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Introduction Geology Principles of relative dating Planetology Archaeology See also References Citations.

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Relative dating.

Relative dating — Before the advent of absolute dating in the 20th century, archaeologists and geologists were largely limited to the use of relative dating techniques. It estimates the order of prehistoric and geological events were determined by using basic… … Wikipedia.

relative dating — /ˈrɛlətɪv deɪtɪŋ/ (say reluhtiv dayting) noun Archaeology a system of dating objects from the strata of rocks in which they are found, the lower strata antedating the ones closer to the surface. Compare absolute dating … Australian English dictionary.

relative dating — noun a) A method of determining the age of a fossil by comparing its placement with that of fossils in other layers of rock. b) An act of so doing … Wiktionary.

Dating methodologies in archaeology — Dating material drawn from the archaeological record can be made by a direct study of an artifact or may be deduced by association with materials found in the context the item is drawn from or inferred by its point of discovery in the sequence… … Wikipedia.

Dating methodology (archaeology) — Dating material drawn from the archaeological record can made by a direct study of an artifact or may be deduced by association with materials found in the context the item is drawn from or inferred by its point of discovery in the sequence… … Wikipedia.

Relative — can refer to: *Kinship, the principle binding the most basic social units society. If two people are connected by circumstances of birth, they are said to be relatives Physics*Relativity as a concept in physics (for example Albert Einstein s… … Wikipedia.

Dating (disambiguation) — Dating refers to seeking and arranging meetings with potential intimate partners. Dating may also refer to: In chronology absolute dating, for example using radiocarbon dating relative dating, for example using stratigraphy or tree rings dating… … Wikipedia.

dating — I In geology and archaeology, the process of determining an object s or event s place within a chronological scheme. Scientists may use either relative dating, in which items are sequenced on the basis of stratigraphic clues (see stratigraphy) or … Universalium.

Dating — This article is about the form of courtship. For other uses, see Dating (disambiguation). Double Date redirects here. For the episode of How I Met Your Mother , see Double Date (How I Met Your Mother). For the episode of The Office , see Double… … Wikipedia.

Radiometric dating — (often called radioactive dating) is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates.[1]… … Wikipedia.

Absolute dating — is the process of determining a specific date for an archaeological or palaeontological site or artifact. Some archaeologists prefer the terms chronometric or calendar dating, as use of the word absolute implies a certainty and precision that is… … Wikipedia.

Difference Between Absolute and Relative Dating.

The main difference between absolute and relative dating is that the absolute dating is a technique to determine the numerical age of a rock or a fossil whereas the relative dating is a technique that determines the relative age. Furthermore, absolute dating can be done with the use of radiometric dating while relative age is determined with respect to other layers.

Absolute dating and relative dating are two techniques used in geology to evaluate the age and the period of a fossil or rock.

Key Areas Covered.

Key Terms.

Absolute Dating, Amino Acid Dating, Dendrochronology, Methods of Dating, Numerical Dating, Radiometric Dating, Relative Dating, Thermoluminescence.

What is Absolute Dating.

In geology, absolute dating is a technique that determines the exact numerical age of a historical remaining. Since it evaluates the exact age of the sample, absolute ageing is also called numerical dating . The four techniques used in absolute dating are radiometric dating, amino acid dating, dendrochronology, and thermoluminescence.

Radiometric dating : It determines the age of the sample by measuring the amount of a particular radioactive isotope present in the sample. The age can be determined by the rate of decay of that particular isotope. The type of radioactive isotope used depends on the type of sample. One of the most popular and widely used types of radioactive isotope in this type of techniques is the carbon-14.

Figure 1: Radiocarbon Date Calibration Curve.

What is Relative Dating.

Relative dating is the technique used to determine the age by comparing the historical remaining to the nearby layers. It is a less advanced technique when compared to absolute dating. Some methods used in relative dating are stratigraphy, biostratigraphy, and cross dating.

Stratigraphy : This technique assumes that the lowest layer is the oldest while the topmost layer is the youngest layer. It is one of the oldest methods of relative dating.

Figure 2: Igneous Rock Layers.

Similarities Between Absolute and Relative Dating.

Absolute and relative dating are the two types of techniques used to determine the age of a historical remaining. Both techniques help to understand the order of formation of the historical remaining.

Difference Between Absolute and Relative Dating.

Definition.

The absolute dating refers to a technique used to determine the exact age of the artefact or a site using methods such as carbon dating while relative dating refers to a technique used to determine which object or item is older in comparison to the other one.

Significance.

Absolute dating determines the numerical age while relative dating arranges the fossils in an order.

Methods.

The four methods involved in absolute dating are radiometric dating, amino acid dating, dendrochronology, and thermoluminescence while biostratigraphy, stratigraphy, and cross dating are involved in the relative dating.

Precision.

The precision in absolute ageing is high while the precision of the relative ageing is low.

Quantitative/Qualitative.

Absolute age is a quantitative measurement while relative age is a qualitative measurement.

Work Better for.

Absolute dating works better for igneous and metamorphic rocks while relative dating works better for sedimentary rocks having layered arrangement of sediments.

Cost and Time.

Absolute dating is expensive and takes time while relative dating is less-expensive and efficient.

Conclusion.

Absolute dating is the technique that determines the exact age of a historical remaining while relative dating gives the order of age of several samples. Therefore, absolute dating is a quantitative measurement while relative dating is a qualitative measurement. The main difference between absolute and relative dating is the precision of the measurement.

Reference:

1. “Absolute Dating.” Science Learning Hub, Available Here 2. “Relative Dating.” Science Learning Hub, Available Here.

Image Courtesy:

1. “Radiocarbon Date Calibration Curve” By HowardMorland – Own work based on information from: Reimer, P.J., et al. C.E. (2004). “IntCal04 Terrestrial radiocarbon age calibration”. Radiocarbon 46: 1029-58. (CC BY-SA 3.0) via Commons Wikimedia 2. “Relative dating of fossils” By Jillcurie – Own work (CC BY-SA 3.0) via Commons Wikimedia.

About the Author: Lakna.

Lakna, a graduate in Molecular Biology & Biochemistry, is a Molecular Biologist and has a broad and keen interest in the discovery of nature related things.

Relative Vs. Absolute Dating: The Ultimate Face-off.

Our planet inherits a large number of artifacts and monuments bestowed upon us by older historic civilizations. These remains are subjected to dating techniques in order to predict their ages and trace their history. This ScienceStruck post enlists the differences between the absolute and relative dating methods.

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Did You Know?

Although both relative and absolute dating methods are used to estimate the age of historical remains, the results produced by both these techniques for the same sample may be ambiguous.

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Geological specimens that are unearthed need to be assigned an appropriate age. To find their age, two major geological dating methods are used. These are called relative and absolute dating techniques. Absolute dating, also called numerical dating, arranges the historical remains in order of their ages. Whereas, relative dating arranges them in the geological order of their formation.

The relative dating techniques are very effective when it comes to radioactive isotope or radiocarbon dating. However, not all fossils or remains contain such elements. Relative techniques are of great help in such types of sediments.

Relative Dating Vs. Absolute Dating.

Relative Dating.

➤ It determines if an object/event is younger or older than another object/event from history. ➤ Relative dating is qualitative. ➤ This technique helps determine the relative age of the remains. ➤ It is less specific than absolute dating. ➤ Relative dating is comparatively less expensive and time-efficient. ➤ It works best for sedimentary rocks having layered arrangement of sediments.

The following are the major methods of relative dating.

Stratigraphy: The oldest dating method which studies the successive placement of layers. It is based on the concept that the lowest layer is the oldest and the topmost layer is the youngest.

Biostratigraphy: An extended version of stratigraphy where the faunal deposits are used to establish dating. Faunal deposits include remains and fossils of dead animals.

Cross dating: This method compares the age of remains or fossils found in a layer with the ones found in other layers. The comparison helps establish the relative age of these remains.

Fluorine dating: Bones from fossils absorb fluorine from the groundwater. The amount of fluorine absorbed indicates how long the fossil has been buried in the sediments.

Absolute Dating.

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➤ It determines the age of a rock/object using radiometric techniques. ➤ Absolute dating is quantitative. ➤ This technique helps determine the exact age of the remains. ➤ It is more specific than relative dating. ➤ Absolute dating is expensive and time-consuming. ➤ It works best for igneous and metamorphic rocks.

The following are the major methods of relative dating.

Radiometric dating: This technique solely depends on the traces of radioactive isotopes found in fossils. The rate of decay of these elements helps determine their age, and in turn the age of the rocks.

Amino acid dating: Physical structure of living beings depends on the protein content in their bodies. The changes in this content help determine the relative age of these fossils.

Dendrochronology: Each tree has growth rings in its trunk. This technique dates the time period during which these rings were formed.

Thermoluminescence: It determines the period during which certain object was last subjected to heat. It is based on the concept that heated objects absorb light, and emit electrons. The emissions are measured to compute the age.

Differentiation Using a Venn Diagram.

A Venn diagram depicts both dating methods as two individual sets. The area of intersection of both sets depicts the functions common to both. Take a look at the diagram to understand their common functions.

When we observe the intersection in this diagram depicting these two dating techniques, we can conclude that they both have two things in common:

1.Provide an idea of the sequence in which events have occurred. 2.Determine the age of fossils, rocks, or ancient monuments.

Although absolute dating methods determine the accurate age compared to the relative methods, both are good in their own ways.

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Relative dating.

Relative dating is the science of determining the relative order of past events (i.e., the age of an object in comparison to another), without necessarily determining their absolute age, (i.e. estimated age). In geology rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another. Prior to the discovery of radiometric dating which provided a means of absolute dating in the early 20th century, archaeologists and geologists were largely limited to the use of relative dating techniques to determine the age of geological events.

Though relative dating can only determine the sequential order in which a series of events occurred, not when they occur, it remains a useful technique especially in materials lacking radioactive isotopes. Relative dating by biostratigraphy is the preferred method in paleontology, and is in some respects more accurate (Stanley, 167–69). The Law of Superposition, which states that older layers will be deeper in a site than more recent layers, was the summary outcome of 'relative dating' as observed in geology from the 17th century to the early 20th century.

The regular order of occurrence of fossils in rock layers was discovered around 1800 by William Smith. While digging the Somerset Coal Canal in southwest England, he found that fossils were always in the same order in the rock layers. As he continued his job as a surveyor, he found the same patterns across England. He also found that certain animals were in only certain layers and that they were in the same layers all across England. 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.

Contents.

1 Principles of relative chronology 1.1 Uniformitarianism 1.2 Intrusive relationships 1.3 Cross-cutting relationships 1.4 Inclusions and components 1.5 Original horizontality 1.6 Superposition 1.7 Faunal succession 1.8 Lateral continuity 1.9 Inclusions of Igneous rocks 1.10 Included fragments 2 Archaeology 3 Linguistics 4 Planetology 5 See also 6 Works cited 7 References.

Principles of relative chronology.

Methods for relative dating were developed when geology first emerged as a formal science. Geologists still use the following principles today as a means to provide information about geologic history and the timing of geologic events.

Uniformitarianism.

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. [1] A fundamental principle of geology advanced by the 18th century Scottish physician and geologist James Hutton, is that "the present is the key to the past." In Hutton's words: "the past history of our globe must be explained by what can be seen to be happening now." [2]

Intrusive relationships.

The principle of intrusive relationships concerns crosscutting intrusions. In geology, when an igneous intrusion cuts across a formation of sedimentary rock, it can be determined that the igneous intrusion is younger than the sedimentary rock. There are a number of different types of intrusions, including stocks, laccoliths, batholiths, sills and dikes.

Cross-cutting relationships.

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. Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault. [3]

Inclusions and components.

The principle of inclusions and components states that, with sedimentary rocks, if inclusions (or clasts ) are found in a formation, then the inclusions must be older than the formation that contains them. 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.

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). [3]

Superposition.

The law of superposition states that a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it. This is because it is not possible for a younger layer to slip beneath a layer previously deposited. The only disturbance that the layers experience is bioturbation, in which animals and/or plants move things in the layers. however, this process is not enough to allow the layers to change their positions. 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. [3]

Faunal succession.

The principle of faunal succession is based on the appearance of fossils in sedimentary rocks. 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 evolution, the principles of succession were developed independently of evolutionary thought. 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 strata), and that not all fossils may be found globally at the same time. [4]

Lateral continuity.

The principle of lateral continuity states that layers of sediment initially extend laterally in all directions; in other words, they are laterally continuous. 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. 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.

If sufficient sedimentary material is available, it will be deposited up to the limits of the sedimentary basin. Often, the sedimentary basin is within rocks that are very different from the sediments that are being deposited, in which the lateral limits of the sedimentary layer will be marked by an abrupt change in rock type.

Inclusions of Igneous rocks.

Melt inclusions are small parcels or "blobs" of molten rock that are trapped within crystals that grow in the magmas that form igneous rocks. In many respects they are analogous to fluid inclusions. Melt inclusions are generally small - most are less than 100 micrometres across (a micrometre is one thousandth of a millimeter, or about 0.00004 inches). Nevertheless, they can provide an abundance of useful information. Using microscopic observations and a range of chemical microanalysis techniques geochemists and igneous petrologists can obtain a range of useful information from melt inclusions. Two of the most common uses of melt inclusions are to study the compositions of magmas present early in the history of specific magma systems. This is because inclusions can act like "fossils" - trapping and preserving these early melts before they are modified by later igneous processes. In addition, because they are trapped at high pressures many melt inclusions also provide important information about the contents of volatile elements (such as H 2 O, CO 2 , S and Cl) that drive explosive volcanic eruptions.

Sorby (1858) was the first to document microscopic melt inclusions in crystals. The study of melt inclusions has been driven more recently by the development of sophisticated chemical analysis techniques. Scientists from the former Soviet Union lead the study of melt inclusions in the decades after World War II (Sobolev and Kostyuk, 1975), and developed methods for heating melt inclusions under a microscope, so changes could be directly observed.

Although they are small, melt inclusions may contain a number of different constituents, including glass (which represents magma that has been quenched by rapid cooling), small crystals and a separate vapour-rich bubble. They occur in most of the crystals found in igneous rocks and are common in the minerals quartz, feldspar, olivine and pyroxene. The formation of melt inclusions appears to be a normal part of the crystallization of minerals within magmas, and they can be found in both volcanic and plutonic rocks.

Included fragments.

The law of included fragments is a method of relative dating in geology. Essentially, this law states that clasts in a rock are older than the rock itself. [5] One example of this is a xenolith, which is a fragment of country rock that fell into passing magma as a result of stoping. Another example is a derived fossil, which is a fossil that has been eroded from an older bed and redeposited into a younger one. [6]

This is a restatement of Charles Lyell's original principle of inclusions and components from his 1830 to 1833 multi-volume Principles of Geology , which states that, with sedimentary rocks, if inclusions (or clasts) are found in a formation, then the inclusions must be older than the formation that contains them. 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.

Archaeology.

Relative dating methods in archaeology are similar to some of those applied in geology. The principles of typology can be compared to the biostratigraphic approach in geology.

Linguistics.

Relative dating is often employed in historical linguistics, most typically in study of historical phonology and of loanwords.

Planetology.

Relative dating is used to determine the order of events on objects other than Earth; for decades, planetary scientists have used it to decipher the development of bodies in the Solar System, particularly in the vast majority of cases for which we have no surface samples. Many of the same principles are applied. For example, if a valley is formed inside an impact crater, the valley must be younger than the crater.

Craters themselves are highly useful in relative dating; as a general rule, the younger a planetary surface is, the fewer craters it has. If long-term cratering rates are known to enough precision, crude absolute dates can be applied based on craters alone; however, cratering rates outside the Earth-Moon system are poorly known.(Hartmann, 258)

relative dating.

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1. Relative age dating.

Chapter contents:

It may surprise you to learn that geologists were able to determine much of the history of the Earth and its life without knowing anything about the actual ages of the rocks that they studied. Through use of absolute age dating techniques (which were developed during the 20th century; see Section 2), they were able to later assign dates in years before the preset to important events in Earth's history. But, before that, they relied upon a different approach to first determine the sequence of important events in Earth's past: relative age dating .

Very simply, relative age dating has to do with determining whether one geological or paleontological event happened before or after a second event. For example:

Did rock layer A form before or after rock layer B? Did trilobites live before or after the dinosaurs?

Relative age dating has to do with determining the temporal ordering of events in Earth's past. Geologists employ a handful of simple principles in relative age dating; two of the most important of these are are the principles of superposition and cross-cutting relationships . A third key principle-- faunal succession- -is reviewed in Section 3.

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 (1638-1686) in the 17th century.

Portrait of Nicholas Steno (public domain; Wikimedia Commons).

The principle of superposition is simple, intuitive, and is the basis for relative age dating. It states that rocks positioned below other rocks are older than the rocks above.

The image below shows a sequence of Devonian-aged (

380 Ma) rocks exposed at the magnificent waterfall at Taughannock Falls State Park in central New York. The rocks near the bottom of the waterfall were deposited first and the rocks above are subsequently younger and younger.

Taughannock Falls near Trumansburg, New York, illustrating the Principle of Superposition. Image by Jonathan R. Hendricks. This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Superposition is observed not only in rocks, but also in our daily lives. Consider the trash in your kitchen garbage can. The trash at the bottom was thrown out earlier than the trash that lies above it; the trash at the bottom is therefore older (and likely smellier!). Or, think about a stack of old magazines or newspapers that might be sitting in your home or garage: most likely, the newspapers at the bottom of the pile have dates on them that are older than the newspapers at the top of the pile.

The photograph below was captured at Volcano National Park on the Big Island of Hawaii. Use superposition to determine which is older: the road or the lava flow? How do you know?

A photograph from Volcano National Park, Big Island of Hawaii. Image by Jonathan R. Hendricks. This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

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.

Imagine cutting a slice of bread from a whole loaf. Because of cross-cutting relationships, the cut that divides the slice from the rest of the loaf is younger than the loaf itself (the loaf had to exist before it could be cut).

When investigating rocks in the field, geologists commonly observe features such as igneous intrusions or faults that cut through other rocks. Because these features are the ones doing the cutting, we know that they are younger than the rocks that they cut into.

Have a look at the photographs below, which show the curb of a road in a neighborhood in Hollister, California. You can see that the curb is offset: the bottom half does not line up with the top half. As it turns out, the famous San Andreas fault runs below the curb at this location, which has caused the curb to be broken and displaced. 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. Image by Jonathan R. Hendricks. This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

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? Image by Jonathan R. Hendricks. This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Let's work through the imaginary example above.

First, we know from the principle of superposition that rock layer F is older than E, E is older than D, D is older than C, and C is older than B.

Oldest F, E, D, C, B Youngest.

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.

Oldest F, E, D, C, B, H Youngest.

Third, we observe that the fault A cuts across and displaces rock layers B-F. It is therefore younger than B-F. Because the fault does not cut across H, we do not know if it is older or younger than that rock unit.

Oldest F, E, D, C, B, (H or A) Youngest.

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).

Oldest F, E, D, C, B, (H or A), G Youngest.

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:

Oldest F, E, D, C, B, (H or A), G, I Youngest.

Relative dating.

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 stratigraphy (layers of rock are called strata). Relative dating does not provide actual numerical dates for the rocks.

Next time you find a cliff or road cutting with lots of rock strata, try working out the age order using some simple principles:

Sedimentary rocks are normally laid down in order , one on top of another. In a sequence, the oldest is at the bottom, the youngest is at the top. This is the principle of ‘superposition’.

Most sedimentary rocks are laid down in flat (horizontal) layers, although these can later tilt and fold. This is the principle of ‘horizontality’. Layers of sedimentary rock extend sideways in the same order . A later event, such as a river cutting, may form a gap, but you can still connect the strata. This is the principle of ‘lateral continuity’.

Fossils and relative dating.

Fossils are important for working out the relative ages of sedimentary rocks. Throughout the history of life, different organisms have appeared, flourished and become extinct. Many of these organisms have left their remains as fossils in sedimentary rocks. Geologists have studied the order in which fossils appeared and disappeared through time and rocks. This study is called biostratigraphy.

Fossils can help to match rocks of the same age, even when you find those rocks a long way apart. This matching process is called correlation, which has been an important process in constructing geological timescales.

Some fossils, called index fossils, are particularly useful in correlating rocks. For a fossil to be a good index fossil, it needs to have lived during one specific time period, be easy to identify and have been abundant and found in many places. For example, ammonites lived in the Mesozoic era. If you find ammonites in a rock in the South Island and also in a rock in the North Island, you can say that both rocks are Mesozoic. Different species of ammonites lived at different times within the Mesozoic, so identifying a fossil species can help narrow down when a rock was formed.

Correlation can involve matching an undated rock with a dated one at another location. Suppose you find a fossil at one place that cannot be dated using absolute methods. That fossil species may have been dated somewhere else, so you can match them and say that your fossil has a similar age. Some of the most useful fossils for dating purposes are very small ones. For example, microscopic dinoflagellates have been studied and dated in great detail around the world. Correlation with them has helped geologists, such as Professor James Crampton, date many New Zealand rocks, including those containing dinosaurs.

Activity idea.

Bring relative dating principles to life with the activity Rock layers and relative dating. Students begin by observing a photograph and a diagram of rock layers near Whanganui, watch an animation about how the layers were formed, then use an interactive labelling diagram to work out the order in which the rocks were created. The activity offers literacy opportunities as well as practice using the science capability 'Interpret representations'.

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