In geology, the law of superposition, states that the sequence of layers observed in sedimentary rocks marks the time of deposition of the layers. The lowest layer is the oldest layer of deposition and the ones above it are successive younger layers of deposition according to the law of superposition definition. Thus, the principle of superposition geology is one of the important concepts for explaining the geological stratigraphy used widely in the fields of geology, archaeology and other fields related to it.
Law of Superposition Definition
The law of superposition was first put forward by the Danish scientist Nicolas Steno. It was later propagated in English literature by William Smith, who also used the principle of superposition geology in making the first-ever geologic map of Britain.
In a very simple manner, the law of superposition states that whenever there is any geological stratification, the first strata deposited or formed will be the oldest one and it will be followed by the subsequent younger strata which will form because of newer and newer deposition over time. But the condition for this law to be true is that the strata of the rock should be undeformed because of any of the exogenic processes such as weathering and erosion. This is the most important concept in stratigraphic dating but starts with a few assumptions that the law of superposition holds true and the strata/layer formed due to deposition cannot be older than the mineral content of the strata/layer.
The superposition geology is essential not only in dating of the strata but also in the scientific dating of the fossils. Unless and until the sedimentary rocks are not deformed beyond 90° the lowest layer in the strata will be the oldest and the highest layer will be the newest. Thus, the palaeontologists and paleobotanists can identify the relative ages of any of the fossils that are found in the different strata. For example, the fossils of the most archaic organisms will be found in the lowest strata/layer of the rock. Thus, the law of stratigraphy is also applicable to the dating of the fossils as well. The superposition geology also helps in our understanding of the relationship between the species found in the same as well as different layers of the rock.
The law of superposition definition can be clearly understood when applied to a rock such as the one shown in the image below:
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Considerations and Limitations of Superposition Geology
One of the limitations of the principle of superposition geology is that it cannot be applied as it is to some close fields such as archaeology. The stratigraphic superposition in archaeology is different from the law of stratigraphy in geology. This is because the processes involved in the laying down of archaeological strata are different from the geological processes that lead to a structural layering that is required to explain the law of superposition. Also, the man-made intrusions may not be in a chronological order which is the foremost and basic assumption made to describe the law of superposition geology. Also, the deformation due to man-made activities is not horizontal as is the case for the natural strata. Some of the archaeological strata may be formed by the undercutting of the previous or older strata. The best example that shows this difference is the silt back-fill of an underground drainage system. The silt back-fill of the underground drain would belong to a time later or more recent as compared to the layer above it.
While determining the archaeological history a degree of interpretation is required because there might have been changes over time to lower strata. For example, considering the construction of a two-storey building of an ancient civilisation, the materials and the composition of the walls, doors and windows, murals, etc. might have been changed after the creation of such or other similar structures on the first floor. Therefore, the dating of items cannot be done accurately according to the principle applied on superposition rocks in geology and may be wildly misleading.
One of the most important limitations of the law of superposition in geology is that the sedimentary rocks that are being studied shouldn't be deformed or at least not deformed beyond 90°. The original stratification that was achieved by various natural processes can be disrupted and deformed by a number of factors which also include the interference from animals and vegetation, and crystallization limestone as these processes contribute to the weathering and erosion of the strata over a significant period of time. The reason that the law of superposition is primarily concerned with sedimentary rocks is that these rocks are formed by the deposition of fragments of rocks and minerals over a period of time one above the other. Such a stratification scheme may not be the same for the surface-forming igneous rocks depositions such as the lava flows, and the ash falls. Thus, under these conditions superposition geology may not be successfully applicable.