Scientists use relative dating to determine

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.


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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.

WHO'S ON FIRST? RELATIVE DATING (Student Activity)

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.

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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.

FOSSILS: how fossils are dated

That stack of numbers on the right side of the diagram is comforting; it seems like we've got a good handle on the history of the Moon if we can label it so neatly. 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. The public wouldn't know what I meant if I said "Nectarian" or "Imbrian. If the ages are so uncertain for the Moon, what about the ages of Mars and Mercury? I'll leave those for another day Earth , the Moon , geology , explaining science , Late Heavy Bombardment.

Relative dating

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Relative and absolute ages in the histories of Earth and the Moon: The Geologic Time Scale

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. Courtesy Paul Spudis The Moon's major impact basins A map of the major lunar impact basins on the nearside left and farside right.

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. Red marks individual impact basins. The brown splotch denotes ebbing and flowing of mare volcanism. Alan Shepard checks out a boulder Astronaut Alan B. Note the lunar dust clinging to Shepard's space suit. What is the law of superposition and how can it be used to relatively date rocks? Related questions What is the principle of Uniformitarianism and how is it important to the relative dating of rocks?

What is the age of inclusions found in a rock relative to the rock in which they are found? What is the principle of cross-cutting relations and why is it important for relative dating? What forces can disturb relative dating? What is meant by dating rocks relatively rather than absolutely? How can fossils be used to determine the relative ages of rock layers? The principle of Uniformitarianism states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time.


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  • The principle of intrusive relationships concerns crosscutting intrusions. In geology, when an igneous intrusion cuts across a formation of sedimentary rock , it can be determined that the igneous intrusion is younger than the sedimentary rock. There are a number of different types of intrusions, including stocks, laccoliths , batholiths , sills and dikes. The principle of cross-cutting relationships pertains to the formation of faults and the age of the sequences through which they cut.


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    • WHO'S ON FIRST? A RELATIVE DATING ACTIVITY.
    • Faults are younger than the rocks they cut; accordingly, if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault. Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault. The principle of inclusions and components explains that, with sedimentary rocks, if inclusions or clasts are found in a formation, then the inclusions must be older than the formation that contains them.

      For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer. A similar situation with igneous rocks occurs when xenoliths are found. These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix.

      Geologic Age Dating Explained - Kids Discover

      As a result, xenoliths are older than the rock which contains them. The principle of original horizontality states that the deposition of sediments occurs as essentially horizontal beds. Observation of modern marine and non-marine sediments in a wide variety of environments supports this generalization although cross-bedding is inclined, the overall orientation of cross-bedded units is horizontal.

      The law of superposition states that a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it. This is because it is not possible for a younger layer to slip beneath a layer previously deposited. This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed. The principle of faunal succession is based on the appearance of fossils in sedimentary rocks.

      As organisms exist at the same time period throughout the world, their presence or sometimes absence may be used to provide a relative age of the formations in which they are found. Based on principles laid out by William Smith almost a hundred years before the publication of Charles Darwin 's theory of evolution , the principles of succession were developed independently of evolutionary thought.

      The principle becomes quite complex, however, given the uncertainties of fossilization, the localization of fossil types due to lateral changes in habitat facies change in sedimentary strata , and that not all fossils may be found globally at the same time. The principle of lateral continuity states that layers of sediment initially extend laterally in all directions; in other words, they are laterally continuous. As a result, rocks that are otherwise similar, but are now separated by a valley or other erosional feature, can be assumed to be originally continuous.

      Layers of sediment do not extend indefinitely; rather, the limits can be recognized and are controlled by the amount and type of sediment available and the size and shape of the sedimentary basin. Sediment will continue to be transported to an area and it will eventually be deposited. However, the layer of that material will become thinner as the amount of material lessens away from the source. Often, coarser-grained material can no longer be transported to an area because the transporting medium has insufficient energy to carry it to that location.

      In its place, the particles that settle from the transporting medium will be finer-grained, and there will be a lateral transition from coarser- to finer-grained material.