The Law of Superposition

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.

The principle of superposition simply says that when sediments are deposited, those which are deposited first will be at the bottom, and so the lower sediments will be the older. This is because sediment is deposited from above, because gravity operates in a downward direction, and because sediment does not readily pass through other sediment.

The Law of Superposition is a geologic principle, first observed and named by Friedrich von Schiller in 1785.

The Principle of Superposition Sedimentary layers - Argentina

When we see the rock formation in paralleled form, the layered sequence of the material such as lava flow or sedimentary layers constitutes a clear shape. In this shape, the bottom layer shows the oldest material and the upper layer shows the youngest material. In the same way, all the layers going from bottom to top are successively younger.

It is compulsory to examine the rock material before applying the law of superposition. The rocks’ layers are usually formed due to depositional events like lava eruptions. However, this is not necessary for all types of rock structures. The older-to-newer relationship (law of superposition) is different in volcanic and metamorphic rocks.

The law of superposition doesn’t work on igneous rocks. The layers in the igneous rocks are formed due to the pressure from the uppermost layer to the lowest layer. Hence, every layer is chaotic. The age of layers isn’t clear in igneous rocks.

Geological illustration of the Law of Superposition: Older rock layers are typically found beneath younger layers in undisturbed sedimentary rock sequences.

The law of superposition can be applied to sedimentary rocks because the nature of fossils can accurately specify the age of the rock. The rock layers found in mountains and hills are the best example of the law of superposition. The rock layers are always in contact with each other, specifying that the layers are close in time and the older layer is at the bottom. The contact usually happens for two reasons; first, the younger layer has flowed into the older layer, or the older rocks intruded into, the younger ones.

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.

Superposition: In a sequence of strata, the oldest bed is on the bottom, and the youngest on top. Pouring sand into a glass illustrates this point.

 

Exceptions and Considerations:

Disturbances: The sequence can be disturbed by geological processes like folding, faulting, or intrusions. If a rock sequence has been overturned or significantly disturbed, the law might not apply directly without additional geological evidence to reconstruct the original order.

Intrusions: Igneous intrusions, like dikes or sills, can cut across existing layers. Here, the intrusion is younger than the rocks it cuts through, regardless of where it appears in the vertical sequence.

Erosion: Erosion can remove layers, creating gaps in the geological record known as unconformities. These gaps represent missing time in the rock record.

Use in Field Geology:

When geologists examine a cross-section of the Earth, whether in a road cut, cliff face, or drill core, they use this law to piece together the history of that location. They look for signs of disturbance or intrusion to understand if and how the original sequence might have been altered.

Correlation: By combining the Law of Superposition with other methods like index fossils or radiometric dating, geologists can correlate rock layers from different locations, creating a more comprehensive geological history.

Relative Dating: This law allows geologists to determine the relative ages of rock layers. By observing the order in which the layers are stacked, one can infer which layers formed first, second, third, and so on, without knowing their exact ages in years.

Fossil Succession: Combined with the study of fossils (biostratigraphy), the Law of Superposition helps in establishing a relative timeline of life on Earth. Fossils found in lower layers will generally be older than those found in upper layers.

Relative Dating: Principles and Examples