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How do we tell the difference between geologic ages?

You can see the different layers of rock laid down over time in Utah's slot canyon in Grand Staircase-Escalante National Monument.
You can see the different layers of rock laid down over time in Utah's slot canyon in Grand Staircase-Escalante National Monument.
(Image: © Shutterstock)

From the emergence of life to mass extinctions, Earth has gone through incredible changes in its 4.6 billion years. With so much history, how can researchers keep track of what happened when? 

The system many scientists have settled on is the International Geologic Time Scale (laid out here in the International Chronostratigraphic Chart), which breaks geologic time into five units. From the longest to the shortest and most precise, those units are eons, eras, epochs, periods and ages. 

The various stages of geologic time are "defined by visible changes in the fossil record," according to Jacquelyn Gill, associate professor of paleoecology and plant ecology at the University of Maine. Fossils are a handy tool in this dating work for a few reasons. For one, life has likely been around for about 90% of Earth's existence, so the history of Earth is paralleled by the history of life. Fossils are also useful because changes in the fossil record reflect changes in ecology, that is, the relationships between living things and their environment. These changes in Earth’s ecology tend to reflect the major events in the planet’s history, Gill said.

Related: Can rocks grow?

One important moment in geologic time was the transition from the Mesozoic era to the Cenozoic era about 65 million years ago. The change was spurred by the asteroid impact that eventually killed the nonavian dinosaurs

"That was one bad afternoon that took time to play out fully," Gill told Live Science. The effects of that bad day have rippled through the tens of millions of years since. It ushered in our current era and allowed mammals and flowering plants to thrive. 

"If you [were to] time travel, that's [the earliest point] when you'd know you were on planet Earth," Gill said of the early Cenozoic era, "at least until some weird mammal ran by."

But you don't have to get your hands on a time machine to appreciate the magnitude of the change between the Mesozoic and the Cenozoic. Careful study of a rock layer can be enough to help a researcher figure out its age. For example, the impact credited with ending the Mesozoic era and beginning the Cenozoic era is marked with a layer containing unusually high levels of iridium, which is much more common in meteors than in the Earth’s crust. 

The 2020 International Chronostratigraphic Chart

The International Chronostratigraphic Chart of 2020. (Image credit: International Commission on Stratigraphy, http://www.stratigraphy.org)

Other changes in mineral and element ratios can offer evidence of Earth's tumultuous history, too. For instance, a little more than 5 million years ago, tectonic activity closed the Strait of Gibraltar, eventually causing the Mediterranean Sea to shrink and its mineral content to spike. This event occurred during the Messinian age and marked the end of the Miocene epoch and the beginning of the Pliocene epoch, about 5.3 million years ago. 

"Salt and gypsum deposits are now observable in many of the countries that surround the Mediterranean and are now exposed as rocks above the modern sea level," because of tectonic activity in the millions of years since it happened for the last time, according to Karl Wegmann, an associate professor of geology at North Carolina State University.

Recent advances in geochronology, or rock dating, allow geologists to measure radioactive decay and "assign very precise absolute ages to geologic events," Wegmann told Live Science via email. To do this, geochronologists calculate the age of rocks by comparing the proportions of certain isotopes, or elements that have a different number of neutrons in their nuclei than normal. All of these approaches fit together like a jigsaw puzzle to give researchers a common language for discussing the distant past.

The geologic time scale is a scientific tool, but it's also an artifact of history. Objectively measuring properties like radioactive decay can tell researchers when layers of rock formed, but it's up to scientists, who are often building on the work of their predecessors, to decide how to slice and dice the data into geologic time frames. One of the most contentious questions in chronostratigraphy is how to define our own time.

"The Holocene is kind of an arbitrary epoch," Gill said. The Holocene epoch began about 12,000 years ago when Earth began warming after the last ice age. But according to Gill, the end of that ice age, even though it coincided with the transition to a new epoch, isn't of greater geological importance than the end of the ice ages before it. 

Even today, scientists are still naming new windows of time, including the Chibanian age, named after a Japanese prefecture where the sediment defining the age was found. Many scientists and others claim that humans’ recent effect on the planet merits the declaration of a new age, the Anthropocene, while other scholars say the Capitalocene more accurately conveys the social systems that have so greatly affected the planet since the Industrial Revolution. 

Editor's Note: This story was updated to correct Karl Wegmann's title. He is an associate professor at North Carolina State University, not the University of North Carolina.

Originally published on Live Science

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  • TorbjornLarsson
    admin said:
    Earth's history is long. How do scientists keep track of what happened when?

    How do we tell the difference between geologic ages? : Read more

    Integrated genomic and fossil evidence agree that life evolved way before the first 10 % of Earth's history ended, likely right after the global ocean reformed after the Moon forming impact https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6152910/pdf/emss-78644.pdf ].

    Which brings me to the suggestion, not yet accepted by IGUS, to define convenient pre-Hadean solar system geological ages https://en.wikipedia.org/wiki/Hadean#Subdivisions ]. The common Chaotian Eon of the planetary disk dust and rock assembly would split into separate planetary geology. For Earth Chaotian would end then the proto-Earth body, labeled Tellus, would be hit by the Theia impactor to gain its last 10 % of mass and a big Moon https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100036717_2010036774.pdf ].
    Reply
  • Rod mcfadden
    Science has given way too much energetic credence to the idea of an asteroid impact destroying the dinosaur epoc... an asteroid could not have possibly done as much damage as is reported...if one did possibly hit in the yucatan area as storied..the area looks more like a senote ut an impact crater...we must not ignore the iridium left behind...from the use of expended atomic weapons...yes they are an old device..past great technologies used to inhabit this planet and war among them selves sadly enough..and doing great damage to this diamond in the vast galactic sea of space..we best at using common sense when looking at big events..so the science and political establishment does not get to let to us any more.
    Reply
  • Broadlands
    The first 10% of Earth's history is supposed to be anoxic. The biggest difficulty with the GOE and the anoxic atmosphere-ocean hypothesis is the simple fact that cyanobacteria cannot live and grow (even survive) in an ocean saturated with reduced iron. There have been no successful experiments simulating such conditions. This alone falsifies the GOE and any phylogenetic theories about the LUCA that rely on that.
    Reply
  • William Pennat
    The change was spurred by the asteroid impact that eventually killed the nonavian dinosaurs. -- from the story

    I'm sorry but, again, calling birds 'dinosaurs' angers the heck out of me. Birds are NOT dinosaurs in any real sense of the word. Birds evolved FROM dinosaurs and so belong to the same biological clade. But then, mammals evolved from therapsids and belong to that clade but we don't go around calling mammals "therapsids". Calling birds dinosaurs started as a kind of paleontological joke/fad a couple of decades ago when the clade system became widely used because some paleontogist(s) likely thought it was cute and clever. So of course, science journalists took up the fad. But it is misleading and bad biology (as well as bad science journalism).
    Reply
  • Broadlands
    William Pennat said:
    The change was spurred by the asteroid impact that eventually killed the nonavian dinosaurs. -- from the story

    I'm sorry but, again, calling birds 'dinosaurs' angers the heck out of me. Birds are NOT dinosaurs in any real sense of the word. Birds evolved FROM dinosaurs and so belong to the same biological clade. But then, mammals evolved from therapsids and belong to that clade but we don't go around calling mammals "therapsids". Calling birds dinosaurs started as a kind of paleontological joke/fad a couple of decades ago when the clade system became widely used because some paleontogist(s) likely thought it was cute and clever. So of course, science journalists took up the fad. But it is misleading and bad biology (as well as bad science journalism).

    This is just a semantic argument. Misleading but irrelevant to telling the difference between one geological event and another.
    Reply
  • peter303
    admin said:
    Earth's history is long. How do scientists keep track of what happened when?

    How do we tell the difference between geologic ages? : Read more
    The oldest method was to note the different stratigraphic layers, e.g. alternating sandstone, limestone, volcanic and plutonic layers. The change in layers represents a change in geological processes. These changes may occur on a regional or global scale. Internal fossils, if any, constrain the age and allow one to tie different regions together. Stable radiometric minerals, if any, allow radiometric dating and correlations across regions.

    Early geologists in Britain, France and Germany developed the method of stratigraphic dating in the 1700s. Radiometric dating was not developed until the 1900s.

    Field geologists, when geologically mapping a new area, create a reference stratigraphic column for that region. It may be substantially the same as nearby geologic regions, but with local deviations. In some cases the column might be considerably different from nearby columns due to crossing a plate tectonic boundary where there were large horizontal motions between the plates over the geologic ages.
    Reply
  • TorbjornLarsson
    Rod mcfadden said:
    Science has given way too much energetic credence to the idea of an asteroid impact destroying the dinosaur epoc... an asteroid could not have possibly done as much damage as is reported...if one did possibly hit in the yucatan area as storied.

    The impact is a well tested observation. with the geological residues (iridium, shocked quartz, cenote ring, seafloor ring, strewn field) and the latest drill results (show the event sequence). The effects would suffice according to many papers.

    The debate was not that there was am immediate change to a new geological era and that it was caused by an impact (iridium boundary layer), it was if the impact was alone. That was recently shown to be the case by finding well preserved sediments so giving finely resolved dating. The other factors made their stuff at the wrong dating (some 0.2 Myrs separation, IIRC).
    Reply
  • TorbjornLarsson
    Broadlands said:
    The first 10% of Earth's history is supposed to be anoxic. The biggest difficulty with the GOE and the anoxic atmosphere-ocean hypothesis is the simple fact that cyanobacteria cannot live and grow (even survive) in an ocean saturated with reduced iron. There have been no successful experiments simulating such conditions. This alone falsifies the GOE and any phylogenetic theories about the LUCA that rely on that.

    Earth was observably anoxic for 50 % of its age, based on observation. A recent isotopic analysis show that oxygenation can have been coeval with the ending of high mantle/crust heat flow with thin, sluggish plate tectonics and starting the growth of continents https://www.nature.com/articles/s41561-020-0538-9 ]. Anoxic Earth, since "igh-temperature fluid–rock interactions strip 16O from hydrother-mal fluids, ...".

    The cyanobacteria ancestor lineage is very old, and it was and is poisoned by oxygen as everything else so the GOE meant it too saw 99 % species extinction. Its modern fair oxygen tolerance (though RuBisCO that can't separate much between O2 and its target CO 2is a bummer) evolved.

    "Integrated genomic and fossil evidence illuminates life’s early evolution and eukaryote origins", https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6152910/pdf/emss-78644.pdf; but see the image from https://phys.org/news/2018-08-timescale-evolution-life-earth.html !!! For some reason the mounted image gets kicked out from LifeScience server, so you have to go to the link. It shows the dating for cyanobacteria, GOE and chloroplasts in a cleaner manner than the original paper.

    The above phylogeny show that nothing is falsified by the geology but instead supports the evolutionary tree (since, well, evolution). By the way, and as a reflection on the article description of life and geology coevolving, the integrated evidence credibility range for evolution of LUCA is ~50 Myrs right after the Moon forming impact. It's coevolution all the way down.
    Reply
  • TorbjornLarsson
    William Pennat said:
    I'm sorry but, again, calling birds 'dinosaurs' angers the heck out of me. Birds are NOT dinosaurs in any real sense of the word. Birds evolved FROM dinosaurs and so belong to the same biological clade. But then, mammals evolved from therapsids and belong to that clade but we don't go around calling mammals "therapsids". Calling birds dinosaurs started as a kind of paleontological joke/fad a couple of decades ago when the clade system became widely used because some paleontogist(s) likely thought it was cute and clever. So of course, science journalists took up the fad. But it is misleading and bad biology (as well as bad science journalism).

    I believe the terminology has been adopted in most papers. You can't get a consistent terminology mapping between taxonomy, morphological/fossil species and genomic biological species - and it's even worse for prokaryote OTU clustering. What we can do is to make useful taxonomy. Birds as dinosaurs, mammals as archosaurs, ..., tetrapods as fishes et cetera is useful if you want to illustrate evolutionary relationships and avoid confusing paraphyly.
    Reply