Mr. Joanis’ Science Class! ~ ESES ~ WEEK 8

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Principles of Relative Dating The 2nd Half of Geochronology

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Imagine slicing into the crust of the Earth, and sliding that slice out to see what it looks like. Would there be more than one type of rock in that slice? Would that slice of Earth contain many different rocks? How would they be arranged? That slice of Earth might look a little like the diagram to the right: Many layers of different types of rocks, with “splashes” of other rocks embedded throughout. Why does Earth look like this? Why are there layers of different kinds of rock, how did those layers get there, why are some straight while others are crooked, and when did they all get there? Here, we’ll learn how to analyze cross-sections of Earth to determine the answers to questions like those.

Previously, we learned about how to use radiologic dating to determine the ages of certain materials, based on the amounts of radioactive isotopes they contain. Radiologic dating is a method of absolute dating: Determining the precise age of an object. Through absolute dating, you can determine the actual dates that certain rocks formed. This is one of the tools that geologists can use to put together a timeline of geological events (the creation of rocks and other geological formations in the planet’s crust). Relative dating is another useful tool for putting together these types of timelines. Relative dating involves determining the relative order of past events or the relative order of the creation of geological features. Here, the word “relative” means “compared to other things.” Relative dating processes can’t tell you exactly when certain events happened, and it can’t tell you exactly when certain geological features formed; instead, relative dating can only tell you the order in which those geological features and events formed or took place. Relative dating can’t tell us how much time passed between any two events, just which event happened last. Together, the science of using both relative dating and absolute dating techniques to construct timelines of geological events and rock formations is called geochronology (“geo” = earth, “chrono” = time, “logy” = science).

Relative to the other apples, the middle-right apple looks quite old.

There is one guiding principle that geological relative dating is based upon, and six key principles that explain how one can determine the relative ages of geological events and rock formations.

Guiding Principle – Uniformitarianism

Uniformitarianism is the idea that the geological processes that are happening today are the same processes that have shaped Earth over time. Essentially, it is the assumption that the things we can currently observe worked the same way before we were around to observe them. Without making this assumption about the Earth, we wouldn’t be able to draw any conclusions about how the layers of Earth formed or how certain rock formations came about being. We must assume that all of Earth’s processes are the same and have the same effects, or we can not put together any timelines. If we reject the idea of uniformitarianism, then the only things we can use as evidence to make geological claims are the processes that we see happening before our very eyes. In other sciences like chemistry, that might be fine; however, geological processes occur very, very slowly over the course of thousands if not millions of years. Humans haven’t been around long enough to personally witness the formation of all of the planet’s crustal features. Most of what we know about geology is based on observations and our acceptance of the principle of uniformitarianism.

Principle No. 1 – Original Horizontality

Original horizontality is the idea that the deposition of sediments occurs horizontally (flat). In other words, layers in the Earth’s crust are formed spread out from left-to-right. Sedimentary rocks form when sediments (small particles, like grains of sand) settle out and compact into rock. As sediments settle on the Earth’s surface, they spread horizontally. We can observe this “horizontal spread” occur in real-time by observing how sediments deposit in riverbeds – river currents will carry “piles” of sediment and over time will spread that sediment evenly. Wind can also carry sediments, spreading them horizontally over non-marine environments. Even looking at how sand flows down an upturned hourglass shows us how sediment “flattens out” naturally. Instead of piling up into a single column, the sand that flows out of the top of an hourglass forms a mostly flat surface with a light “hill.”

In summation: All sedimentary rock forms initially as horizontal layers – always!

Principle No. 2 – Superposition

Superposition is the idea that any given rock layer in an undisturbed sequence of sedimentary rock layers is younger that the one beneath it, and older than the one above it. In other words, the deeper down you “dig” from the surface, the older the rock layers you’ll see. There’s an important exception in the expanded definition: Those rock layers must be undisturbed for this to be true. There are many events that can “disturb” a rock layer, like an earthquake or other seismic event. Many of the principles that follow this one deal with what those “disrutbances” do, and how they end up effecting the way we determine the ages of certain rock layers.

Something very important to remember is that all of these principles, including the guiding principle of uniformitarianism, must be taken together in order to fully understand and engage in the science of geochronology. If we consider both principles 1 and 2 at the same time – which we absolutely should – we can make some simple and correct statements about the ages of the rocks in the diagram to the right. All of the layers of earth there are sedimentary rock, and they originally deposited horizontally (with a slight curvature because of the unevenness of the layer each successive layer formed upon). Because there aren’t any disturbances in these rock layers (there aren’t any fractures, intrusions, or large areas of erosion), we can confidently make the statement that the green layer is the youngest, the pink layer is the second-youngest, and the yellow layer is the oldest.

Principle No. 3 – Cross Cutting

This principle is also sometimes referred to as “faulting,” because cross-cuts in the earth also go by the name “faults.” The principle of cross cutting states that any faults in rock layers must be younger than the rocks that they cut. In other words, any faults (or “cuts” through layers of rock), must have happened after the layers that they cut through were formed. This makes sense, from a logical point of view. You cannot cut something that doesn’t exist, so when seismic events like earthquakes create faults in rock, it can only effect rocks that are already there. Following the logic of all of the rules that we understand so far, we can confidently say a few things about the diagram up above: First, we know that the limestone is the oldest layer of rock in this sequence. Then, the siltstone is the second-oldest. The fault must be younger than both the siltstone and the limestone because it cuts through both of those layers, meaning that the siltstone and the limestone must have existed before the fault was created. The basalt is not cut by the fault, which means that the basalt must have formed after whatever earthquake or event created the fault. Given our reasoning, we can reasonably and correctly say that the limestone formed first, the siltstone formed second, the fault formed third, and the basalt formed fourth.

We also came across the term “uplift” in our readings before this worksheet. The fault in the diagram above has uplifted a portion of the limestone and siltstone here. As you can see, the limestone reaches higher-up on the right-hand side of the fault. Earthquakes and other seismic events that create faults will uplift the rocks on one side of the fault. The siltstone to the right of the fault is “cut short” likely because of erosion that evened-out the surface of the siltstone layer after the fault occurred.

Principle No. 4 – Intrusions

Intrusions are any deposits of igneous rock that form within existing layers of rock. When we discussed how igneous rocks form, we learned that they can form underground when magma cools and hardens in cavities or seeps into cracks that exist in existing rock layers.

This principle of geochronology states that any intrusions in rock layers must be younger than the rock layers that they cut through or intrude between. In the example diagram to the right, we can see that an intrusion has formed between existing layers of rock (the orange and beige rocks in this diagram are sedimentary rocks). The red intrusion has formed both a dike (a thin, vertical intrusion) and a sill (a thin, horizontal intrusion that pushes in between two existing layers of sedimentary rock). The red “layer” of igneous rock here is the youngest of all the rocks in this diagram. Below is another example of an intrusion in existing layers of sedimentary rock.

Principle No. 5 – Lateral Continuity

The principle of lateral continuity implies that deposited sedimentary layers extend laterally in all directions. The consequence of this is that rock layers separated by a valley or other feature caused by erosion can be assumed to have been originally continuous, meaning that we can assume such layers were originally part of a single layer and are therefore the same age. The Grand Canyon is a great place to see evidence of lateral continuity. On either side of the canyon, you can see stripes of rock that are the same color at the same heights. The Grand Canyon was created by flowing water the gradually eroded parts of the rock layers away, cutting the original, continuous layers in two. The principle of lateral continuity can be applied to all canyons. In the diagram below, we can see that there are two gray layers of stone separated by a canyon. Even though these gray stone layers don’t touch (they are not continuous), we can assume that they were continuous at some point. When we make this assumption, we can say that the left half of the grey rock layer and the right half of the grey rock layer are the same age.

Principle No. 6 – Included Fragments

The final principle, the principle of included fragments, states that chunks of rock embedded within layer of sedimentary rock must be older than the surrounding rock. These chunks of included rock (sometimes called clasts or fragments) can become “trapped” in other layers of sedimentary or rock as sediments build up. The diagram below breaks down how you can use this principle to determine the relative ages of rock fragments within close-by layers. Although the granite and the sandstone rocks both exist at the same elevation, we can tell that the granite is younger than the sandstone. We know this because of the existence of a sandstone fragment embedded within the granite. The schist is the oldest of all three of these layers, as there are included fragments of schist within the sandstone. The existence of a schist fragment within the sandstone fragment embedded in granite further proves the fact that the schist layer is the oldest of the three.

Answer the following questions, based on the knowledge and examples provided in this packet: