8.2: Uniformitarianism and Relative Age Dating

Uniformitarianism

Our ability to use evidence left behind in rocks to interpret and reconstruct events that happened thousands, millions, or billions of years ago is dependent upon geology’s most fundamental assumption: uniformitarianism. Uniformitarianism is the assumption that the chemical and physical laws of nature have not changed over the course of Earth’s long history. The laws have remained constant, or “uniform” in their operation.This means, for example, if geoscientists determine rates of plate motion are a few centimeters per year, there has likely never been a time that pieces of the lithosphere were zooming around the planet at 50 kilometers per hour! While geoscientists cannot have direct observations of Earth processes occurring billions of years ago, there is no evidence to suggest the basic principles of biology, chemistry, and physics should be different than today.

Figure \(\PageIndex{1}\): Portrait of James Hutton by Sir Henry Raeburn (public domain; Wikimedia Commons).

The beauty of uniformitarianism is that it allows us to use observations of the modern world to understand Earth’s history. We credit James Hutton (1726-1797) with development of the assumption of uniformitarianism. Hutton was a careful observer of the landscapes around him and he paid close attention to the slow rates at which rocks naturally erode, and the similarly, slow rates at which new layers of sediment are formed. Given such slow rates, what were the implications for interpreting the time it must have taken to form the countless thick layers of rocks exposed at Earth’s surface? Hutton’s observations led him to conclude that geological time must be unimaginably vast. He viewed Earth as constantly changing and renewing itself, a view radically different from the orthodoxy of the day, which commanded that Earth had been created a few thousand years ago just as we see it today.

Figure \(\PageIndex{2}\): Portrait of Charles Lyell by Alexander Craig (1840) (public domain; Wikipedia).

While Hutton developed the concept of uniformitarianism, Charles Lyell (1797-1875) made the idea famous in his influential book Principles of Geology, first published in 1830. Based on many observations and examples, he convinced many people that geological processes act slowly and continuously. We now know that this is not strictly true: the intensities and rates of some processes have changed over Earth’s history (for example, the interior of the Earth used to be much hotter than it is today, as it once contained much more heat-generating radioactive material) and Earth’s history has been occasionally punctuated by catastrophic events (such as the asteroid impact that sealed the fate of most dinosaurs 66 million years ago). But, we still accept the uniformitarian view that the chemical and physical laws of nature have nevertheless remained constant over time.

Fundamentally, the work of Hutton and Lyell solidified the uniformitarian view that “the present is the key to the past.” This means that by studying how Earth processes work today, we can also make sense of how Earth operated in the past, as well as predict its future. Because of this, uniformitarianism is the single most important and fundamental idea in all of geology.

Principles of Relative Dating

It may surprise you to learn that geologists were able to determine much of the history of the Earth and its life without knowing anything about the actual ages of the rocks that they studied. Through use of numerical (absolute) dating techniques (which were developed during the 20th century; discussed later), they were able to later assign dates in years before present to important events in Earth’s history. But, before that, they relied upon a different approach to first determine the sequence of important events in Earth’s past: relative dating.

Figure \(\PageIndex{3}\): Grand Canyon, Arizona. (CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0); By Niagara66, https://commons.wikimedia.org/wiki/File:Grand_Canyon_National_Park_06.jpg.)

The principles of relative dating are largely applied to unraveling the geologic history of the thin blanket of sedimentary rock that covers much of continental land surfaces. Most of what we know about the development, diversity, and evolution of complex life is preserved in this thin blanket. The thickness of the sedimentary rock layers, or strata, varies quite a bit. In the northern Canadian Shield areas, which have been stripped clean of sedimentary cover during the past several million years of ice age glaciations, the cover is essentially zero. In locations uninterrupted by recent mountain building or the effects of glaciation, like the Gulf Coast of the United States, this cover can be substantial, up to 20 km thick (~12 miles)! The average cover is about 1800m thick (just over 1 mile). The thickness of the sedimentary rock layers exposed in the Grand Canyon is close to the average at 1850m. This extent of visual exposure in the canyon is not found elsewhere in the U.S. which is what makes it so spectacular.

Figure \(\PageIndex{4}\): Sedimentary rock layers exposed at Quebrada de Cafayate, Salta (Argentina). (CC BY-SA 2.0, https://commons.wikimedia.org/w/inde...curid=24779211; by travelwayoflife https://www.flickr.com/photos/travelwayoflife/6164348161/in/photostream/)

The following relative dating principles apply to this covering of strata which is largely sedimentary in origin. However, igneous rock may also be included in the layers when it forms horizontally, in layers parallel to the Earth’s surface, as in lava flows, pyroclastic flows and ash falls.

Very simply, relative dating has to do with determining whether one geological event happened before or after a second event. For example:

Did rock layer A form before or after rock layer B?

Did the crinoid from Indiana (in figure \(\PageIndex{5}\) below) live before or after the trilobite from Oklahoma (in figure \(\PageIndex{6}\) below)?

Figure \(\PageIndex{5}\): Fossil Trilobite. Cravat Member, Bois d’Arc formation, Lower Devonian (Held.) age, Clarita, Coal Co., Oklahoma, U.S.A. (CC BY-SA 3.0; Moussa Direct Ltd., https://commons.wikimedia.org/w/inde...?curid=4461171)

Figure \(\PageIndex{6}\): Fossil Crinoid. Agaricocrinus americanus, Carboniferous, Indiana. (CC BY-SA 3.0; https://commons.wikimedia.org/wiki/User:Vassil.)

We can answer these questions with a handful of principles that help guide our thinking when establishing how old one geologic feature is relative to another. Relative dating has to do with determining the ordering of events in Earth’s past. Geologists employ six simple principles in relative dating; the most important of which is the principle of superposition.

Principle of Superposition

Figure \(\PageIndex{7}\): Portrait of Nicholas Steno (public domain; Wikimedia Commons).

Nicholas Steno (1638-1686) was a well traveled Danish anatomist and one of the earliest scientists associated with studying paleontology and geology. He was the first to study relationships between layers of rock and the association of sediment, minerals and fossils to the surrounding and containing rock. He recognized the stories that rock could tell through their history of formation read as clues found within each rock. Steno’s principle of superposition is simple, intuitive, and is the basis for relative dating of geological formations. It states that rocks positioned below other rocks are older than the rocks above. Superposition is observed not only in rocks, but also in our daily lives. Consider the trash in your kitchen garbage can. The trash at the bottom was thrown out earlier than the trash that lies above it; the trash at the bottom is therefore older.

Figure \(\PageIndex{8}\): Taughannock Falls near Trumansburg, New York, illustrating the Principle of Superposition. (This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License; Image by Jonathan R. Hendricks.)

Figure \(\PageIndex{8}\) demonstrates this principle as it shows a sequence of Devonian-aged (~380 Ma) rocks exposed at the waterfall at Taughannock Falls State Park in central New York. The rocks near the bottom of the waterfall were deposited first and the rocks above are subsequently younger and younger.

The photograph below was captured at Volcano National Park on the Big Island of Hawaii. Use superposition to determine which is older: the road or the lava flow? How do you know?

Figure \(\PageIndex{9}\): Volcano National Park, Big Island of Hawaii. (This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License; Image by Jonathan R. Hendricks.)

Principle of Faunal Succession

Figure \(\PageIndex{10}\): Sequence and correlation of fossils from different locations. (CC BY-SA; https://commons.wikimedia.org/wiki/File:Fossils.png by: Kurt Rosenkrantz)

William Smith (1769 – 1839) worked as a surveyor in the coal-mining and canal-building industries in southwestern England in the late 1700s and early 1800s. While doing his work, he had many opportunities to look at the Paleozoic and Mesozoic sedimentary rocks of the region, and he did so in a way that few had done before. Smith noticed the textural similarities and differences between rocks in different locations, and more importantly, he showed that fossils could be used to correlate rocks of the same age. Smith is credited with formulating a fundamental concept and basis for the development of the geologic time scale – the principle of faunal succession. Smith found the same ordering of fossil species from place to place; Fossil A was always found below Fossil B, which in turn was always found below Fossil C, and so on. By documenting these sequences of fossils, Smith was able to correlate rock layers (strata) from place to place. He established that rock layers in two different places were the same age based upon the fact that they include the same distinctive types of fossils.

Principle of Original Horizontality

The principle of original horizontality states that sediments are deposited by gravity in layers parallel to the Earth’s surface. This applies also to volcanic igneous rocks that have been formed by lava flows, pyroclastic flows and ashfall deposits. Layers that accumulate along the edges of basins may be slightly sloped in toward the basin. This principle is particularly useful when rock layers have been deformed by the forces of tectonic activity. Students must learn to “unfold the folds” or reconstruct faulted and contorted layers that were originally horizontal to the Earth’s surface to understand their geologic history. Check out the contorted folds in the sedimentary rock layers in the photo below. From the picture it is difficult to put all the rock layers back in their originally horizontal position. A geologist would walk this area, and any adjacent areas of exposed rock layers, to make sketches, take samples and pictures and use their imagination to attempt to conceptually “unravel” the deformed layers to understand the complete story of what happened from the original deposition of sediments to get them to their current appearance.

Figure \(\PageIndex{11}\): Folds exposed in a roadcut along Highway 14 in Palmdale, California, adjacent to San Andreas fault. (With permission by: Garry Hayes and Susan Hayes from: http://geotripperimages.com/Tectonic_Processes/Folds.htm)

Principle of Lateral Continuity

The principle of lateral continuity states that layers of sediment initially extend outward in all directions; in other words, they are laterally continuous. As a result, rocks that are otherwise similar, but are now separated by a valley or other erosional feature (for instance, Grand Canyon), can be assumed to be originally continuous.

Layers of sediment do not extend indefinitely or infinitely. Instead, limits are controlled by the amount and type of sediment available, and the size and shape of the sedimentary basin. As sediment is transported to an area, thickest deposits are found closest to the source, and gradually thin as the distance increases.

In the panoramic photo below, the originally laterally continuous layers of sedimentary rock have been cut into by the Colorado River at Dead Horse Point State Park in southern Utah.

Figure \(\PageIndex{12}\): By Jean-Christophe BENOIST (Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=10152833)

Principle of Cross-Cutting Relationships

The principle of cross-cutting relationships states that any geological feature that cuts across, or disrupts, another feature must be younger than the feature that is disrupted. In the photo below, a fault disrupts the once continuous layers of sedimentary rock . The dark red “marker bed” can be used to see the displacement that has occurred on either side of the fault (e.g. the block on the right dropped down). The fault “cross-cuts” the layers of sedimentary rock. This indicates that the sedimentary layers pre-date the fault; the fault is the younger feature.

Figure \(\PageIndex{13}\): Normal fault offsetting layers in the Noonday Dolomite, Mosaic Canyon, Death Valley, California. (With permission by: Garry Hayes and Susan Hayes from: http://geotripperimages.com/images/DSC01776%20Normal%20fault%20in%20Mosaic%20Canyon.jpg)

Figure \(\PageIndex{14}\): Above: A thin basaltic dike cross-cutting volcanic sediments of Paleogene age at Lagganulva, on the Isle of Mull, Scotland. (By permission, John Faithful, University of Glasgow, Scotland.) Below: Feeder dike intruding tephra with basaltic lava flow above, Iceland. (By: Marli Miller from https://geologypics.com/geological_entries/gallery. For educational purposes.)

Another example of a cross cutting relationship includes an igneous intrusive feature known as a dike. In Figure \(\PageIndex{14}\), hot magma forced its way into a fracture, or weakness, in pre-existing rock of the crust. The hot magma also “baked” the pre-existing rock through the process of contact metamorphism. These two clues from this exposure help us understand that the dike is younger than the rock it cross-cuts.

A final possibility occurs when a new pattern, such as tectonic cleavage or folding overprints a pre-existing structure, such as sedimentary bedding or metamorphic foliation. The pattern of folding cuts across (reconfigures) the old pattern. Here is an example from Connecticut, showing both this “overprinting” variety of cross-cutting, as well as a traditional igneous dike:

Figure \(\PageIndex{15}\): What sequence of steps were needed to make this Connecticut outcrop? (Photograph by Gregory Simones; reproduced with permission)

Figure \(\PageIndex{16}\): The sequence of steps needed to make the Connecticut outcrop. (Original photograph by Gregory Simones; reproduced with permission)

Principle of Inclusions

The principle of inclusions states that any rock fragments that are included in another body of rock must be older than the rock in which they are included. The photo below provides a good example. Both are igneous rock. The pink rock is granite while the black rock is basalt. A xenolith (fragment of rock trapped in another type of rock) of the granite is included in the mass of basalt rock. How did this happen? Basalt intruded into the pre-existing pink granite. The injection of mafic magma caused a piece of the solid granite to break off, rotate a bit, and then get locked in place as an “inclusion” within the basalt as the dark magma cooled.

Figure \(\PageIndex{17}\): Included fragment (xenolith) of granite within basalt near Georgeville, Nova Scotia. (CC BY-SA 3.0; By M.C. Rygel)

Another interesting feature is apparent in this photo. The pink xenolith displays a much deeper, more rosey color which is also seen along most of the contact between the granite and basalt. This deeper, rosier color is due to the baking of the granite by the basalt. This is referred to as contact metamorphism and is also a relative dating principle. The rock that has been “baked” must have been in place prior to the intrusion of the hot magma/lava which caused the baking. You can’t cook something that doesn’t yet exist.

The following gigapixel panorama is a fascinating example of an inclusion embedded into pyroclastic ash flow deposits. The photo is from Kilbourne Hole in southeast New Mexico. The black basalt volcanic bomb was ejected during a volcanic eruption and landed in a recent pyroclastic ash flow deposit that had yet to solidify. You can easily see how this volcanic bomb deformed the layers of ash as it landed and became embedded. The lower ash layers obviously had to be there prior to the volcanic bomb landing and embedding. Later ash layers buried the bomb.

The Grand Canyon Example

The Grand Canyon of the Colorado River in the U.S. state of Arizona illustrates many of the stratigraphic principles discussed thus far. The photo below shows layers of rock on top of one another in order, from the oldest at the bottom to the youngest at the top, an illustration of the principle of superposition. The predominant white layer just below the canyon rim is the Kaibab Limestone. This layer is laterally continuous with the upper white layers of strata in the far distance, even though the intervening canyon separates them. The rock layers exhibit the principle of lateral continuity, as they are found on both sides of the Grand Canyon, ten miles apart.

Figure \(\PageIndex{18}\): Many people consider Grand Canyon National Park the world’s premiere geologic landscape and a “geologic wonder”. The eroded cliffs reveal 1.7 billion years of fossils, volcanic activity, and geologic history. (United States Public Domain; National Park Service from: https://www.nps.gov/articles/age-of-rocks-in-grand-canyon.htm.)

Figure \(\PageIndex{19}\): A schematic cross-section of the strata exposed in the Grand Canyon. (United States Public Domain; Callan Bentley after National Park Service from: https://www.nps.gov/articles/age-of-rocks-in-grand-canyon.htm.)

Figure \(\PageIndex{19}\) shows a cross-section of the rocks exposed on the walls of the Grand Canyon, illustrating the principle of cross-cutting relationships, superposition, and original horizontality. In the lower parts of the Grand Canyon are the oldest sedimentary formations, with igneous and metamorphic rocks at the very bottom. (For this reason, these crystalline rocks are sometimes called “the basement.”) The principle of cross-cutting relationships shows the sequence of these events. The metamorphic schist (#1) is the oldest rock formation and the cross-cutting granite intrusion (#3) is younger. As seen in the figure, the other layers on the walls of the Grand Canyon are numbered with #18 being the youngest, of all of the rocks exposed in the canyon, #18 was the last to form. This illustrates the principle of superposition. The Colorado River carves through the Colorado Plateau, exposing the horizontal strata, that follows the principle of original horizontality. These rock strata have been barely disturbed from their original deposition, except by a broad regional uplift.

Key Terms

• principle of cross-cutting relationships – a geologic principle stating that a rock or geologic feature (such as a fault or intrusion) that cuts across another rock body must be younger than the rock it cuts through
• principle of faunal succession – the concept that fossils succeed one another in a definite, recognizable order, allowing rock layers to be dated and correlated by the fossils they contain
• principle of inclusions – states that any rock fragments (inclusions) contained within another rock must be older than the rock in which they are enclosed.
• principle of lateral continuity – sedimentary layers originally extend in all directions until they thin out or are interrupted by a barrier, meaning that separated layers on opposite sides of a valley were once continuous
• principle of original horizontality – states that layers of sediment are originally deposited in horizontal or nearly horizontal layers so tilting must have occurred after deposition
• principle of superposition – in an undisturbed sequence of sedimentary rocks, the oldest layers are at the bottom and the youngest layers are at the top
• relative dating – a method of determining the chronological order of past events by comparing rock layers and fossils, without determining their exact numerical age
• uniformitarianism – the principle that the geologic processes we see shaping the Earth today (such as erosion, sedimentation, and volcanism) have operated in the same way throughout Earth’s history
• xenolith – a fragment of rock enclosed within another, usually a piece of country rock trapped within an igneous intrusion such that the fragment is older than what it is trapped in