The Law of Superposition is a geologic principle, first observed and named by Friedrich von Schiller in 1785. It states that rock layers are always in contact with younger rock layers (but not with older ones), because younger rock layers are younger and therefore more closely related in time to the older layers. By applying the Law of Superposition, geologists can determine the relative ages of rock strata in order to reconstruct history, and also help geologists determine the age of geologic structures such as mountains and volcanoes, that appear in the middle of existing rock layers. The Law of Superposition can be seen in both vertical and horizontal layers of rock, such as on the Earth's surface.
In vertical layers
The Law of Superposition is demonstrated in the layers of rock found in mountains and hills on the Earth's surface. These layers of rock are always seen in contact with each other. This means that the layers are close in time, or younger than the layer above. The contact usually occurs because the younger layer has flowed into the older layer, although the contact can also be due to the older rock being intruded into the younger layer.
The relationship is demonstrated on Earth's surface through a comparison of stratigraphic columns (diagrams that display layers of sedimentary rock that are stacked in sequence in a vertical direction) from different parts of the world. These layers can be seen directly when they are exposed to the Earth's surface. The horizontal extent of the rock layers is very similar in most places, despite a variety of changes in conditions over time, such as erosion and deposition.
The Law of Superposition can also be seen in rocks that are not stacked in a vertical sequence. This includes layered sedimentary rocks (stratified rocks that are not in a layer-by-layer arrangement) and horizontally layered rocks.
Rocks can also be seen in contact in a vertical direction if they form part of the same rock body.
Rocks with the same stratigraphic position or sequence in a vertical column can be found at different depths in a single rock body. This relationship is also demonstrated by the fact that all the rocks in a rock body were originally in a layer-by-layer position, but became separated in time and position due to changes in the physical and chemical properties of the rock that took place during their evolution. The Law of Superposition is not valid in all cases.
Relation to Geomorphology
The law of superposition explains why streams follow specific paths across surfaces such as rivers, where the layers are often more or less vertical. Water-deposited material that is deposited horizontally tends to be weathered into flat, horizontal layers which may even be completely horizontal. However, the layers of a valley are younger than the layers above them, and as a result, the valley bed slopes downward as it is weathered.
The law is also very useful in explaining the formation of glacial features such as, for example, the U-shaped course of the River Rhine, which is made possible by the greater thickness of older sedimentary layers that overlie the younger ones. In the Himalayas, the highest mountains occur in places where layers have steeply tilted relative to each other, which tends to create more and taller mountains.
Law of superposition in sedimentary rocks
The law of superposition in sedimentary rocks is based on the observation that the oldest rocks tend to be closest to the surface of the earth (i.e. the closest to the surface that erosion will reach). A rock can be described as young if it is made up of smaller and more poorly sorted material and old if it is made up of larger, more well-sorted material. So, in effect, the surface of the ground is made up of the surface of the younger material, overlain by layers of older material.
The law is the basic principle behind many kinds of stratigraphy, which can be described as the study of the temporal distribution of rock strata. The law can also be used in the field of sedimentary geology to explain many kinds of rock features.
In many cases, the law of superposition may be seen to be the most important fundamental principle behind a particular feature, rather than the sole explanation. For example, the shape of rivers can be explained through a combination of the law of superposition and the law of sedimentary basin development.
The law of superposition does not mean that all geologic features can be explained solely by superposition. For example, the folding of the strata in a mountain range will often be explained by the law of folding. Likewise, when looking at the layers of oil shale, there is a law of maximum age that can be explained as a matter of the most recently exposed rock being of the oldest age. However, generally, many geologic structures will be most naturally explained through a combination of the laws of superposition and the law of sedimentary basin development, rather than through the law of superposition alone.
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.
Superposition geology is essential not only in the 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, 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 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.