Added: Alvie Volk - Date: 03.02.2022 08:24 - Views: 11002 - Clicks: 9593
There are two types of age determinations. Geologists in the late 18th and early 19th century studied rock layers and the fossils in them to determine relative age. William Smith was one of the most important scientists from this time who helped to develop knowledge of the succession of different fossils by studying their distribution through the sequence of sedimentary rocks in southern England.
It wasn't until well into the 20th century that enough information had accumulated about the rate of radioactive decay that the age of rocks and fossils in of years could be determined through radiometric age dating. This activity on determining age of rocks and fossils is intended for 8th or 9th grade students. It is estimated to require four hours of class time, including approximately one hour total of occasional instruction and explanation from the teacher and two hours of group team and individual activities by the students, plus one hour of discussion among students within the working groups.
Explore this link for additional information on the topics covered in this lesson: Geologic Time. Students not only want to know how old a fossil is, but they want to know how that age was determined. Some very straightforward principles are used to determine the age of fossils. Students should be able to understand the principles and have that as a background so that age determinations by paleontologists and geologists don't seem like black magic.
This activity consists of several parts. Objectives of this activity are: 1 To have students determine relative age of a geologically complex area. A single watch or clock for the entire class will do. After students have decided how to establish the relative age of each rock unit, they should list them under the block, from most recent at the top of the list to oldest at the bottom. The teacher should tell the students that there are two basic principles used by geologists to determine the sequence of ages of rocks.
They are: Principle of superposition: Younger sedimentary rocks are deposited on top of older sedimentary rocks. Principle of cross-cutting relations: Any geologic feature is younger than anything else that it cuts across. For example, U is an unstable isotope of uranium that has 92 protons and neutrons in the nucl eus of each atom.
Through a series of changes within the nucleus, it emits several particles, ending up with 82 protons and neutrons. This is a stable condition, and there are no more changes in the atomic nucleus. A nucleus with that of protons is called lead chemical symbol Pb.
The protons 82 and neutrons total This particular form isotope of lead is called Pb U is the parent isotope of Pb, which is the daughter isotope. Many rocks contain small amounts of unstable isotopes and the daughter isotopes into which they decay. Where the amounts of parent and daughter isotopes can be accurately measured, the ratio can be used to determine how old the rock is, as shown in the following activities. That chance of decay is very small, but it is always present and it never changes. In other words, the nuclei do not "wear out" or get "tired". If the nucleus has not yet decayed, there is always that same, slight chance that it will change in the near future.
Atomic nuclei are held together by an attraction between the large nuclear particles protons and neutrons that is known as the "strong nuclear force", which must exceed the electrostatic repulsion between the protons within the nucleus. In general, with the exception of the single proton that constitutes the nucleus of the most abundant isotope of hydrogen, the of neutrons must at least equal the of protons in an atomic nucleus, because electrostatic repulsion prohibits denser packing of protons. But if there are too many neutrons, the nucleus is potentially unstable and decay may be triggered.
This happens at any time when addition of the fleeting "weak nuclear force" to the ever-present electrostatic repulsion exceeds the binding energy required to hold the nucleus together. In other words, during million years, half the U atoms that existed at the beginning of that time will decay to Pb This is known as the half life of U- Many elements have some isotopes that are unstable, essentially because they have too many neutrons to be balanced by the of protons in the nucleus.
Each of these unstable isotopes has its own characteristic half life. Some half lives are several billion years long, and others are as short as a ten-thousandth of a second. On a piece of notebook paper, each piece should be placed with the printed M facing down. This represents the parent Relative age of rocks.
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 Relative age of rocks represent a change to the daughter isotope.
Then, count the 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 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 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 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: 1 Why didn't each group get the same ? Is it the single group'sor 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 Relative age of rocks 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.
The task now for each team is to determine how many timed intervals that is, how many half-lives the set of pieces they are looking at has experienced. The half life of U is million years. Both the team that turned over a set of pieces and the second team that examined the set should determine how many million years are represented by the proportion of U and Pb present, compare notes, and haggle about any differences that they got. Right, each team must determine the of millions of years represented by the set that they themselves turned over, PLUS the of millions of years represented by the set that another team turned over.
Using the same reasoning about proportions as in Part 2b above, students can determine how old the pegmatite and the granite are. They should write the ages of the pegmatite and granite beside the names of the rocks in the list below the block diagram Figure 1. This makes the curve more useful, because it is easier to plot it more accurately. That is especially helpful for ratios of parent isotope to daughter isotope that represent less than one half life. Students should write the age of the volcanic ash beside the shale, siltstone and basalt on the list below the block diagram. What is it?
Why can't you say exactly what the age of the rock is? Why can you be more precise about the age of this rock than you could about the ages of the Relative age of rocks that has the trilobites and the rock that contains acritarchs and bacteria? Note for teachers: Based on cross-cutting relationships, it was established that the pegmatite is younger than the slate and that the slate is younger than the granite.
Therefore, the slate that contains the acritarch and bacteria is between million years and million years old, because the pegmatite is million years old and the granite is million years old. The slate itself cannot be radiometrically dated, so can only be bracketed between the ages of the granite and the Relative age of rocks. The trilobite-bearing limestone overlies the quartz sandstone, which cross-cuts the pegmatite, and the basalt cuts through the limestone.
Therefore the trilobites and the rock that contains them must be younger than million years the age of the pegmatite and older than million years the age of the basalt. The limestone itself cannot be radiometrically dated, so can only be bracketed between the ages of the granite and the pegmatite. The Triceratops dinosaur fossils are approximately 70 million years old, because they are found in shale and siltstone that contain volcanic ash radiometrically dated at 70 million years. Any Triceratops found below the volcanic ash may be a little older than 70 million years, and any found above may be a little younger than 70 million years.
The age of the Triceratops can be determined more closely than that of the acritarchs and bacteria and that of the trilobites because the rock unit that contains the Triceratops can itself be radiometrically dated, whereas that of the other fossils could not.Relative age of rocks
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Dating Rocks and Fossils Using Geologic Methods