I’ve got one wallet-sized version of the 1983 Geological Society of America time scale in my field pouch, and another page-sized version that used to be taped to the wall in front of my desk. I relied on it a lot, especially when starting work in a new field area. In the early 90’s, when I was trying to break into the world of Appalachian tectonics research, I started by trying to figure out where the problems were in the models of how the metamorphic rocks of New England came to be. I spent a lot of time reading papers from different subdisciplines – the records of old mountain belts are partially recorded in the basins where eroded sediments collect, in volcanic rocks erupted before plates collide, in granites that intrude the deeper parts of the crust, and in the metamorphic rocks that tell how hot and how deep rocks were in the past. Some of those papers used fossils to tell time, some used radioactive isotopes, and some used complicated reasoning based on both stratigraphy and geochronology. Some of the papers told time using stratigraphic subdivisions: Emsian, Eifelian. Some used numbers: 390 million years old. Figuring out what happened when required a translation key, and the GSA time scale was a good one.
Lots of new radiometric ages of important stratigraphic boundaries, plus lots of discussions by the International Commission on Stratigraphy, have made my well-worn time scales obsolete. (Even by the mid-90’s, my paper copy had all sorts of notes about corrected ages of boundaries between periods.) And computers have made it easier to distribute color images. So now, more than 25 years later, GSA has a new time scale:
Some geobloggers will complain that this time scale is already out-of-date, because it includes the Tertiary (which has been abandoned, poor thing, by the International Commission on Stratigraphy). Personally, I’m glad it’s still there, because the old terms make it possible to read the old literature and figure out what it means. This is, after all, a translation tool – something that is needed so that paleomagnetists and stratigraphers and geochronologists and people who use all of their data can talk to one another. And I like the addition of periods like the Cryogenian – periods that I know about, but haven’t worked in, and don’t know ages of.
I’m curious what other people think of it. How about those colors? Is it confusing, the way that all four eras (Cenozoic, Mesozoic, Paleozoic, Precambrian) take up the same amount of space, even though the Precambrian is about 9 times as long as all the others put together?