It determines the period during which certain object was last subjected to heat.
What is Relative Dating
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.
Relative Dating Techniques Explained. How are Waterfalls Formed. Types of Metamorphic Rocks.
How are Rivers Formed? What Tools do Archaeologists Use. Why is Archaeology Important. Deepest Part of the Ocean. Interesting Facts About Hurricanes. How do Tornadoes Form. Who Invented the Battery. Lab Safety Rules for Kids. How Does a Diode Work? Who Invented the Computer? Uses of Radioactive Isotopes. 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 Scale , fully 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 in , most 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 AD , the 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. 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.
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.
Relative Vs. Absolute Dating: The Ultimate Face-off
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.
What is Absolute Dating
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 basins , and 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. 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. Mare Ingenii, the "Sea of Cleverness," is a small area of mare basalt dark filling an impact basin that is itself inside the South Pole-Aitken Basin on the Moon's farside. 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. 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. 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. 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.