Superposition is one of the fundamental principles of stratigraphy. It is also a statement of the obvious (or seems to be). You will learn all about the concept in this lesson.
What is Superposition?
Superposition is the name for the observation that given a stack of layers, or beds, of sedimentary rocks, older beds are found below younger beds (as long as there has been no disturbance of the beds). Although often referred to as law of superposition, it also is known as the principle of superposition.
A Basic Principle of Stratigraphy
The study of how sedimentary rock layers form and their relationships to one another is a branch of geology known as stratigraphy. Superposition is one of the four fundamental principles that geologists rely upon for interpreting the sequence of events that created a stack of sedimentary rock layers. The other three are original horizontality, lateral continuity, and cross-cutting relationships.
The idea was first formally applied to understanding sedimentary rocks in the late 1660s, by Nicolas Steno, a Danish Catholic bishop and scientist. He surmised that layers of sedimentary rock formed from sediment that settled out of water over time in horizontal layers, what we call the principle of original horizontality. Based on that premise, he concluded that unless the layers of sediment or the resulting beds of rock were somehow overturned or otherwise disturbed following deposition, older rock layers would always be found below younger ones in a layer-cake sequence of beds.
Why Superposition is Useful
Accepting the logic of superposition allowed geologists to start constructing the sequence of events that formed layer upon layer of sedimentary rocks. This method of assigning ages based on position in a sedimentary sequence came to be known as relative age dating. The assignment of relative ages to beds of sedimentary rocks permitted assigning relative ages to the fossils in them, too. That in turn allowed people in different places to begin to identify beds of similar relative age based on their fossil content.
When It Does and Doesn’t Apply
Steno’s rule does not apply all the time or to all rock types. As the definition states, it works where layers of sedimentary rock are more-or-less in their original depositional sequence. But during mountain building, tectonic forces can bend or displace sedimentary rock layers in such a way that older beds end up atop younger ones. Superposition does not generally apply to igneous or metamorphic rocks. But, layers of lava flows would follow the rule, and it is even possible that metamorphic rocks formed from sedimentary rocks might preserve the original sequence of beds.
Let’s review. The law of superposition was proposed in the late 17th century by Nicolas Steno. It states that in a sequence of sedimentary rock layers, the oldest beds are at the bottom, unless the layers have been overturned by tectonics. Superposition is one of the fundamental concepts of sedimentary geology and allowed geologists to determine the relative ages of beds in a sequence of sedimentary rocks. Though primarily applied to sedimentary rocks, some igneous and metamorphic might also follow the rule.