RELATIVE VS. ABSOLUTE DATING Transcript of RELATIVE VS. ABSOLUTE DATING. RELATIVE VS. EXAMPLE: Absolute Dating. What is relative and absolute dating and how do archaeologists employ both? Relative Dating Examples geological principle that in any pile of sedimentary rocks that have not been disturbed by folding or overturning, each bed is older. There's no absolute age-dating method that works from orbit, and Relative-age time periods are what make up the Geologic Time Scale . For example, the Imbrium impact basin on the Moon spread ejecta all over the place.
This method compares the age of remains or fossils found in a layer with the ones found in other layers.
The comparison helps establish the relative age of these remains. Bones from fossils absorb fluorine from the groundwater. The amount of fluorine absorbed indicates how long the fossil has been buried in the sediments. This technique solely depends on the traces of radioactive isotopes found in fossils. The rate of decay of these elements helps determine their age, and in turn the age of the rocks.
Physical structure of living beings depends on the protein content in their bodies. The changes in this content help determine the relative age of these fossils. Each tree has growth rings in its trunk. This technique dates the time period during which these rings were formed. It determines the period during which certain object was last subjected to heat.
It is based on the concept that heated objects absorb light, and emit electrons. The emissions are measured to compute the age. Differentiation Using a Venn Diagram A Venn diagram depicts both dating methods as two individual sets.
The area of intersection of both sets depicts the functions common to both. Take a look at the diagram to understand their common functions. When we observe the intersection in this diagram depicting these two dating techniques, we can conclude that they both have two things in common: Provide an idea of the sequence in which events have occurred. Determine the age of fossils, rocks, or ancient monuments.
Although absolute dating methods determine the accurate age compared to the relative methods, both are good in their own ways.
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. 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. The chapter draws on five decades of work going right back to the origins of planetary geology.
The Moon's history is divided into pre-Nectarian, Nectarian, Imbrian, Eratosthenian, and Copernican periods from oldest to youngest. The oldest couple of chronostratigraphic boundaries are defined according to when two of the Moon's larger impact basins formed: There were many impacts before Nectaris, in the pre-Nectarian period including 30 major impact basinsand there were many more that formed in the Nectarian period, the time between Nectaris and Imbrium.
The Orientale impact happened shortly after the Imbrium impact, and that was pretty much it for major basin-forming impacts on the Moon. I talked about all of these basins in my previous blog post. Courtesy Paul Spudis The Moon's major impact basins A map of the major lunar impact basins on the nearside left and farside right. There was some volcanism happening during the Nectarian and early Imbrian period, but it really got going after Orientale.
Vast quantities of lava erupted onto the Moon's nearside, filling many of the older basins with dark flows. So the Imbrian period is divided into the Early Imbrian epoch -- when Imbrium and Orientale formed -- and the Late Imbrian epoch -- when most mare volcanism happened. People have done a lot of work on crater counts of mare basalts, establishing a very good relative time sequence for when each eruption happened. The basalt has fewer, smaller craters than the adjacent highlands. Even though it is far away from the nearside basalts, geologists can use crater statistics to determine whether it erupted before, concurrently with, or after nearside maria did.
Over time, mare volcanism waned, and the Moon entered a period called the Eratosthenian -- but where exactly this happened in the record is a little fuzzy.
Tanaka and Hartmann lament that Eratosthenes impact did not have widespread-enough effects to allow global relative age dating -- but neither did any other crater; there are no big impacts to use to date this time period. 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.
Relative Vs. Absolute Dating: The Ultimate Face-off
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. 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.
Relative Vs. Absolute Dating: The Ultimate Face-off
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!
Relative dating - Wikipedia
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.