Relative age dating of rocks

Using relative and radiometric dating methods, geologists are able to answer the question: how old is Relative dating to determine the age of rocks and fossils.
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Relative Dating Practice 2015

We know that the curb was originally straight when it was first constructed. The fault cut the curb and is thus younger than the curb itself. A curb in Hollister, California that is offset by the San Andreas fault.

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The cartoon below shows an imaginary sequence of rocks and geological events labeled A-I. Using the principles of superposition and cross-cutting relationships, can you reconstruct the geological history of this place, at least based upon the information you have available? An imaginary cross-section, showing a series of rock layers and geological events A-I.

A is a fault. B-F are sedimentary rock layers. G and H are both igneous intrusions. Finally, I is an erosional surface. Based on the principles of superposition and cross-cutting relationships, what are the relative ages of these rocks and events? Second, we observe that rock layer H which is an igneous intrusion cuts into rock layers B-F.

It is therefore younger than B-F. Third, we observe that the fault A cuts across and displaces rock layers B-F. Fourth, we see that G, another igneous intrusion, cuts across A-H; it is therefore younger than all of these note that G is not displaced by A, the fault. Finally, we note an erosional surface, I, at the top of the sequence and immediately below the corn field that cuts both A and G.

I is therefore younger than both A and G. Putting this all together, we can determine the relative ages of these rock layers and geological events:.

Given the information available, we cannot resolve whether H is older than A or, vice versa. This problem could be resolved, however, if we were to observe A cutting across H i.

Relative dating - Wikipedia

What geological principle states that rocks at the bottom of a sequence are older than the rocks above? What dating approach is used to evaluate the ordering of past geological events? Think about the principle of cross-cutting relationships. If a fault cuts across a rock layer, is the fault older or younger than the rock layer?

What dating approach is used to determine the age of a geological sample in years before the present date? Did rock layer A form before or after rock layer B? Did trilobites live before or after the dinosaurs? Principle of superposition Just as uniformitarianism is the key underlying assumption of geology, the science's most fundamental principle is superposition, developed by Danish anatomist Nicholas Steno in the 17th century. Principle of cross-cutting relationships The principle of cross-cutting relationships states that a rock unit or other geological feature, such as a fault that is cut by another rock unit or feature must be older than the rock unit or feature that does the cutting.

Let's work through the imaginary example above. Putting this all together, we can determine the relative ages of these rock layers and geological events: Absolute age dating Previous Section: 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 number 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 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.

Relative dating

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.


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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.

MATERIALS REQUIRED FOR EACH GROUP

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 minutes , or at the end of the second timed interval 4 minutes , and so on. After all the timed intervals have occurred, teams should exchange places with one another as instructed by the teacher.


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  • PURPOSE AND OBJECTIVES;

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 number of millions of years represented by the set that they themselves turned over, PLUS the number of millions of years represented by the set that another team turned over. Pb atoms in the pegmatite is 1: 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. For the block diagram Figure 1 , if a geochemical laboratory determines that the volcanic ash that is in the siltstone has a ratio of U If the ratio in the basalt is 7: Students should write the age of the volcanic ash beside the shale, siltstone and basalt on the list below the block diagram.