be determined through radiometric age dating or Relative and Absolute Age Dating. Activity: To start, place all your activity pieces with U facing up. Determine the answers for the other Team's pieces and discuss the difference. Day 1: Relative dating of rock layers To connect this activity with rock layers, students next observe . terward, students compare and contrast the charts . compared to absolute dating? (Relative dating is a comparison of two rock layers. Relative dating involves things like higher layers are most recent than rocks lower in the sequence. This can be extended to the known.
The History of Planet Earth Geologists estimate the age of rocks using a variety of techniques. Absolute dating attempts to determine the numerical age of an object.
Relative dating techniques place rocks in their sequential order of formation. Absolute dating is primarily accomplished through a technique called radiometric dating. All matter is composed of chemical elements, and each element is distinguished by a specific number of protons. For example, an atom of the element carbon has six protons. While all carbon atoms have six protons, they may vary in their number of neutrally charged neutrons. These variants are called isotopes.
Some isotopes are considered to be radioactive because they decay over time and emit ionizing radiation in the form of energy and particles. The rate of decay of a radioactive isotope is measured in terms of its half-life, or the amount of time required for a material to decrease by one-half. Scientists can use this information to calculate the absolute age of an object containing a particular radioactive isotope such as carbon Carbon has a half-life of 5, years.
After an organism dies, it stops absorbing new carbon from the environment, and the isotope begins to decay at an exponential rate. Only half of the original carbon isotope will remain in the fossil 5, years after the organism died. Only half of that, or one-quarter of the original isotope, will remain 5, years after that, or 11, years after the organism died. Scientists can analyze the rate of radioactive decay in a fossil and use that information to calculate the date when the organism died.
For more detail on radioactivity isotopes and radioactive decay calculations, see Module 2 Unit 4: Unlike the continuous ticking clock of the "chronometric" scale measured in years before the year ADthe 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.
Compare and contrast relative and absolute dating? | Yahoo Answers
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. 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.
Exploring Our Fluid Earth
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.
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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.
- Compare and contrast relative and absolute dating?
- Relative and absolute ages in the histories of Earth and the Moon: The Geologic Time Scale
- Relative Vs. Absolute Dating: The Ultimate Face-off
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.