When I talk about my age, or my birthday, or similar topics, I have a base line of a documented day I was born and started breathing the atmosphere. When we talk about the age of a tree, we may be talking about a known planting date or the number of annual growth rings counted near the base. For a manufactured item, it might be known from its container, or make and model from a car, and can often be inferred at least approximately from stylistic changes through the years. In each case, there is a particular point in time that we consider a starting point: birth, first growth, manufacturing.
With something like an entire planet, it gets a little more complicated.
There's a new geotweep, Geologic Time, who is planning to tweet our planet's history as it would occur over the course of time if its ~4.54 billion years were compressed into one calendar year. I signed up to follow the other day because it sounds like a fun exercise and because while, yes, I know a great deal about earth history, I expect there will be any number of facts and tidbits that are new to me.
So when Geologic Time tweeted earlier (in a piece that has now apparently been taken down) that the proto-earth was currently in the process of accretion- planetesimals and rocky material falling together to form the young planet- I tweeted back asking "So what defines when protoplanet becomes 'Earth?'" In other words (I meant) what was the moment, event, or datum of some kind, that geoscientists use to say, "Before this point, it was proto-earth; after this point it was earth." Because there has to be a starting point, right?
Well, the short answer appears to be, "No, not exactly." The accretion process was messy and hot. The proportion of radionuclides- which decay and release more heat- was much higher than today. So there are no datable rocks from our planet's earliest history, and thus nothing on our planet that can be dated. The oldest known rocks are about 4 billion years old, and the oldest known mineral grains are detrital zircons from the Jack Hills of Australia, at about 4.4 billion years. The latter value represents an absolute minimum age for the earth.
My assumption had been that there had been more work done with the lunar samples. I know a lot of work and analysis has been done with those over the last 25 years, and I haven't followed it very carefully. I was also quite aware that the precision of radiometric dating had improved a great deal during that interval. As it turns out though, the planetary midnight of 1/1 is still the same as it was when I was a student: the age of the material that created the earth. Meteorites. I've been reading a bit on this topic since it came up this morning, and I'm not going to go into great detail, but essentially, we have the lower bound of about 4.4 billion years and an upper bound of meteorite ages of about 4.55 billion years. The earth had to have formed between those two times.
Wikipedia did offer one bit of information that was new to me, though. "Nevertheless, ancient Archaean lead ores of galena have been used to date the formation of Earth as these represent the earliest formed lead-only minerals on the planet and record the earliest homogeneous lead-lead isotope systems on the planet. These have returned age dates of 4.54 billion years with a precision of as little as 1% margin for error." So there are two independent and consistent paths that get us to the currently accepted age: meteorites and Archaean galena. One particularly interesting inference that I'm tentatively drawing from all this is that the time from protoplanetary disk to fully accreted planets must have been quite short in the grand scheme of things- maybe ten million to a hundred million years, tops.
The two links included above are both to Wikipedia, but two other sites I found particularly informative were Talk Origins, which appears to be primarily to provide ammunition for those refuting young-earthers, and Palaeos.com, The Hadean Age. The latter provides a description and discussion of what happened when during the first eon of the planet's history, and why we think so. From a distance of billions of years, it makes for fascinating and exciting reading, but I'm glad I wasn't there for it. Both have references from as recently as 2007, so while they may not be utterly current, they have definitely helped me come more up to date than I was a few hours ago.
How to Look at a Rock
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