My Time Scale: Introduction to Relative and Absolute Dating Activity and since the relative age of rock layers can be determined by superposition, rock layers. As we learned in the previous lesson, index fossils and superposition are effective methods of Define the difference between absolute age and relative age. There's no absolute age-dating method that works from orbit, and On Earth, we have a very powerful method of relative age dating: fossil assemblages. . the lunar surface during the mission's second extravehicular activity.
- Geologic Age Dating Explained
Dinosaurs disappeared about 65 million years ago. That corn cob found in an ancient Native American fire pit is 1, years old. How do scientists actually know these ages? Geologic age dating—assigning an age to materials—is an entire discipline of its own.
My Time Scale: Introduction to Relative and Absolute Dating Activity
In a way this field, called geochronology, is some of the purest detective work earth scientists do. 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. This is called the Rule of Superposition. This rule is common sense, but it serves as a powerful reference point.
Relative and absolute ages in the histories of Earth and the Moon: The Geologic Time Scale
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? Tanaka and Hartmann suggest that the decline in mare volcanism -- and whatever impact crater density is associated with the last gasps of mare volcanism -- would be a better marker than any one impact crater.
Most recently, a few late impact craters, including Copernicus, spread bright rays across the lunar nearside. Presumably older impact craters made pretty rays too, but those rays have faded with time. Rayed craters provide another convenient chronostratigraphic marker and therefore the boundary between the Eratosthenian and Copernican eras.
The Copernican period is the most recent one; Copernican-age craters have visible rays. The Eratosthenian period is older than the Copernican; its craters do not have visible rays. Here is a graphic showing the chronostratigraphy for the Moon -- our story for how the Moon changed over geologic time, put in graphic form.
Basins and craters dominate the early history of the Moon, followed by mare volcanism and fewer craters. Red marks individual impact basins. The brown splotch denotes ebbing and flowing of mare volcanism. Can we put absolute ages on this time scale? Well, we can certainly try.
The Moon is the one planet other than Earth for which we have rocks that were picked up in known locations. We also have several lunar meteorites to play with. Most moon rocks are very old. All the Apollo missions brought back samples of rocks that were produced or affected by the Imbrium impact, so we can confidently date the Imbrium impact to about 3. And we can pretty confidently date mare volcanism for each of the Apollo and Luna landing sites -- that was happening around 3.
Not quite as old, but still pretty old. Alan Shepard checks out a boulder Astronaut Alan B.
Note the lunar dust clinging to Shepard's space suit. The Apollo 14 mission visited the Fra Mauro formation, thought to be ejecta from the Imbrium impact.
Geologic Age Dating Explained - Kids Discover
Beyond that, the work to pin numbers on specific events gets much harder. There is an enormous body of science on the age-dating of Apollo samples and Moon-derived asteroids. We have a lot of rock samples and a lot of derived ages, but it's hard to be certain where a particular chunk of rock picked up by an astronaut originated.
The Moon's surface has been so extensively "gardened" over time by smaller impacts that there was no intact bedrock available to the Apollo astronauts to sample. And it's impossible to know where a lunar meteorite originated. So we can get incredibly precise dates on the ages of these rocks, but can't really know for sure what we're dating. Consequently, there is a lot of uncertainty about the ages of even the biggest events in the Moon's history, like the Nectarian impact.
There's some evidence suggesting that it's barely older than Imbrium, which means that there was a period of incredibly intense asteroid impacts -- the Late Heavy Bombardment. There are other people who argue that the rocks we think are from the Nectaris are either actually from Imbrium or were affected by Imbrium, so that we don't actually know when Nectaris happened and consequently can't say for sure whether the Late Heavy Bombardment happened.
Dating lunar asteroids doesn't help; none have been found that are older than 3. It seems like there's a lot of evidence supporting the idea that it happened, and there's a workable explanation of why it might have happened, but there's a problematic lack of geologic record for the time before it happened.
But we do the best we can with what we've got. Here is the same diagram I showed above, but this time I've squished and stretched parts of it to fit a linear time scale on the right. I drew in a billion years' worth of lines for the boundary between the Eratosthenian and Copernican ages, because we really don't have data that tells us where precisely to draw that line.
Look how squished the Moon's history is! Almost all the cratering happened in the bottom bit of the diagram. The volcanism pretty much ended halfway through the Moon's history. For more than two billion years -- half the diagram -- almost no action.
A crater here, a little squirt of volcanism there. But it's really not nearly as neat as the crisp lines on this diagram make it seem. Most of the events on the list could move up and down the absolute time scale quite a lot as we improve our calibration of the relative time scale. When I write for magazines, my editors always ask me to put absolute numbers on the dates of past events. I absolutely hate absolute ages in planetary science, because their precision is illusory, even for a place like the Moon for which we have quite a lot of returned samples.
It gets much, much worse for other worlds. Relative ages are more accurate, among scientists anyway. To the new list, add information about exactly how many years ago the event occurred 4. The list so far is what geologists refer to as a relative or sequential timeline. Discuss the difference between relative time, ordering events by which happened first, and absolute time, the number of years ago each event happened. A relative timeline can be observed from rock layers just by reading it from bottom to top.
But is it possible to observe the actual, absolute number of years ago a rock layer formed just by looking at it?
You can determine absolute time for events from your own life because you can remember each event, but nobody can remember the exact, absolute age of a rock. Scientists use special tools to find the absolute, number of years ago a given rock layer was formed.
My Time Scale: Introduction to Relative and Absolute Dating Activity
More on this in later lessons. After the discussion about relative and absolute time, students can categorize events into a hierarchical series of time periods.Relative Vs Absolute Dating
These will form the largest divisions of your personal time scale and will be called eons. Students should draw a horizontal line across the eon, era and period columns, dividing their table according to the criteria they chose. They should name the categories with a 1 word label that ends in the suffix —ian or —ic. In the eon column, they should write down the names they chose.
Now students should divide each of the eons into 2 or 3 eras based on a different criteria. Students should divide their tables by drawing a line across the era and period columns according to the criteria they chose and should write in the name of the eras. Finally, students should divide each era into 2 or 3 periods, if that is possible.
Students should divide their tables by drawing a line across the period column and should write in the name of the period. If you have remaining time, students can color their tables or share them with one another in small groups.
Sources Activity adapted from http: