Principles of Relative Dating

The process of determining the relative order of past geologic events without necessarily determining their absolute (numeral) age is known as relative dating. This method was developed in the 18th century when geology first emerged as a natural science. To determine the sequence of geologic events, several principles must be followed. Though these principles will seem common sense once you learn them, they were ground breaking in many ways, and are still used today in the field of geology.

Uniformitarianism

The principle of uniformitarianism was first stated by James Hutton in the 18th century, and is a fundamental underpinning of our understanding of geologic processes.  Hutton realized the events in the earths past can be understood by observing the processes we see happening today. Natural processes such as erosion and deposition, plate tectonics, and the laws of gravity and isostasy produce the features of the Earth. These processes have not changed. For example, the processes that transport and deposit sediment in a river are the same today as they were a billion years ago (Figure 4.2.1). Therefore the principle of uniformitarianism is expressed as “the present is the key to the past” which guides the principles of stratigraphy.

The Principle of Superposition

In a series of undisturbed layered rocks, each successive layer above is younger than the layer below it. Therefore, the oldest rocks are at the bottom of the sequence and the youngest are at the top. This is pretty straight forward in practice. In Figure 4.2.2 (left) you can a cliff with horizontal layers of sandstone. The light colored layer at the base of the cliff was deposited first because it is at the bottom, and each layer above was deposited in order from bottom to top. Making the uppermost layer the youngest.

The Principle of Original Horizontality

Sediments deposited or precipitated on the Earth’s surface are under the influence of gravity, and will accumulate in horizontal layers (Figure 4.2.2 left). This is also generally true of most lava flows or volcanic ash that is erupted and deposited at Earth’s the surface. Therefore, if rocks are tilted, folded or metamorphosed, then these events must have followed deposition and lithification. For example, in Figure 4.2.2 (right), rocks that were originally deposited in horizontal layers like the cliffs in Figure 4.2.2 (right). They were then subsequently subjected to heat and pressure, resulting in transformation to contorted, tilted, and folded marble.

The Principle of Lateral Continuity

Sediments are deposited laterally in continuous layers. Therefore if horizontal strata are dissected by erosional features, such as a valley, they can be interpreted to be the same strata (Figure. 4.2.3 left). Imagine you are at Dead Horse Point near Moab, Utah (Figure 4.2.3 right) or even the rim of the Grand Canyon. The horizontal layers of rock that are dissected by the Colorado River can be interpreted to be the same stratigraphic layers on the other side of canyon. Furthermore, you can begin to put together  the relative age of geologic events of such a view, even from the few principles we have learned: 1) Each layer was deposited in order from bottom to top, so the oldest layers are at the bottom, youngest at the top, 2) the layers are still more or less horizontal so, so no major geologic event has tilted or contorted these rocks, 3) subsequent to depositing all of the layers you can see at this view point, the Colorado River cut down through the layers. You can see this starts to become simple logic. The Colorado River couldn’t cut down through layers if they didn’t exist. Therefore, all the layers that Colorado River cuts down through had to have been deposited before the Colorado River began cutting down through them and forming this canyon.

The Principle of Cross-Cutting Relationships

Any geologic feature that crosscuts or modifies another feature must be younger than the rocks it cuts through. The cross-cutting feature is the younger feature because there must be something previously there to cross-cut.  Cross-cutting features can include folds, faults, and igneous intrusions.  They can also include events like metamorphism (Figure 4.3.4).

The Principle of Baked Contacts

This principle is similar to the Principle of Cross-Cutting Relationships. Magma will heat/metamorphose or “bake” the rocks it comes in contact with. Therefore if the rocks surrounding an igneous rock have a “baked contact”, they must have been present before the magma cooled (Figure 4.3.5).

The Principle of Inclusions

The principle of inclusions states that any rock fragments that are included in rock must be older than the rock in which they are included. For example, a xenolith in an igneous rock or a clast in sedimentary rock must be older than the rock that includes it (Figure 4.3.6).

Exercise 4.1 Cross-Cutting Relationships

Can you put all of the principles of relative age dating to work and solve the geologic puzzle in Figure 4.3.7? Describe in order the sequence of events that occurred in the diagram, listing them in order from oldest to youngest.

See Appendix 3 for Exercise 4.1 answers.

Unconformities

An unconformity represents an interruption in the process of deposition of sedimentary rocks. Recognizing unconformities is important for understanding time relationships in sedimentary sequences. An example of an unconformity is shown in Figure 4.3.8. The Proterozoic rocks of the Grand Canyon Group have been tilted and then eroded to a flat surface prior to deposition of the younger Paleozoic rocks. The difference in time between the youngest of the Proterozoic rocks and the oldest of the Paleozoic rocks is close to 300 million years. Tilting and erosion of the older rocks took place during this time, and if there was any deposition going on in this area, the evidence of it is now gone.

There are four types of unconformities, as summarized in Table 4.1, and illustrated in Figure 4.3.9.

Media Attributions

• Physical Geology Laboratory by Cindy Kearns and Elizabeth Johnson is licensed under CC BY SA 4.0, adapted from the original by Ryan B. Anderson.
• Physical Geology-2nd Edition, by Steven Earle is licensed under CC BY 4.0, Modified from the original by Ryan B. Anderson.
• Figures 4.3.1, 4.3.2, 4.3.3 (right), 4.3.4, 4.3.6: © Steven Earle. CC BY.
• Figure 4.3.3 (left) Woudloper. Public Domain.
• Figure 4.3.5. © John Faithfull. CC BY.
• Figure 4.3.7: Woudloper. CC BY SA.
• Figures 4.3.8, 4.3.9: © Steven Earle. CC BY.