Steno’s law of superposition states that any rock bed in an undisturbed or undeformed sedimentary rock sequence is older than the one above and younger than the one below it.
It is one of the most basic principles in archeology, stratigraphy, geology, or Earth scientists to determine relative rock strata age. The law uses common sense and logical arguments that you cannot have an upper layer before laying the lower one. So, it means that the lower is older than the upper.
Learn more about the law of superposition, including its definition and an example to illustrate it. We will also let you know its significance, especially in relative dating, who proposed it, and any limitations.
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Historial background
Nicolaus Steno (1638-1686), born Niels Stensen, was a Danish geologist, anatomist, and later a catholic priest who proposed the law of superposition. He published it in his dissertation De Solido Intra Solidum Naturaliter Contento Dissertationis Prodromus (popularly known as Prodromus) in 1669.
The term superposition comes from the word super, meaning above, and positum, meaning to place. So, the law is about the strata position, i.e., a younger rock layer is always above the older one and vice versa.
Years later, William Smith (1816–1819) popularized the law by including it in English literature. He also used it to make Britain’s first geologic map.
Besides this law, Steno had other principles valuable in knowing the relative age of a rock strata sequence, i.e., the cross-cutting relationships, lateral continuity, and original horizontality.
Lastly, even with technological advancement that has opened a window to calculating absolute rock ages, the superposition law remains a basic geology principle.
What is the law of superposition in geology?
The principle or law of superposition states that in an undisturbed or undeformed sedimentary sequence, any stratum or layer is older than the one above and younger than the one below it. Therefore, it means that the bottommost bed is the oldest and the uppermost bed is the youngest.
The law basis is on the simple fact that sediment deposits happen horizontally or flat and sequentially or chronologically from bottom to top. Why? Because you cannot have deposition beneath an existing layer.
Therefore, the oldest layer will be at the bottommost part, and successive ones become younger as you move upwards. The only assumption to make is that the strata haven’t overturned.
You can also use this law for lava flows or other bedded pyroclastic volcanic rocks like ash beds. However, it doesn’t apply to metamorphic rocks as change depends on conditions, and different layers can form simultaneously, not necessarily in stratigraphic order.
Below is a video to explain it further:
Lastly, please don’t confuse this law with the principle of superposition, which states that in any linear system, the net response or effect from two or more stimuli will equal the sum of each stimulus acting individually.
Examples to illustrate the law of superposition
To illustrate the law of superposition, assume you have ten books with different colors, i.e., red, blue, green, etc.
Now start arranging them one on top of the other, each after some time interval. If you started with a red book, you could not add the blue beneath it. However, instead, you can only add the blue on top of the red and a little later afterward. This explanation has the basics of what the law of superposition states.
What about deformation? Imagine overturning the whole stack downward to demonstrate what disturbances or deformation may do. You will have taken the book you last staked to the bottom.
There can be as many other examples or activities to demonstrate this principle. Also, you will find some downloadable worksheets online to help you set up exercises or examples.
Significance
By studying a sedimentary rock outcrop, we can piece together historical events as they happened. Why? Because the various strata keep a record of happenings, including living things.
Also, this law and five others, i.e., inclusion, fossil succession, original horizontality, cross-cutting relationship, and lateral continuity, form the basis of relative age. How? By studying the arrangement of rock layers, we can sort them chronologically and say which is older than the other.
It is not only stratigraphers who use this principle. Paleobotanists and paleontologists can also use the law of superposition to chronologically arrange fossils they find as they dig through rock layers. All they need to record is the strata that a fossil is found, and they can compute relative age.
With this knowledge, these experts can determine the successive existence of certain species. Also, if found on the same strata, they coexisted together. The general rule is that the lowest strata will have the most ancient or archaic life forms.
Dealing with deformed or overturned sedimentary sequence
Earthquakes, mountain building, and other tectonic movements may tilt, fold, overturn, or fault a sedimentary sequence. Also, chemical alterations, erosion, magma intrusion, limestone crystallization, human/animal activities, etc., may disturb or deform the strata.
Such happenings will interfere with the applicability of the superposition rule, making it hard to tell the relative age of rock layers or sequence of events. So, you may end up with misleading information.
For the case where the sedimentary sequence is deformed or overturned, you can use the strata’s geometric features, including lithological comparison with nearby strata. Also, study any fossils present.
For instance, it is possible to conclude that an originally graded rock will have the finest grain on the top. However, if overturned, the coarsest will be at the top.
Also, sedimentary features like ripple marks, cross-bedding, or mud cracks facing down indicate that said strata overturned downwards. For fossils, the assumption is that the simplest form is the oldest.
Lastly, if folded, using this law of superposition, we can conclude that the superposed fold is younger than the structure or layer that folds it.
Limitations of the superposition law
Using the law of superposition, it may be hard to know the relative age of rock if deformation was severe, i.e., over 90 degrees tilting or rocks overturned during tectonic plate movements, unless there are evident surficial features. Some people will consider this an exception, not a limitation.
Secondly, while it is important to get the relative age of sedimentary rock sequence, it cannot give information on absolute age. Also, you cannot know the time interval between strata as it can be short or take millions of years.
A possible explanation for the variation in the time interval in the stratigraphic record is that deposition may not always be continuous. Also, unconformities may be a reason for discontinuities or breaks.
Also, determining the relative age of formations with a wide space separation is difficult. For instance, you cannot tell if the mudstone in Tuscany younger, older, or similar in age were to a similar sequence in the UK.
Lastly, care must be taken when superposition in archaeology, particularly as you excavate through strata. Why? Because the archaeological strata laying process may differ from the geological. Also, manmade intrusion/activities may not be chronological from top to bottom.
References
- Tarbuck, E. J., Lutgens, F. K., & Tasa, D. (2017). Earth: An introduction to physical geology (12th ed.). Pearson
- Levin, H. L. (1991). The Earth Through Time (3rd ed.). Saunders College Pub.
- Grotzinger, J. P., & Jordan, T. H. (2014). Understanding earth (7th ed.). W.H. Freeman and Company.
- Kusky, T. M. (2005). Encyclopedia of earth science (1st ed.). Facts on File.
- Law of superposition. (2022, September 23). In Wikipedia. https://en.wikipedia.org/w/index.php?title=Law_of_superposition&oldid=1111957748