Relative age dating of sedimentary rocks

Sedimentary and metamorphic rocks and age determination

relative age dating of sedimentary rocks

relative age determination is based on fossils and only sedimentary rocks contain fossils. Relative dating is used to arrange geological events, and the rocks they are important for working out the relative ages of sedimentary rocks. For example, a xenolith in an igneous rock or a clast in sedimentary rock relationships outlined above, determine the relative ages of these three rock types.

But this sediment doesn't typically include the necessary isotopes in measurable amounts. Fossils can't form in the igneous rock that usually does contain the isotopes.

The extreme temperatures of the magma would just destroy the bones. So to determine the age of sedimentary rock layers, researchers first have to find neighboring layers of Earth that include igneous rock, such as volcanic ash. These layers are like bookends -- they give a beginning and an end to the period of time when the sedimentary rock formed. By using radiometric dating to determine the age of igneous brackets, researchers can accurately determine the age of the sedimentary layers between them.

Using the basic ideas of bracketing and radiometric dating, researchers have determined the age of rock layers all over the world. This information has also helped determine the age of the Earth itself. While the oldest known rocks on Earth are about 3. Based on the analysis of these samples, scientists estimate that the Earth itself is about 4.

In addition, the oldest known moon rocks are 4. Since the moon and the Earth probably formed at the same time, this supports the current idea of the Earth's age. You can learn more about fossils, dinosaurs, radiometric dating and related topics by reading through the links below. 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 number of neutrons must at least equal the number 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.

Relative Dating - Example 1

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.

Relative Age Determination

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 age dating of sedimentary rocks

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.

Are only sedimentary rocks used for relative age determination? Why?

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.

relative age dating of sedimentary rocks

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.