Relative dating of rock

The Laws of Relative Dating — Mr. Mulroy's Earth Science

relative dating of rock

Geologists are able to 'read' the rock layers using relative and absolute dating techniques. Relative dating arranges geological events – and. Relative dating is the science of determining the relative order of past events without necessarily determining their. Relative dating is used to arrange geological events, and the rocks they leave behind, in a sequence. The method of reading the order is called.

He also found that certain animals were in only certain layers and that they were in the same layers all across England.

Relative dating

Due to that discovery, Smith was able to recognize the order that the rocks were formed. Sixteen years after his discovery, he published a geological map of England showing the rocks of different geologic time eras. Principles of relative dating[ edit ] Methods for relative dating were developed when geology first emerged as a natural science in the 18th century.

Geologists still use the following principles today as a means to provide information about geologic history and the timing of geologic events. Uniformitarianism[ edit ] The principle of Uniformitarianism states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time. In geology, when an igneous intrusion cuts across a formation of sedimentary rockit can be determined that the igneous intrusion is younger than the sedimentary rock.

There are a number of different types of intrusions, including stocks, laccolithsbatholithssills and dikes. Cross-cutting relationships[ edit ] Cross-cutting relations can be used to determine the relative ages of rock strata and other geological structures.

The principle of cross-cutting relationships pertains to the formation of faults and the age of the sequences through which they cut.

Faults are younger than the rocks they cut; accordingly, if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault.

relative dating of rock

Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault. For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer. A similar situation with igneous rocks occurs when xenoliths are found.

These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix. As a result, xenoliths are older than the rock which contains them.

Original horizontality[ edit ] The principle of original horizontality states that the deposition of sediments occurs as essentially horizontal beds. Observation of modern marine and non-marine sediments in a wide variety of environments supports this generalization although cross-bedding is inclined, the overall orientation of cross-bedded units is horizontal. This is because it is not possible for a younger layer to slip beneath a layer previously deposited.

relative dating of rock

This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed. As organisms exist at the same time period throughout the world, their presence or sometimes absence may be used to provide a relative age of the formations in which they are found.

Based on principles laid out by William Smith almost a hundred years before the publication of Charles Darwin 's theory of evolutionthe principles of succession were developed independently of evolutionary thought.

relative dating of rock

The principle becomes quite complex, however, given the uncertainties of fossilization, the localization of fossil types due to lateral changes in habitat facies change in sedimentary strataand that not all fossils may be found globally at the same time.

As a result, rocks that are otherwise similar, but are now separated by a valley or other erosional feature, can be assumed to be originally continuous. Layers of sediment do not extend indefinitely; rather, the limits can be recognized and are controlled by the amount and type of sediment available and the size and shape of the sedimentary basin.

  • How does the law of crosscutting relationships help scientists determine the relative age of rocks?

Sediment will continue to be transported to an area and it will eventually be deposited. However, the layer of that material will become thinner as the amount of material lessens away from the source. Often, coarser-grained material can no longer be transported to an area because the transporting medium has insufficient energy to carry it to that location. In its place, the particles that settle from the transporting medium will be finer-grained, and there will be a lateral transition from coarser- to finer-grained material.

The lateral variation in sediment within a stratum is known as sedimentary facies. 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 number 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 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 of rock

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.

Relative Dating 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 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.

relative dating of rock