5 geologic principles for relative dating

It's called the Principle of Original Horizontality , and it just means what it sounds like: Of course, it only applies to sedimentary rocks. Recall that sedimentary rock is composed of As you can imagine, regular sediments, like sand, silt, and clay, tend to accumulate over a wide area with a generally consistent thickness. It sounds like common sense to you and me, but geologists have to define the Principle of Original Horizontality in order to make assumptions about the relative ages of sedimentary rocks.

Once we assume that all rock layers were originally horizontal, we can make another assumption: This rule is called the Law of Superposition. Again, it's pretty obvious if you think about it. Say you have a layer of mud accumulating at the bottom of a lake. Then the lake dries up, and a forest grows in. More sediment accumulates from the leaf litter and waste of the forest, until you have a second layer. The forest layer is younger than the mud layer, right? And, the mud layer is older than the forest layer.

When scientists look at sedimentary rock strata, they essentially see a timeline stretching backwards through history. The highest layers tell them what happened more recently, and the lowest layers tell them what happened longer ago. How do we use the Law of Superposition to establish relative dates? Let's look at these rock strata here:. We have five layers total. Let's say we find out, through numerical dating, that the rock layer shown above is 70 million years old. We're not so sure about the next layer down, but the one below it is million years old.

Can we tell how old this middle layer is? Not exactly, but we do know that it's somewhere between 70 and million years old. Geologists use this type of method all the time to establish relative ages of rocks. What could a geologist say about that section of rock? Following the Principle of Original Horizontality, he could say that whatever forces caused the deformation, like an earthquake, must have occurred after the formation of all the rock strata.

Geologic Principles & Relative Dating

Since we assume all the layers were originally horizontal, then anything that made them not horizontal had to have happened after the fact. We follow this same idea, with a few variations, when we talk about cross-cutting relationships in rock. Let's say, in this set of rock strata, that we found a single intrusion of igneous rock punching through the sedimentary layers. We could assume that this igneous intrusion must have happened after the formation of the strata.

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If it had happened before the layers had formed, then we wouldn't see it punching through all the layers; we would only see it going through the layers that had existed at the time that it happened. The newer layers would have formed a cap overtop. The Principle of Cross-Cutting Relationships states that rock formations that cut across other rocks must be younger than the rocks that they cut across.

The same idea applies to fault lines that slide rock layers apart from each other; a fault that cuts across a set of strata must have occurred after the formation of that set. Geologists find the cross-cutting principle especially useful for establishing the relative ages of faults and igneous intrusions in sedimentary rocks.


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Sometimes, geologists find strange things inside the strata, like chunks of metamorphic or igneous rock. These items are called inclusions - foreign bodies of rock or mineral enclosed within another rock.

Because the sedimentary rock had to have formed around the object for it to be encased within the layers, geologists can establish relative dates between the inclusions and the surrounding rock. Inclusions are always older than the sedimentary rock within which they are found. Other times, geologists discover patterns in rock layers that give them confusing information.

There may be a layer missing in the strata, or a set of sedimentary rock on top of metamorphic rock. These interfaces between discontinuous layers of rock are called unconformities. They complicate the task of relative dating, because they don't give an accurate picture of what happened in geologic history.


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  4. For example, say we have a layer missing from the rock strata. That layer may have eroded away before the next layer was built upon the exposed surface. So, we'll never know what type of rock used to be there or what fossils it may have held. One famous example of an unconformity is the Great Unconformity of the Grand Canyon. It clearly shows the interface between two types of rock: The sandstones lie horizontally, just as they did when they were originally laid down. But, the shales are all deformed and folded up.

    The tops of their folds are completely gone where the sandstones have replaced them. What can we make of this giant unconformity? Can we establish any relative ages between the rock strata or the cause of their formations?

    Relative dating

    Well, following the Principle of Cross-Cutting Relationships, we can tell that whatever deformed the shales - probably an earthquake - must have occurred before any of the upper sandstones were deposited. In fact, we can put together a timeline. The shales were deposited first, in a horizontal position, and then there was an earthquake that made them all fold up. Then, the tops were eroded off until the rock was basically flat, and then the sandstones were deposited on top of everything else.

    With only a few geologic principles, we've established the relative dates of all the phenomena we see in the Great Unconformity. Geologists establish the relative ages of rocks mostly through their understanding of stratigraphic succession. The Principle of Original Horizontality states that all rock layers were originally horizontal. The Law of Superposition states that younger strata lie on top of older strata. The Principle of Cross-Cutting Relationships states that intrusions and faults that cut across rock are necessarily younger than that rock.

    Inclusions , or foreign bodies, found inside rock are necessarily older than that rock. Using microscopic observations and a range of chemical microanalysis techniques geochemists and igneous petrologists can obtain a range of useful information from melt inclusions. Two of the most common uses of melt inclusions are to study the compositions of magmas present early in the history of specific magma systems.

    This is because inclusions can act like "fossils" — trapping and preserving these early melts before they are modified by later igneous processes. In addition, because they are trapped at high pressures many melt inclusions also provide important information about the contents of volatile elements such as H 2 O, CO 2 , S and Cl that drive explosive volcanic eruptions. Sorby was the first to document microscopic melt inclusions in crystals. The study of melt inclusions has been driven more recently by the development of sophisticated chemical analysis techniques.

    Scientists from the former Soviet Union lead the study of melt inclusions in the decades after World War II Sobolev and Kostyuk, , and developed methods for heating melt inclusions under a microscope, so changes could be directly observed. Although they are small, melt inclusions may contain a number of different constituents, including glass which represents magma that has been quenched by rapid cooling , small crystals and a separate vapour-rich bubble. They occur in most of the crystals found in igneous rocks and are common in the minerals quartz , feldspar , olivine and pyroxene.

    The Grand Canyon and Relative Dating

    The formation of melt inclusions appears to be a normal part of the crystallization of minerals within magmas, and they can be found in both volcanic and plutonic rocks. The law of included fragments is a method of relative dating in geology. Essentially, this law states that clasts in a rock are older than the rock itself. Another example is a derived fossil , which is a fossil that has been eroded from an older bed and redeposited into a younger one. This is a restatement of Charles Lyell 's original principle of inclusions and components from his to multi-volume Principles of Geology , which states that, with sedimentary rocks , if inclusions or clasts are found in a formation , then the inclusions must be older than the formation that contains them.

    These foreign bodies are picked up as magma or lava flows , and are incorporated, later to cool in the matrix.