My son Toby told me a story the other day that I thought was a perfect metaphor for how geologic reasoning works. Some friends of his had recently cleaned out the heating vents in their house, and they were horrified to find the layers of crayons, toys, legos, chess pieces, and lost marbles interspersed with dust and cat hair. As they went through the various vents of their house, they began to notice a pattern: most of the debris had built up since New York State went into lockdown in March of 2020. The family had simply been at home more, and the two parents (both professors) had less time to supervise the children. In their home, the topmost layer of debris could be called the “COVID Layer.”
They noticed other patterns too. For example, many of the items that accumulated in the vents had been deposited there during Christmas break or after birthday parties. And, of course, as they cleaned deeper and deeper into the vents, they found older and older items. Eventually, they encountered objects that had never been theirs––an old dice, a scrap of perforated printer paper, a glossy flower magnet they’d never seen before. These were the artifacts left by previous owners.
In geology, we tend to think of sediments accumulating in layers like pieces of a cake. However, to determine ages of those layers, we need to use a combination of forensic tools that include radiometric decay rates and “cross-correlation” of different layer types. That is, suppose a large volcanic eruption occurred 20 million years ago, and that ash from this eruption could be found from Wyoming all the way to Europe and Eastern Africa. Because we know the age of this eruption, whenever we encounter this layer of ash, we know that sediments below it must be older than 20 million years, and the sediments above would be younger (assuming no other processes had disrupted the layers).
Now imagine the year is 2030 and you are running a successful vent cleaning business. In homes where the owners had not cleaned their vents for over 10 years, you would probably notice the “COVID layer:” a conglomerate of crayons, colored pencils, scraps of paper, and small toys. Just like the ash from a massive volcano, you’d be able to observe this layer across many different homes across New York, the US as a whole, and in fact the entire world!