybe even a video. I’m hoping it will be fun for everyone!