How can we tell the age of rocks and fossils?

Written by Ichiko Sugiyama and Dr. Kärt Paiste

Knowing the age of different rock layers is paramount to reconstructing past environmental changes and is accomplished by dating rocks with various analytical techniques. Geologic dating methods can be placed into two broad categories: 1) relative dating and 2) absolute dating. Today, we will focus on understanding relative dating and how this can bring forward insights about our planet.

Figure 1. Valaste Waterfall near Ontika in Estonia exposes a succession of Upper Cambrian claystone (Cambrian: 541 to 485 million years ago) and sandstone, as well as Lower Ordovician (Ordovician: 485 to 443 million years ago) phosphoritic and glauconitic sandstone, mudstone, and limestone (Photo credit: Dr. Kärt Paiste).

Relative dating: Determining the relative age of rocks and fossils

Relative dating is one of the geologic methods used to determine the relative order of events by comparing variations in rock units or fossil fauna (i.e., ancient animal life) and flora (i.e., ancient plant life) they contain. Relative dating follows the theory of “Uniformitarianism” that was initially developed by a Scottish geologist, James Hutton, in his 1785 book, “Theory of the Earth, with proofs and illustrations.” This philosophical idea laid the foundation of geology and makes an assumption that processes and natural laws that operate in the present also operated in the past. As time always moves forward younger sediments (i.e., a precursor to a rock; refer to the rock cycle) lay atop older ones – just like a layered cake (Figure 2). Such rock successions are referred to as strata or stratum for singular, which means a series of layers of rocks on the ground. The study of rock strata is called, “stratigraphy,” and branches into litho- and bio- stratigraphy. Lithostratigraphy is a study of rock layers and its compositions, and biostratigraphy is a study of fossils (remains or traces of ancient life) in rock layers.

Figure 2. Stratigraphy is just like a layered cake (image credit: Ichiko Sugiyama)!

“In order to understand the past, we must understand the present.”
– Carl Sagan

From studies of modern water bodies, we know that sediments precipitate over time in horizontal layers that laterally extend until they meet a physical boundary. We also know that changes in the physical or chemical conditions predictably cause variations in the structure of rock units. For example, as the sea level rises, sand or gravel deposits at the coast of a beach will be replaced by fine mud so that over time layers of sand will be overlain by muds (Figure 3). If the change in sea level occurred globally, we would expect to see a similar order of sediment layers at various geographic locations and consider such rock successions similar in age. In addition, rock strata commonly contain fossils of animals and plants that lived during sediment accumulation but became extinct at some point in time. Therefore, rocks containing the same fossils have similar ages and are correlatable in time, even if the rock type and locality are different.

Figure 3. As the sea level rises, the sand and gravel will be eventually overlain by fine sand or mud (image credit: Ichiko Sugiyama).

Rule of thumb: Understanding the order of events in a stratum

Figure 4. This is an example of strata showcasing the boundary between different rock layers, in Pinhay Bay, England, U.K.

Understanding stratigraphic sequences require knowledge about facies correlation – a skill every geologist acquires. ‘Correlation’ is just a fancy word for matching similar rocks that share physical or chemical attributes, and ‘facies’ is a fancy word for a body of rock with specific textural or chemical characteristics. For example, in most cases, if a similar order of rocks in a succession is found elsewhere, the chances are that these rock strata are closer in age than others and can be correlated. There are many general principles used to correlate rocks, as seen below.

1. Principle of superposition

The principle of superposition states that within a sequence of rock layers, the oldest layer is at the bottom, and the youngest layer is at the top (Figure 5). For example, we can assume a similar age for limestones bearing coral fossils that became extinct 200 million years ago but are found thousands of kilometers apart even if we don’t know their exact age.

Figure 5. The principle of superposition (image credit: Ichiko Sugiyama).

2. Principle of horizontality

The principle of horizontality states that due to gravity, layers of sediments (solid natural materials, e.g., mineral grains) are deposited initially horizontally (Figure 6). This pretty much means that sedimentary rocks lay on top of each other like a stack of pancakes, unless other physical forces (like cutting through a pancake stack) alter the original layering and cause faulting or folding of the sedimentary beds.

Figure 6. The principle of horizontality (image credit: Ichiko Sugiyama).

3. Principle of lateral continuity

The principle of lateral continuity states that layers of sediments extend laterally (i.e., side-to-side) in all directions until they meet a physical obstacle (Figure 7). In other words, if we observe similar sedimentary rock bodies sporadically in a confined area, we can assume that initially, these rock bodies formed a continuous layer.

Figure 7. The principle of lateral continuity (image credit: Ichiko Sugiyama).

4. Principle of cross-cutting relations

The principle of crosscutting relations states that a geologic feature that cuts another is the younger of the two features (Figure 8). In the lower illustration, a dike (i.e., a vertical intrusion of magmatic rock) cross-cuts sedimentary layering and, thus had to form after rock formation.

Figure 8. The principle of cross-cutting relations (image credit: Ichiko Sugiyama).

5. Principle of faunal succession

The principle of faunal succession states that different strata may contain a particular type of fossil by which the rocks may be correlated. The fossil record has taught us that plants and animals evolve over time by changing aspects of their bodies or becoming entirely new species. Thus, the fossil record is cluttered with smaller- and larger-scale extinction events but also periods of major biological innovation. Therefore, geologists can use the appearance, presence, and disappearance of fossils as a way to correlate strata, even those deposited in different locations on Earth (Figure 9).

Figure 9. The principle of faunal succession (image credit: Ichiko Sugiyama).

The significance of understanding the age of rocks and fossils

Figure 10. Fossilized Trilobites in rocks.

Understanding the relative order of events of rocks and fossils provides insights about Earth History, from the evolutionary pathway of life (i.e., fossil record), understanding the evolution of the atmosphere to changes in sea level in the past. Even though relative dating cannot provide the exact age of rocks and fossils, it is one of the many powerful tools geologists use to decipher the mystery of the Earth’s past.