Read the lab materials and define the following terms and concepts / answer the Discuss the difference between relative age dating and absolute age dating. -Lab: Relative Dating- Name______________. Background: Sequencing events establishes a relative age of a stratum. The process of . Absolute age-. Background: Relative dating gives an approximate age of something Radiometric dating methods give absolute ages ranging from decades to billions of years.
This represents the parent isotope. The candy should be poured into a container large enough for them to bounce around freely, it should be shaken thoroughly, then poured back onto the paper so that it is spread out instead of making a pile. This first time of shaking represents one half life, and all those pieces of candy that have the printed M facing up represent a change to the daughter isotope.
Then, count the number of pieces of candy left with the M facing down. These are the parent isotope that did not change during the first half life. The teacher should have each team report how many pieces of parent isotope remain, and the first row of the decay table Figure 2 should be filled in and the average number calculated. The same procedure of shaking, counting the "survivors", and filling in the next row on the decay table should be done seven or eight more times.
Each time represents a half life. Each team should plot on a graph Figure 3 the number of pieces of candy remaining after each of their "shakes" and connect each successive point on the graph with a light line. AND, on the same graph, each group should plot points where, after each "shake" the starting number is divided by exactly two and connect these points by a differently colored line.
After the graphs are plotted, the teacher should guide the class into thinking about: Is it the single group's results, or is it the line based on the class average? U is found in most igneous rocks.
Unless the rock is heated to a very high temperature, both the U and its daughter Pb remain in the rock. A geologist can compare the proportion of U atoms to Pb produced from it and determine the age of the rock.
The next part of this exercise shows how this is done. Each team is given a piece of paper marked TIME, on which is written either 2, 4, 6, 8, or 10 minutes. The team should place each marked piece so that "U" is showing. This represents Uranium, which emits a series of particles from the nucleus as it decays to Lead Pb- When each team is ready with the pieces all showing "U", a timed two-minute interval should start.
During that time each team turns over half of the U pieces so that they now show Pb This represents one "half-life" of U, which is the time for half the nuclei to change from the parent U to the daughter Pb A new two-minute interval begins. Continue through a total of 4 to 5 timed intervals. That is, each team should stop according to their TIME paper at the end of the first timed interval 2 minutesor at the end of the second timed interval 4 minutesand so on.
After all the timed intervals have occurred, teams should exchange places with one another as instructed by the teacher. Nonconformity marks erosion clear down to crystalline basement rocks below the sedimentary basin. Often this is the basal unconformity for an entire sedimentary basin. The rocks below are deformed igneous and metamorphic that are hundreds of millions to billions of years older than the overlying sedimentary succession.
Work through examples 8. Cross sections represent the layered view of the Earth and its rocks as seen at a roadcut, along a stream valley or in a sequence of boreholes in the subsurface. There are 4 cross sections,3 hypothetical ones but one real one from the Inner Gorge of the Grand Canyon.
Note the legend blocks below so you can tell igneous intrusions which baked the rocks they cut from eroded plutons covered by gravel of their own detritus. Generally sediments are deposited in horizontal layers.
If they are dipping tilted there has been enough time after their deposition and lithification to uplift and erode them in a mountain building episode. In geological sequences, unconformities for erosion or non deposition actually represent more missing time than is represented by the deposition of a single layer or sequence.
Relative and absolute ages in the histories of Earth and the Moon: The Geologic Time Scale
Therefore, we include unconformities in the sequence as they represent the biggest blocks of time. Often an unconformity is more than one type depending on what the underlying layer is like. For example a flat lying strata that overlies both dipping beds and crystalline rocks is both an angular unconformity and a non-conformity. The present day erosional surface nonconformity on the Canadian Shield is missing on average 2.
The Tertiary Period is further subdivided into: Paleocene, Eocene, Oligocene, Miocene and Pliocene. Fossils are unique impressions or mineral replacements of once living biota. The average lifetime of a species, be it a microscopic species of plankton or a massive mammal, is about 2 Ma. Thus recognizing and telling one fossil species from another gives a pretty well defined interval of time. Groups of closely related animals live longer than individual species.
For example there have been horse-shoe crabs since the Lower Paleozoic, but there have been hundreds if not thousands of different species. Marine strata preserve the best fossil records. Paleontologists use the overlapping and sequential range zones of different groups of fossils.
Identifying species with easily distinguished shapes is the key to interpreting relative ages.
Now that we have used volcanic ash beds or lavas intercalated with sediments to assign absolute radiomentric ages, we know the Cambrian began million years ago. Finally, using the absolute age scale along the left edge of the diagram, assign an absolute age to the fossil assemblage pictured.
Here we are using long lived groups and our precision is less but the general idea is the same. For example, if a stratum contained Shark teeth and the brachiopod Chonetes, we can say it was sometime between Late Devonian and Latest Permian for relative age and between Ma and Ma from the overlap of the 2 range zones for its absolute radiometric age. 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. 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.
Lab 8: Relative and Absolute Geological Dating Lab
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.
- Lab 8: Relative and Absolute Geological Dating Lab
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.
DETERMINING AGE OF ROCKS AND FOSSILS
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.