Geologists often need to know the age of material that they find. This is different to relative dating, which only puts geological events in time order. Most absolute dates for rocks are obtained with radiometric methods. There are two basic approaches: relative age dating, and absolute age To determine the relative age of different rocks, geologists start with. Unfortunately, those methods don't work on all rocks, and they don't work at There's no absolute age-dating method that works from orbit, and.
When you say that I am 38 years old or that the dinosaurs died out 65 million years ago, or that the solar system formed 4. We use a variety of laboratory techniques to figure out absolute ages of rocks, often having to do with the known rates of decay of radioactive elements into detectable daughter products.
Unfortunately, those methods don't work on all rocks, and they don't work at all if you don't have rocks in the laboratory to age-date. There's no absolute age-dating method that works from orbit, and although scientists are working on age-dating instruments small enough to fly on a lander I'm looking at you, Barbara Cohennothing has launched yet.
So that leaves us with relative ages. Relative ages are not numbers. They are descriptions of how one rock or event is older or younger than another. Relative age dating has given us the names we use for the major and minor geologic time periods we use to split up the history of Earth and all the other planets.
Relative-age time periods are what make up the Geologic Time Scale.
Earth Floor: Geologic Time
The Geologic Time Scale is up there with the Periodic Table of Elements as one of those iconic, almost talismanic scientific charts. Long before I understood what any of it meant, I'd daydream in science class, staring at this chart, sounding out the names, wondering what those black-and-white bars meant, wondering what the colors meant, wondering why the divisions were so uneven, knowing it represented some kind of deep, meaningful, systematic organization of scientific knowledge, and hoping I'd have it all figured out one day.
This all has to do with describing how long ago something happened. But how do we figure out when something happened?
There are several ways we figure out relative ages. The simplest is the law of superposition: We have no idea how much older thing B is, we just know that it's older. That's why geologic time is usually diagramed in tall columnar diagrams like this.
Just like a stack of sedimentary rocks, time is recorded in horizontal layers, with the oldest layer on the bottom, superposed by ever-younger layers, until you get to the most recent stuff on the tippy top.
On Earth, we have a very powerful method of relative age dating: Paleontologists have examined layered sequences of fossil-bearing rocks all over the world, and noted where in those sequences certain fossils appear and disappear. When you find the same fossils in rocks far away, you know that the sediments those rocks must have been laid down at the same time.
Relative and absolute ages in the histories of Earth and the Moon: The Geologic Time Scale
The more fossils you find at a location, the more you can fine-tune the relative age of this layer versus that layer. Of course, this only works for rocks that contain abundant fossils.
Conveniently, the vast majority of rocks exposed on the surface of Earth are less than a few hundred million years old, which corresponds to the time when there was abundant multicellular life here. Look closely at the Geologic Time Scale chartand you might notice that the first three columns don't even go back million years. That last, pink Precambrian column, with its sparse list of epochal names, covers the first four billion years of Earth's history, more than three quarters of Earth's existence.
Most Earth geologists don't talk about that much. Paleontologists have used major appearances and disappearances of different kinds of fossils on Earth to divide Earth's history -- at least the part of it for which there are lots of fossils -- into lots of eras and periods and epochs. When you talk about something happening in the Precambrian or the Cenozoic or the Silurian or Eocene, you are talking about something that happened when a certain kind of fossil life was present.
Major boundaries in Earth's time scale happen when there were major extinction events that wiped certain kinds of fossils out of the fossil record. This is called the chronostratigraphic time scale -- that is, the division of time the "chrono-" part according to the relative position in the rock record that's "stratigraphy".
The science of paleontology, and its use for relative age dating, was well-established before the science of isotopic age-dating was developed. Nowadays, age-dating of rocks has established pretty precise numbers for the absolute ages of the boundaries between fossil assemblages, but there's still uncertainty in those numbers, even for Earth. In fact, I have sitting in front of me on my desk a two-volume work on The Geologic Time Scalefully pages devoted to an eight-year effort to fine-tune the correlation between the relative time scale and the absolute time scale.
The Geologic Time Scale is not light reading, but I think that every Earth or space scientist should have a copy in his or her library -- and make that the latest edition. In the time since the previous geologic time scale was published inmost of the boundaries between Earth's various geologic ages have shifted by a million years or so, and one of them the Carnian-Norian boundary within the late Triassic epoch has shifted by 12 million years. With this kind of uncertainty, Felix Gradstein, editor of the Geologic Time Scale, suggests that we should stick with relative age terms when describing when things happened in Earth's history emphasis mine: For clarity and precision in international communication, the rock record of Earth's history is subdivided into a "chronostratigraphic" scale of standardized global stratigraphic units, such as "Devonian", "Miocene", "Zigzagiceras zigzag ammonite zone", or "polarity Chron C25r".
Unlike the continuous ticking clock of the "chronometric" scale measured in years before the year ADthe chronostratigraphic scale is based on relative time units in which global reference points at boundary stratotypes define the limits of the main formalized units, such as "Permian".
The chronostratigraphic scale is an agreed convention, whereas its calibration to linear time is a matter for discovery or estimation. We can all agree to the extent that scientists agree on anything to the fossil-derived scale, but its correspondence to numbers is a "calibration" process, and we must either make new discoveries to improve that calibration, or estimate as best we can based on the data we have already. To show you how this calibration changes with time, here's a graphic developed from the previous version of The Geologic Time Scale, comparing the absolute ages of the beginning and end of the various periods of the Paleozoic era between and I tip my hat to Chuck Magee for the pointer to this graphic.
Fossils give us this global chronostratigraphic time scale on Earth. On other solid-surfaced worlds -- which I'll call "planets" for brevity, even though I'm including moons and asteroids -- we haven't yet found a single fossil.
Something else must serve to establish a relative time sequence.
That something else is impact craters. Earth is an unusual planet in that it doesn't have very many impact craters -- they've mostly been obliterated by active geology.
Geologic Age Dating Explained
Venus, Io, Europa, Titan, and Triton have a similar problem. On almost all the other solid-surfaced planets in the solar system, impact craters are everywhere. The Moon, in particular, is saturated with them. We use craters to establish relative age dates in two ways. If an impact event was large enough, its effects were global in reach.
For example, the Imbrium impact basin on the Moon spread ejecta all over the place. Any surface that has Imbrium ejecta lying on top of it is older than Imbrium. Any craters or lava flows that happened inside the Imbrium basin or on top of Imbrium ejecta are younger than Imbrium.
Imbrium is therefore a stratigraphic marker -- something we can use to divide the chronostratigraphic history of the Moon. Apollo 15 site is inside the unit and the Apollo 17 landing site is just outside the boundary.
There are some uncertainties in the positions of the boundaries of the units. The other way we use craters to age-date surfaces is simply to count the craters. At its simplest, surfaces with more craters have been exposed to space for longer, so are older, than surfaces with fewer craters.
Of course the real world is never quite so simple.
- Pre/Post-Test Key
- What is the difference between absolute age and relative age of fossils?
There are several different ways to destroy smaller craters while preserving larger craters, for example. Despite problems, the method works really, really well. Most often, the events that we are age-dating on planets are related to impacts or volcanism. Volcanoes can spew out large lava deposits that cover up old cratered surfaces, obliterating the cratering record and resetting the crater-age clock.
When lava flows overlap, it's not too hard to use the law of superposition to tell which one is older and which one is younger. If they don't overlap, we can use crater counting to figure out which one is older and which one is younger. In this way we can determine relative ages for things that are far away from each other on a planet.
Interleaved impact cratering and volcanic eruption events have been used to establish a relative time scale for the Moon, with names for periods and epochs, just as fossils have been used to establish a relative time scale for Earth. There are two basic approaches: Here is an easy-to understand analogy for your students: Absolute age dating is like saying you are 15 years old and your grandfather is 77 years old.
To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones.
Geologic Age Dating Explained - Kids Discover
This is called the Rule of Superposition. This rule is common sense, but it serves as a powerful reference point. Geologists draw on it and other basic principles http: Relative age dating also means paying attention to crosscutting relationships.
Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type. Pretty obvious that the dike came after the rocks it cuts through, right? With absolute age dating, you get a real age in actual years. Based on the Rule of Superposition, certain organisms clearly lived before others, during certain geologic times.
The narrower a range of time that an animal lived, the better it is as an index of a specific time.Laws of Relative Rock Dating
No bones about it, fossils are important age markers. But the most accurate forms of absolute age dating are radiometric methods. This method works because some unstable radioactive isotopes of some elements decay at a known rate into daughter products.
This rate of decay is called a half-life. Half-life simply means the amount of time it takes for half of a remaining particular isotope to decay to a daughter product. Good discussion from the US Geological Survey: There are a couple catches, of course. Not all rocks have radioactive elements. Sedimentary rocks in particular are notoriously radioactive-free zones. So to date those, geologists look for layers like volcanic ash that might be sandwiched between the sedimentary layers, and that tend to have radioactive elements.