Nicolas Steno, a Danish scientist was the first to propose the law of superposition. Later, it was first used by William “Strata” Smith who generated the first geologic map of UK.
According to the law of superposition, “nothing can be older than the material composing it. The material is older and the object composed of it is less old.”
If we elaborate the law of Superposition in detail, it means that the oldest strata lie at the bottom and the newer strata at the top. So, this law is based on the common sense of proof. As we know that the lowest layer has to be laid down first and so, it should be older. The same rule applies and the layers go on to the uppermost one.
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.
When you apply this law to the sedimentary beds of rock, you give 1 number to the lowest layer and then 2nd to the upper and go on in the same sequence until you reach the uppermost layer.
Application of Law of Superposition
Before you apply the law of superposition to a rock having numerous layers, you need to examine the material. The layers are formed due to depositional events such as the eruption of lava, etc. However, it is not necessary for the structure of all the rocks. Some rocks constitute the layers due to the volcano. If you are examining the formation of layers of volcanic rocks, the layers are formed from bottom to top. In case, you are studying the metamorphic rocks, the older to the newer relationship will be observed differently.
In igneous rocks, the layers are formed due to the pressure from the uppermost to the lowest. So, the position of every layer is chaotic. It does not specify whether the upper layer is younger or the lower. By applying the law of superposition, the age of rocks is determined on the basis of age of fossils. The simplest fossils in the rock’s layers are considered to be the older even these are found on the uppermost layer. In the same way, the age of complex fossils is considered to be younger even if they are located in the lowest layer. So, you cannot apply the law of superposition as it is on the igneous rocks.
Shortly, the Law of Superposition does not work on igneous rocks. It is only the nature of fossils in sedimentary layers that can accurately specify the age of rocks. It is not on the basis of the layer division. However, in sedimentary rocks, the formation of the layer is according to the law of superposition.
Application of Law of Superposition – A Few Examples
Here, in figure A, you can see the formation of all sedimentary layers. The uppermost (A) is the newest and the lowest (G) is the oldest. The age of the rock will be determined according to the age of each layer.
It becomes the opposite when applying on igneous rock. See the figure “D” in the image above. It shows the igneous intrusion in all layers. The layers with igneous intrusion are younger than the layer (A) even they lie below.
Here, you can see the pebbles are lying at the uppermost layer of the rock. But we cannot apply the Law of Superposition on igneous rocks. The age of fossils will determine the age of a rock. So, on close examination, the simplest pebble was confirmed to be the older one as compared to the conglomerate rock found in below layer. The simple fossil is considered the oldest one and the complex is considered the newer one. So, the pebble is the older fossil and we will determine its age to determine the age of the rock.