Not meant relative and absolute dating slideshare are not
A few days ago, I wrote a post about the basins of the Moon - a result of a trip down a rabbit hole of book research. In the science of geology, there are two main ways we use to describe how old a thing is or how long ago an event took place. There are absolute ages and there are relative ages. People love absolute ages. An absolute age is a number. When you say that I am 38 years old or that the dinosaurs died out 65 million years ago, or that the solar system formed 4.
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. 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: older Nectaris and younger Imbrium.
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.
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.
Relative and absolute ages in the histories of Earth and the Moon: The Geologic Time Scale
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. So: when did all of this happen, exactly? 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.
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.
Nov 12, Relative & Absolute Dating 3, views. Share; Like Heather Harris, Teacher On SlideShare. 0 From Embeds. 0 Number of Embeds Relative and absolute dating North Gwinnett Middle School. Geologic Time Scale 9 MissWander. Chapter Absolute Age Dating. May 20, Radiometric Dating ? Radioactive decay ? Radioactive isotopes decay from one isotope to another or from one element to another by emitting energy. ? Radiometric dating ? By measuring the of parent and daughter isotopes, age may be calculated. Sep 30, Absolute Dating - the process of establishing the age of an object by determining the number of years it has existed. It's the most precise. Half - life ? A half-life is the amount of time necessary for one-half of the nuclei in a sample (parent) to decay to a stable isotope (daughter).
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.
Jan 08, Geologic Time - Instead, they used relative dating, which placed events in a sequence based on logical assumptions and observations (like knowing that your mom is older than you without knowing her age). - When radioactivity came along, it didn't replace relative dating techniques; it actually supplemented them. Apr 08, Relative Dating 1. Relative Dating Chapter 13 Clues to Earth's Past Relative datingexercises robjmnz Earth Science Relative Dating: Which Came First? robjmnz Fossils to sedimentary rock aalleyne. Earth Science Absolute Dating: A Measure of Time Chris Foltz. Ch11_Geologic_Time_students cresnick. English Espanol. Sep 30, 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.
For more than two billion years - half the diagram - almost no action. A crater here, a little squirt of volcanism there. 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.
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Relative and absolute dating slideshare
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