Relative Vs. Absolute Dating: The Ultimate Face-off
Radiometric dating--the process of determining the age of rocks from the decay of .. Radiometric dating can be compared to an hourglass. how God, Creator of the universe, could care about humans: "When I .. Cornell University Geology Lecture Notes—A large number of pdf files of geology lecture notes are. 14C dating results with the 10Be age sequence from representative boulders; 4) the addition of relative dating techniques to the absolute ones to detect signals. Definition. Absolute and relative dating methods have been used to establish tentative chronologies for rock art. Relative dating refers to non-chronometric.
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. 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. 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.
Relative Vs. Absolute Dating: The Ultimate Face-off
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. Thus, if one measures argon as well as argon, one can calculate and subtract off the air-argon to get an accurate age. One of the best ways of showing that an age-date is correct is to confirm it with one or more different dating Some young-Earth proponents recently reported that rocks were dated by the potassium-argon method to be a several million years old when they are really only a few years old.
But the potassium-argon method, with its long half-life, was never intended to date rocks only 25 years old. These people have only succeeded in correctly showing that one can fool a single radiometric dating method when one uses it improperly. The false radiometric ages of several million years are due to parentless argon, as described here, and first reported in the literature some fifty years ago.
Note that it would be extremely unlikely for another dating method to agree on these bogus ages. Getting agreement between more than one dating method is a recommended practice. Although potassium-argon is one of the simplest dating methods, there are still some cases where it does not agree with other methods.
When this does happen, it is usually because the gas within bubbles in the rock is from deep underground rather than from the air. This gas can have a higher concentration of argon escaping from the melting of older rocks. This is called parentless argon because its parent potassium is not in the rock being dated, and is also not from the air. In these slightly unusual cases, the date given by the normal potassium-argon method is too old.
- Relative and Absolute dating Venn Diagram ( Block Diagram)
However, scientists in the mids came up with a way around this problem, the argon-argon method, discussed in the next section. Even though it has been around for nearly half a century, the argon-argon method is seldom discussed by groups critical of dating methods.
This method uses exactly the same parent and daughter isotopes as the potassium-argon method. In effect, it is a different way of telling time from the same clock. Instead of simply comparing the total potassium with the non-air argon in the rock, this method has a way of telling exactly what and how much argon is directly related to the potassium in the rock.
In the argon-argon method the rock is placed near the center of a nuclear reactor for a period of hours. A nuclear reactor emits a very large number of neutrons, which are capable of changing a small amount of the potassium into argon Argon is not found in nature because it has only a year half-life. This half-life doesn't affect the argon-argon dating method as long as the measurements are made within about five years of the neutron dose.
The rock is then heated in a furnace to release both the argon and the argon representing the potassium for analysis.
Relative and absolute ages in the histories of Earth and the Moon: The Geologic Time Scale
The heating is done at incrementally higher temperatures and at each step the ratio of argon to argon is measured. If the argon is from decay of potassium within the rock, it will come out at the same temperatures as the potassium-derived argon and in a constant proportion.
On the other hand, if there is some excess argon in the rock it will cause a different ratio of argon to argon for some or many of the heating steps, so the different heating steps will not agree with each other.
A typical argon-argon dating plot. Figure 2 is an example of a good argon-argon date. The fact that this plot is flat shows that essentially all of the argon is from decay of potassium within the rock. The potassium content of the sample is found by multiplying the argon by a factor based on the neutron exposure in the reactor.
When this is done, the plateau in the figure represents an age date based on the decay of potassium to argon There are occasions when the argon-argon dating method does not give an age even if there is sufficient potassium in the sample and the rock was old enough to date. This most often occurs if the rock experienced a high temperature usually a thousand degrees Fahrenheit or more at some point since its formation. If that occurs, some of the argon gas moves around, and the analysis does not give a smooth plateau across the extraction temperature steps.
An example of an argon-argon analysis that did not yield an age date is shown in Figure 3. Notice that there is no good plateau in this plot.
In some instances there will actually be two plateaus, one representing the formation age, and another representing the time at which the heating episode occurred. But in most cases where the system has been disturbed, there simply is no date given. The important point to note is that, rather than giving wrong age dates, this method simply does not give a date if the system has been disturbed.
This is also true of a number of other igneous rock dating methods, as we will describe below. In nearly all of the dating methods, except potassium-argon and the associated argon-argon method, there is always some amount of the daughter product already in the rock when it cools.
Using these methods is a little like trying to tell time from an hourglass that was turned over before all of the sand had fallen to the bottom.
One can think of ways to correct for this in an hourglass: One could make a mark on the outside of the glass where the sand level started from and then repeat the interval with a stopwatch in the other hand to calibrate it. Or if one is clever she or he could examine the hourglass' shape and determine what fraction of all the sand was at the top to start with.
By knowing how long it takes all of the sand to fall, one could determine how long the time interval was. Similarly, there are good ways to tell quite precisely how much of the daughter product was already in the rock when it cooled and hardened. Strontium has several other isotopes that are stable and do not decay. The ratio of strontium to one of the other stable isotopes, say strontium, increases over time as more rubidium turns to strontium Rubidium has a larger atomic diameter than strontium, so rubidium does not fit into the crystal structure of some minerals as well as others.
Figure 4 is an important type of plot used in rubidium-strontium dating. A rubidium-strontium three-isotope plot. When a rock cools, all its minerals have the same ratio of strontium to strontium, though they have varying amounts of rubidium. As the rock ages, the rubidium decreases by changing to strontium, as shown by the dotted arrows.
Minerals with more rubidium gain more strontium, while those with less rubidium do not change as much. Notice that at any given time, the minerals all line up--a check to ensure that the system has not been disturbed. This works because if there were no rubidium in the sample, the strontium composition would not change.
The slope of the line is used to determine the age of the sample. As the rock starts to age, rubidium gets converted to strontium.
The amount of strontium added to each mineral is proportional to the amount of rubidium present. The solid line drawn through the samples will thus progressively rotate from the horizontal to steeper and steeper slopes.
From that we can determine the original daughter strontium in each mineral, which is just what we need to know to determine the correct age. It also turns out that the slope of the line is proportional to the age of the rock. The older the rock, the steeper the line will be. To give an example for the above equation, if the slope of a line in a plot similar to Fig. Several things can on rare occasions cause problems for the rubidium-strontium dating method.
One possible source of problems is if a rock contains some minerals that are older than the main part of the rock. This can happen when magma inside the Earth picks up unmelted minerals from the surrounding rock as the magma moves through a magma chamber.
Usually a good geologist can distinguish these "xenoliths" from the younger minerals around them. If he or she does happen to use them for dating the rock, the points represented by these minerals will lie off the line made by the rest of the points.
Another difficulty can arise if a rock has undergone metamorphism, that is, if the rock got very hot, but not hot enough to completely re-melt the rock.
In these cases, the dates look confused, and do not lie along a line. Some of the minerals may have completely melted, while others did not melt at all, so some minerals try to give the igneous age while other minerals try to give the metamorphic age. In these cases there will not be a straight line, and no date is determined. In a few very rare instances the rubidium-strontium method has given straight lines that give wrong ages.
This can happen when the rock being dated was formed from magma that was not well mixed, and which had two distinct batches of rubidium and strontium.
One magma batch had rubidium and strontium compositions near the upper end of a line such as in Fig. In this case, the minerals all got a mixture of these two batches, and their resulting composition ended up near a line between the two batches. This is called a two-component mixing line. It is a very rare occurrence in these dating mechanisms, but at least thirty cases have been documented among the tens of thousands of rubidium-strontium dates made.
If page 9 a two-component mixture is suspected, a second dating method must be used to confirm or disprove the rubidium-strontium date. The agreement of several dating methods is the best fail-safe way of dating rocks. All of these methods work very similarly to the rubidium-strontium method.