Radiometric dating is used to estimate the age of rocks and other objects based on the fixed Relative Dating with Fossils: Index Fossils as Indicators of Time. Thus, when a particular fossil is identified in a sedimentary rock, we can tell whether the rock is 2) Radiometric age determination - analysing the very small quantities of radioactive elements Absolute age dating. – atomic spectroscopy. These include radiometric dating of volcanic layers above or below the fossils or by comparisons to similar rocks and fossils of known ages.

### Dating dinosaurs and other fossils - Australian Museum

Relative dating is used to determine a fossils approximate age by comparing it to similar rocks and fossils of known ages. Absolute dating is used to determine a precise age of a fossil by using radiometric dating to measure the decay of isotopes, either within the fossil or more often the rocks associated with it.

Relative Dating The majority of the time fossils are dated using relative dating techniques. Using relative dating the fossil is compared to something for which an age is already known.

For example if you have a fossil trilobite and it was found in the Wheeler Formation. The Wheeler Formation has been previously dated to approximately million year old, so we know the trilobite is also about million years old.

Scientists can use certain types of fossils referred to as index fossils to assist in relative dating via correlation. Index fossils are fossils that are known to only occur within a very specific age range. Typically commonly occurring fossils that had a widespread geographic distribution such as brachiopods, trilobites, and ammonites work best as index fossils. If the fossil you are trying to date occurs alongside one of these index fossils, then the fossil you are dating must fall into the age range of the index fossil.

Sometimes multiple index fossils can be used. In a hypothetical example, a rock formation contains fossils of a type of brachiopod known to occur between and million years.

The same rock formation also contains a type of trilobite that was known to live to million years ago. Since the rock formation contains both types of fossils the ago of the rock formation must be in the overlapping date range of to million years. Studying the layers of rock or strata can also be useful. Layers of rock are deposited sequentially.

### Radiometric dating - Wikipedia

If a layer of rock containing the fossil is higher up in the sequence that another layer, you know that layer must be younger in age. 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.

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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.

## How Do Scientists Determine the Age of Dinosaur Bones?

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.

### DETERMINING AGE OF ROCKS AND FOSSILS

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 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.