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Dating rocks and minerals is not a simple task, but scientists have been able to solve it. Today, there are various methods available to determine the age of rocks, which are often complex and require specialized education and sophisticated instruments.
Geologists determine the absolute and relative ages of rocks using a combination of different methods, which can be categorized as relative dating and absolute dating methods. The field of study that focuses on dating rocks is known as geochronology.
In this article, we will discuss the methods used to determine the age of rocks, the principles underlying these methods, the distinction between relative and absolute age, how geologists date rocks, and the instruments they employ for this purpose.
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What is the Absolute Age and Relative Age of Rocks and What is the Difference?
The science that studies the geological history of the Earth is called historical geology. Geologists use two terms to describe the age of rocks: relative age and absolute age of rocks.
Absolute age determines the time of rock formation using radiometric methods, typically measured in astronomical units such as years. When scientists operate with the concept of absolute time, they are interested in the precise timing of events.
For example, if geologists determine the absolute age of granite to be 2 billion years, this means that the granite crystallized from magma 2 billion years ago and has persisted to the present day.
However, in many cases, the sequence of geological events, such as the order in which rock layers formed, is more important to geologists. In this context, they refer to the relative age of rocks.
By using specific methods, which we will discuss further, geologists can determine that two adjacent rock layers formed at different times, with one layer being younger or older than another.
In scientific circles, there is a lighthearted legend regarding the difference between absolute and relative geochronology:
Once, a professor of geology was asked, “What is the difference between absolute and relative geochronology?” The professor replied, “The main difference is that relative geochronology is precise, while absolute geochronology is not.” Why is that?
The reason is that absolute geochronology methods have an error range of 1-5%, which may seem small at first glance. However, in the field of geology, which deals with hundreds of millions of years, this error translates into several million years. An error of 1-5 million years can be significant, right?
This is where relative geochronology methods come to help geologists. Since significant accumulations of sedimentary rocks can occur over those millions of years, relative geochronology methods can determine their ages relative to one another and to other rock layers.
Overall, both absolute and relative geochronology is valuable in understanding the geological history of the Earth, with absolute methods providing precise numerical ages and relative methods establishing the order and relationships of geological events and rock layers.
So, let’s continue and discuss the principles used in relative and absolute geochronology and their methods.
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Absolute Dating Methods
As we mentioned before, absolute geochronology uses radiometric (isotopic) methods to determine the age of rock formations. We can distinguish two types of methods in absolute geochronology:
- Primary methods are based on the calculation of time from the process of radioactive decay and are called radiometric dating.
- Secondary methods are based on the study of phenomena resulting from radioactive emissions.
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Primary Methods of Absolute Dating (Radiometric Dating)
Radiometric dating relies on calculations of the ratios between the amounts of radioactive isotopes and their decay products – stable isotopes – within the rocks. However, let’s start by looking at the fundamental principle of isotopic methods, using uranium-lead dating as an example.
This method is a well-established radiometric technique used to determine the absolute age of the oldest Precambrian rocks. For instance, let’s consider a rock formation containing uranium compounds.
It is known that uranium atoms are not stable; they emit energy and particles over time, eventually transforming into the lead (Pb) – a stable element that doesn’t undergo further transformations.
Here are simplified reactions of the radioactive decay of uranium:
238U → 206Pb + 8He
235U → 207Pb + 7He
The nature of these reactions is such that the rate of nuclear decay is constant and unaffected by external factors like temperature or pressure.
Therefore, if the rate of decay of uranium is determined experimentally over a short period of time, it can be accurately extrapolated to longer time intervals.
Consequently, it has been established that in any given sample of uranium (specifically the 238U isotope), half of the atoms comprising it will transform into Pb atoms in 4.5 billion years.
Accordingly, after 9 billion years, only a quarter of the initial amount of 238U will remain, and so on. The time span of 4.5 billion years is referred to as the half-life of 238U.
Therefore, if we have a rock formation containing uranium compounds and if this rock remains undisturbed, all the lead atoms (resulting from the continuous transformation of uranium atoms) will remain within the rock.
Since external factors, as we recall, do not affect the rate of this process, the amount of lead will depend solely on the time during which the rock formation has remained intact.
This last aspect is crucial because any alteration or destruction of the rock formation will cause some of the lead to migrate into the surrounding environment, and in such cases, the proportion of lead within the rock will not correspond proportionally to its age.
If an existing rock formation undergoes certain changes (due to tectonic movements, high temperatures, etc.), it can distort the age data obtained through isotopic methods.
The obtained results of absolute age, in this case, would actually indicate not the absolute age of the rock formation but the time elapsed since the last process that altered the rock.
To conduct research using this method, minerals that contain uranium are used, such as uraninite, monazite, orthite, columbite, and zircon.
These minerals are found in granites, pegmatites, and quartz veins. Samples weighing approximately 0.5 grams are taken for analysis, and the method’s error is around 2-5%.
So, let’s summarize briefly what has been mentioned above:
If a rock formation contains 238U compounds and has remained unchanged since its formation, the amount of accumulated 206Pb isotopes within it will be directly proportional to the age of that rock formation – the older the rock, the more 206Pb it will contain, and the less 238U will be present.
In addition to this method, there are other primary isotopic methods for determining the absolute age of rocks. We will discuss the most popular ones further on.
Uranium-Thorium-Lead Dating (U-Th-Pb)
This method is the most well-studied and accurate radiometric dating method. It operates on the same principle as the uranium-lead (U-Pb) dating we discussed earlier, but it is more precise as it additionally involves calculations based on the isotope thorium (Th).
Here are simplified reactions of the radioactive decay of uranium and thorium:
238U → 206Pb + 8He
235U → 207Pb + 7He
232Th → 208Pb + 6He
As a result, the age is obtained by considering the three decay reactions mentioned above. The values may slightly differ, but by using mathematical methods, the average absolute age of the rock formation can be determined.
Potassium-Argon Dating (K-Ar)
This method relies on the accumulation of the stable isotope of argon (Ar) due to the radioactive decay of potassium-40 (40K). Here are simplified reactions of the process of radioactive decay of potassium:
40K → 40Ca + β,
40K + e → Ar
Through the capture of free electrons, 12% of the potassium isotopes transform into argon, while the remaining potassium decays into calcium through beta radiation. Argon is well retained within the crystal lattice of minerals, making this method quite accurate.
Additionally, this method has the advantage of potassium being a widely distributed chemical element in the Earth’s crust, occurring in over 100 minerals, both igneous (feldspars, micas) and sedimentary (glauconite) rocks.
Radiocarbon Dating (14C)
The carbon (radiocarbon) method is applied to determine the age of the youngest rocks since the half-life of 14C is only 5,500 years. The radioactive isotope 14C is continuously formed in the atmosphere from nitrogen 14N under the influence of cosmic radiation and is assimilated by plants.
When plants die, the assimilation stops, and the isotope begins to decay, converting back to nitrogen. By measuring the amount of carbon that has decayed and the half-life period, the time of plant burial in the rock can be calculated.
This method is also used in archaeological research (for example, determining the burial time of a wooden tomb of an Egyptian pharaoh approximately 2,190 years ago). Additionally, this method is used to determine the age of bones of ancient animals and humans.
In addition to the dating methods mentioned above, there are also well-established methods such as rubidium-strontium, samarium-neodymium, lutetium-hafnium, rhenium-osmium dating methods, etc.
All of these methods are based on the same principles as the previous ones. In each specific case, geologists select the method that can be applied to a particular rock based on the radioactive compound present in that rock.
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Secondary Datind Methods of Absolute Dating
Secondary dating methods of absolute geochronology are based on determining the absolute age of rocks through traces of radioactive decay and are called fission-track dating.
The thing is, even a minuscule amount of radioactive decay in a solid compound, such as a mineral, leaves microscopic damage in the form of tracks. These tracks, which have a thickness of just a few microns, are formed by neutrons emitted by the radioactive atom during decay.
The number of such tracks is directly proportional to the time of radioactive decay and the amount of the radioactive element. By measuring the density of these tracks and the amount of radioactive elements, the age of the rock can be determined.
This method can be applied in the range of 4,000 to 1 billion years. It is a relatively inexpensive method and can be used both as an independent method for age determination and to refine the results obtained from other isotopic geochronology methods.
- Relative Dating Methods
Relative dating deals with the relative age of rocks, which is the age of rocks relative to one another. In other words, relative geochronology determines which layers of rocks formed later and which formed earlier.
All methods of determining relative age can be divided into two groups: non-paleontological and paleontological.
Non-Paleontological (Geologic-Petrographic) Dating
Non-paleontological dating is applied when rocks lack fossilized remains of plants and animals. Here are the most popular non-paleontological methods of determining the relative age of rocks.
Stratigraphic Dating Method
The foundations of the stratigraphic method were laid by the Danish scientist Steno in 1669. He discovered the law of superposition, according to which all layers that lie below are older, meaning they formed earlier than the layers above.
This law is also known as Steno’s principle or as the principle of superposition. It is very straightforward and allows for the determination of the relative age of sedimentary layers in small areas, for example, in a single outcrop with horizontal or gently dipping layers.
Lithological Dating Method (mineralogical-petrographic method)
The lithological (mineralogical-petrographic) method is based on comparing the mineralogical and petrographic composition of rocks, as well as their structural and textural features, color, and the presence of characteristic mineral inclusions in rock layers of different geological sections.
The fundamental principle of this method is that rocks with similar or close mineralogical-petrographic characteristics are rocks of the same age.
This principle can be applied accurately on small scales, but it may not work over large distances because the lithological composition of rocks can also change due to variations in physical and geographical conditions.
This method is widely used in conjunction with the stratigraphic method in areas where sedimentary rocks are found, especially those that do not contain organic remains, as well as in regions with metamorphic and even igneous rocks.
Geophysical Dating Method
The geophysical method is based on studying the physical characteristics of rocks in geological sections and comparing the obtained results.
This method is similar to the mineralogical-petrographic method since it involves dividing the geological sections into separate petrographic horizons, studying their interrelationships, assessing relative age, and identifying similar layers based on their composition as contemporaneous.
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Paleontological (biostratigraphic) Dating Methods
Paleontological methods are based on studying the fossil remains of organisms preserved in layers of sedimentary rocks.
Throughout the Earth’s long geological history, the organic world has undergone complex evolution, with certain groups of organisms inhabiting specific territories or spreading across the entire planet, while others became extinct and were replaced by new, better-adapted forms to the changed physical-geographical environment.
Many organisms, or more specifically, their hard remains (shells, teeth, skeletons, and fragments), were buried in contemporaneous layers of sedimentary rocks and underwent fossilization processes.
However, not all fossil remains can be used as indicators of the age of the rocks in which they are found. Many organisms evolved over extremely long periods of time, and often the same forms were distributed in rocks of different ages.
For determining the relative age of geological rocks, only certain fossils, known as index fossils, are suitable. These are organisms that developed within a relatively short span of time but had a rapid geographic spread.
One of the main paleontological methods is the index fossil method, based on the principle that deposits with the same fossil remains have the same age. This principle is also known as Smith’s principle and has been fundamental in biostratigraphy for a long time.
The main drawback of this analysis is that many species and genera of organisms that are index fossils in one region may not be index fossils in another region of the planet. There are no universal index fossils for the entire planet or for any specific time interval.
The most reliable method is the analysis of fossil assemblages, which takes into account all the fossils found in the sedimentary layer.
This is explained by the fact that each layer contains a specific complex of organisms, which, according to the law of the irreversibility of organic world evolution, is not repeated in other layers.
The advantage of this method is that conclusions about the age of the sections are made based on the study of the entire collection of collected fossil fauna and flora, not just based on key forms.
The phylogenetic method is based on the study of phylogeny – the evolution of the organic world – and is fundamental in modern biostratigraphy. It is believed that descendants are constructed more progressively than ancestral forms, and their remains are found in younger deposits.
The micro-paleontological method and spore-pollen analysis are also widely used in stratigraphy. Microfauna, such as foraminifera, radiolarians, microscopic algae, etc., are studied using the micro-paleontological method to determine the age of rocks in small samples, for example, from boreholes.
The spore-pollen analysis is used to determine the age of primarily continental deposits. In this case, the spores and pollen of plants, which are well-preserved in a fossil state due to their strong shells, are studied.
Based on stratigraphic and paleontological methods, geologists from many countries have conducted extensive work on the division and identification of the thickness of sedimentary rocks from different parts of the Earth.
A stratigraphic scale was compiled, which reflects the sequence of layering of sedimentary rock layers of different relative ages.
Stratigraphic units of different ranks and corresponding geochronological subdivisions were identified, which indicated the time of formation of a particular stratigraphic unit.
According to the scale, the entire geological history is divided into intervals, each of which corresponds to a certain stage in the development of the organic world (eons, eras, periods, epochs).
At the end of each interval, significant changes occurred in the composition of the organic world, including the extinction of some groups of organisms and the emergence of new ones.
Each of the identified geochronological stages corresponds to a certain thickness of sedimentary rocks with its complex of key forms, i.e., stratigraphic subdivisions.
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How Do Geologists Date Rocks? Step-by-Step Guide
As you already understand, there are quite a few methods that geologists use to determine the age of rocks. We have provided a small portion of these methods in this article.
There are also other methods. First of all, it should be noted that a combination of methods is used to date rocks, which reduces the margin of error and provides a more accurate result.
Here’s a step-by-step guide on how geologists date rocks they find during fieldwork:
- Identification of the rock and determination of its genetic type and mineral composition
First, it is necessary to correctly identify the rock and determine its type – whether it belongs to sedimentary, igneous, or metamorphic. During the identification of the rock, geologists determine its mineral composition and pay attention to the presence of fossil remains, if any, in the rock.
- Select the appropriate dating method
Depending on the rock type, mineral composition, and the age range being targeted, geologists choose the most suitable dating methods.
- Prepare rock samples and analyze them based on the selected methods
If isotopic methods for determining absolute age are chosen, a sample is taken from the rock and sent to a specialized laboratory.
There, experts create samples from this specimen, which are then analyzed using specialized equipment. For example, a device used to determine isotopic ratios is called a mass spectrometer.
If relative geochronology methods are chosen, rocks are studied based on the parameters that will later be compared in different rock samples, depending on the selected method.
- Process the data obtained from the instruments and create a report
After obtaining data from the instruments, it is necessary to calculate and determine the potential errors that may have occurred during the research.
Then, a report is created detailing the methods of investigation, instruments used, calculation methods, and possible errors.
What Tools Are Used to Date Rocks?
It’s important to note that different dating methods require different tools and instruments. The selection of specific tools depends on the dating technique being employed and the characteristics of the rocks being studied. Here is a list of tools commonly used:
- Mass Spectrometer
- X-ray Fluorescence (XRF) Spectrometer
- Electron Microscope
- Optical Microscope
- Stratigraphic Tools – geologic compasses, rock hammers, hand lenses, and measuring tapes for detailed mapping and documentation of stratigraphic sequences.
Additionally, technological advancements and innovations continue to improve dating techniques and the tools used in rock dating.
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Absolute dating methods provide more specific origin dates and time ranges, such as an age range in years, while relative dating methods estimate whether an object is younger or older than other things found at the site.
Relative dating does not offer specific dates, it simply allows determining if one artifact, fossil, or stratigraphic layer is older than another.
The methods for determining the absolute age of rocks are based on the phenomenon of radioactive decay. Relative dating methods, on the other hand, are based on comparing the parameters of different rocks with each other.
The different types of rocks and mineral compositions require different dating methods. Relative dating methods are best suited for sedimentary rocks, while absolute dating methods are more appropriate for igneous rocks.
Metamorphic rocks are usually more challenging to date, but in certain cases, radiometric methods are used. However, typically multiple different methods are used simultaneously to determine the age of a rock in order to reduce error.
A stratigraphic scale has been created to correlate the ages of rock layers across different regions of the planet.
Determining the age of rocks requires expertise in this field and often specialization in a particular dating method, as these methods are typically complex and multifaceted.
- A. Novosad – “General Geology”, Rivne, Ukraine 2006.
- O. Ivanik, A. Menasova, M. Krochack – tutorial “General Geology”, Kyiv 2020.
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