Relative dating is a science that geologists and archeologists use to determine the chronological sequence of geological events, i.e., their relative ages. It tells whether a geological event is older or younger than another depending on their interrelationships and not actual numerical years, i.e., absolute age.
In determining relative ages, we use principles of relative dating, i.e., superposition, cross-cutting relationships, and inclusions. Others are original horizontality, lateral continuity, and faunal succession. Also, baked and chilled margins, uniformitarianism, and unconformities will be handy in determining age relationships.
The discovery of radioactivity in the 20th century opened a window to radiometric dating methods that give actual age. However, relative dating remains important to geologists, with the advantage of being easy to apply (it doesn’t need sophisticated equipment).
Today, we will discuss relative dating meaning and how to determine relative ages, i.e., the techniques, laws, or principles to use with an example diagram. We will also examine its significance and how it differs from absolute dating (radiometric).
What is relative dating?
Relative dating is the science of placing events, rocks, minerals, fossils, geological structures, or features in order of their occurrence (chronological sequence) in a geological record, depending on their relationship. One fact is that this dating method doesn’t tell us how long or when they happened but how they followed each other.
For example, relative dating will tell you if a lava flow is older than sedimentary strata, a fault is younger than a granite intrusion or a conglomerate is older than sandstone. However, you will not know when and for how long any events happened, i.e., absolute or actual/numerical age.
On the other hand, relative age tells you if a rock, mineral, fossil, or geological feature/structure is older or younger compared to another. Again, it doesn’t give you numerical age as in the case of absolute or radiometric dating. However, to establish the relative ages, you must acknowledge the sequences or order in which events happened.
Laws or principles of relative dating
We apply various basic relative age dating principles, laws, techniques, or methods in geology to help us establish the order in which events happen, i.e., the chronological sequence in a stratigraphic column. These principles help us determine relative ages or what is older than the other or what preceded the other and interpret geologic history.
The main six principles of relative dating are Steno’s superposition, horizontal originality, lateral continuity, cross-cutting relationships, Charles Lyell’s principle of inclusions, and William Smith’s faunal succession.
These are also the principles of stratigraphy, a branch of geology and Earth Sciences that studies layered rocks (strata) and their arrangement or layering. It also deals with the strata’s origin, composition, and distribution. Stratigraphy’s main branches are biostratigraphy, lithostratigraphy, and chronostratigraphy, including magnetostratigraphy.
Besides the six principles, chilled and baked margins, James Hutton’s doctrine of uniformitarianism and his idea of unconformities are essential in relative dating.
Let’s look at the relative dating or age determination principles and briefly explain how they work or help determine relative age.
1. Principle of superposition
Steno’s law or principle of superposition states that in an undeformed/disturbed sedimentary sequence, a rock layer is older than the one above and younger than the one below it. It is one of the most important principles of relative age determination and arranging rock layers chronologically.
It may require looking at surficial features for deformed, folded, and overturned sedimentary strata. These features include ripple marks, mud cracks, and crossbedding and help establish the correct order of rock layers in a stratigraphic column. Reasons for deformation include plate tectonic activities, erosion, limestone crystallizing, human/animal activities, and chemical alteration.
Besides surficial features, you may have to use other principles like faunal succession, cross-cutting relationships, inclusions, etc., in relative age determination.
2. Principle of horizontal originality
The law or principle of horizontal originality states that sediments are always deposited horizontally or near a horizontal position under gravity. Therefore, sedimentary rock strata should be nearly flat. Any folds or steep tilts happened after the sedimentary rock formation (lithification). Of course, except for the crossbedding.
Original horizontality helps us establish that folding or tilting events happened after the sedimentary strata formed, hence important in relative dating.
3. Principle of lateral continuity
Lateral continuity law or principle states that rock layers will extend continuously in all directions until they thin out, grade into different rock strata, or meet a barrier. This law applies to sedimentary rocks in the same depositional basin.
Lateral continuity helps arrange disjointed strata in a basin in the correct formation sequence or chronological order, essentially relative dating. The rock layer lithology and strata mean that the strata were once continuous, and each layer must be the same age.
However, when applying lateral continuity, don’t forget to identify facies, as they affect lithology correctly. Also, in case of disturbed strata (faulted, folded, or tilted) due to Earth’s movements, carefully study rocks to get a correct chronological order of formation.
4. Principle of cross-cutting relationships
The principle of cross-cutting relationship states that a rock unit or geological feature/structure that cuts or deforms another is younger than the geological feature/structure or body of rock it cuts, penetrates, or bends.
How does it work? Using cross-cutting relationships, we can establish the sequence of events or relative ages. For instance, intrusions (batholiths, sills, dikes, or laccoliths) are younger than the rocks they intrude, and penetrating rock minerals are younger than the rocks they penetrate. Also, folding and faulting events are younger than broken or folded rocks.
5. Principle of inclusions
The law or principle of inclusion states that rock fragments (clasts or xenoliths) enclosed by another rock are older than the rock that surrounds it. This principle helps us determine the relative ages of rocks and minerals, i.e., included fragments are older. Therefore, rocks with rock fragments or minerals of adjacent rocks are younger than the adjacent rocks or minerals.
The inclusions principle identifies intrusions, including sills that may appear like layers with chilled and baked margins.
6. Principle of faunal succession
The principle or law of faunal succession states that sedimentary strata have diagnostic fossils and fossils assemblage that succeed each other vertically in a predictable and reliable order and can be identified over a wide horizontal distance. William Smith, an English geologist and canal surveyor, formulated this law.
The unique succession of fossils in rock layers in time and space is due to unidirectional organic evolution, i.e., it doesn’t repeat. Therefore, we can use characteristic index fossils and fossil assemblages to correlate and assign relative ages to a stratigraphic column. Why? Because rocks containing the same fossils or fossil assemblage are the same age, i.e., formed with the respective organisms were extant. Thus, fossils are independent of lithology or other paleontological observations.
Biostratigraphy is the branch of stratigraphy that uses fossil content to correlate and subdivide and classify rock strata into biozones (biostratigraphic units) before assigning relative age and biocorrelating the rock layers.
To date, biostratigraphy remains a popular relative dating framework relatable to the absolute time scale. Also, it gives high-resolution stratigraphy with a range as short as 10Ka to 100Ka.
7. Unconformities
Unconformities are erosional or non-depositional surfaces separating rock sequences of different ages, i.e., they represent a hiatus. Some unconformity types include angular unconformity, disconformity, non-conformity, and paraconformity. Others are buttress/on lap and blended unconformities.
Since unconformities are younger than the rock layer beneath them and older than the layer above, they can help in relative dating or chronology of events.
8. Chilled and baked margins
Chilled margins are characterized by fine-grained or glassy zones formed by rapid cooling when magma encounters cold rocks as it intrudes. However, the bulk of the magma will cool slowly, forming larger crystals.
On the other hand, seared or baked margins form when extruding magma heats the adjacent cold country rock. The heating results in contact metamorphism and is what characterizes the baked margin.
Both baked and chilled margins are important in relative rock dating. A baked rock is older than an intrusion, while a chilled rock is younger than
9. Uniformitarianism doctrine or principle
James Hutton (1726–1797) proposed the principle of uniformitarianism, and Charles Lyell (1797-1875) amplified it with the aphorism the present is the key to the past. It states that natural laws and processes operating today operated in the past (haven’t changed) but not necessarily with the same intensity or rate.
Using uniformitarianism and applying Charles Darwin’s evolution theory, it is possible to date rocks by assuming that simpler organisms are older than complex ones. However, this may not always be the case and is likely to bring errors. Thus, we don’t consider uniformitarianism an important principle in relative age determination.
Activity: Relative dating examples with diagram
Diagrams or exercises that require establishing relative ages need you to test the six principles of stratigraphy above. Also, you need to consider the concept of unconformities and baked/chilled margin.
The principles we will use are superposition, cross-cutting relationships, and inclusions. Also, we will recognize two unconformities. The sequence of events from oldest to youngest is from 1, 2, 3, 4, unconformity above 3, 5, 6, unconformity above 5, and 7.
Justification
By applying the principle of superposition, layer 1 is older than 2, and 2 is older than 3. All these three are cut by fault 4, meaning fault 4 is younger. However, fault 4 is older than unconformity above strata 3 as it doesn’t cut it.
Unconformity above layer 3 is older than layer 5 by superposition, and magma intrusion 6 is younger than 5 as it has inclusions of 5 and those of layers 1, 2, and 3. However, it is older than unconformity above layer 5 as it doesn’t cut it.
Finally, layer 7 is the youngest. It is above the unconformity above layer 5, and magma intrusion 6 doesn’t cut it.
Relative dating vs. absolute dating
The difference between relative dating and absolute dating is that relative dating places rocks, fossils, and geological events in their chronological sequence, i.e., which is older than the other or compares the age of one to another. In contrast, absolute/radiometric dating gives you the numerical date of origin and time ranges, such as ages in years from the present.
Unlike relative ages that depend on relationships/principles, radiometric dating uses the abundance of radioactive isotopes with known constant decay rates to determine the approximate age of fossils and igneous rocks. Some radiometric dating methods include radiocarbon, potassium-agon, uranium-led, Samarium–neodymium, Rubidium–strontium, and Uranium–thorium.
Significance relative dating
Relative dating is important to geologists, Earth scientists, paleontologists, and other related areas because of the following reasons:
Construction of the geological time scale (GTS)
Geologists developed a relative geological time scale through relative dating, i.e., placing rock layers in their correct stratigraphic position relative to time. Later, after discovering radiometric dating techniques, they could calibrate it to a geological time scale with actual years.
Note that radiometric dating didn’t replace relative dating but only supplemented it. Relative and absolute dating are utilized in making and revising geological time scales.
Discern geological history of the Earth
Relative dating principles are important in correlating rock strata, i.e., showing equivalence of age, not an actual number of years, and placing rocks, geological features, and structures in correct chronological sequence. This helps us get a clear picture of past geological events, including their sequence, which is key in the discerning geological history of the Earth.
Identify economically valuable resources
Relative dating helps us to understand rock sequences, and three-dimensional relationships may help identify potential strata of economically valuable resources like natural gas, oil, minerals, etc.
Frequently asked questions (FAQs)
No. Although relative dating helps determine the relative age of sedimentary and layered igneous rocks, you can use other igneous rocks, including intrusion and metamorphic rocks. For instance, the principle of inclusion and cross-cutting can determine the relative ages of igneous intrusion, while inclusions can determine the relative ages of metamorphic and other igneous rocks.
Radiometric dating is the main method of determining the absolute ages of igneous rocks that employs radioactive decay of radioactive isotopes in the rocks. However, you can use the principle of inclusion relative ages of igneous rocks and cross-cutting relations to get the relative age of intrusions.
Physical features, also known as landforms, refer to valleys, oceans, rivers, glaciers, plains, deserts, mountains, canyons, etc. Most processes that form these features leave characteristic signatures that help determine their relative sequences.
We can establish relative ages and chronological sequence by applying the cross-cutting relationships superposition principle and considering the features’ complexity. Also, geomorphic markers like slope steepness, landform size, and fabric will prove important in determining relative ages and stratigraphic and topographic positions.
One main limitation of relative dating is that it doesn’t give the time of origin or exact age of rocks, fossils, or geological events. Also, it may not be applicable in igneous or metamorphic rocks without some interrelations where you can apply the principles of relative dating.
No. Rock types or rocks with the same lithology don’t always have the same relative ages. Therefore, all limestone isn’t the same age.
In stratigraphy, the geologic column describes the vertical positions of rock units in an area. In relative dating, we use it to give a sequence in which the sedimentary rocks formed from oldest to youngest or their relative ages.
References
- Lutgens, F. K., Tarbuck, E. J., & Tasa, D. (2018). Essentials of geology (13th ed.). Pearson.
- Wicander, R., & Monroe, J. S. (2010). Historical geology: Evolution of Earth and life through time (6th ed.). Brooks/Cole, Cengage Learning.
- Plummer, C. C., Carlson, D. H., & Hammersley, L. (2016). Physical Geology. 15th ed. (15th ed.). McGraw-Hill/Education, Inc.
- Levin, H. L., & King, D. T. (2017). The Earth Through Time (11th ed.). John Wiley & Sons.
- Borradaile, G. J. (2015). Understanding geology through maps (1st ed.). Elsevier.
- Martin, R. (2018). Earth’s evolving systems: The history of planet earth (2nd ed.). Jones & Bartlett Learning.
- Coates, D. R. (1984). Landforms and landscapes as measures of relative time. Developments in Palaeontology and Stratigraphy, 247–267. https://doi.org/10.1016/s0920-5446(08)70075-x