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. 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. 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.
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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? 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. And, unconformities show a discontinuity in the strata, which can only be understood by following the principles of stratigraphy. Geologists utilize all of these laws and principles to establish the relative ages of rocks and the relationships between events that occurred throughout geologic time.
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Explore over 4, video courses. Find a degree that fits your goals. What is Relative Dating? Learn how inclusions and unconformities can tell us stories about the geologic past. We'll even visit the Grand Canyon to solve the mystery of the Great Unconformity! Try it risk-free for 30 days. An error occurred trying to load this video. Try refreshing the page, or contact customer support. Register to view this lesson Are you a student or a teacher?
Chapter 5-Intepreting Earths History
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The Grand Canyon and Relative Dating
Add to Add to Add to. Want to watch this again later? Relative Dating with Fossils: Index Fossils as Indicators of Time. Numerical and Relative Geological Dating. Methods of Geological Dating: Numerical and Relative Dating. What is Relative Age? How to Interpret Events from Natural Phenomena. This follows due to the fact that sedimentary rock is produced from the gradual accumulation of sediment on the surface.
Therefore newer sediment is continually deposited on top of previously deposited or older sediment. In other words, as sediment fills a depositional basins we would expect the upper most surface of the sediment to be parallel to the horizon. Subsequent layers would follow the same pattern.
As sediment weathers and erodes from its source, and as long as it is does not encounter any physical barriers to its movement, the sediment will be deposited in all directions until it thins or fades into a different sediment type. For purposes of relative dating this principle is used to identify faults and erosional features within the rock record. The principle of cross-cutting states that any geologic feature that crosses other layers or rock must be younger then the material it cuts across. Using this principle any fault or igneous intrusion must be younger than all material it or layers it crosses.
Nevertheless, they can provide an abundance of useful information. 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.
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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 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. As a result, xenoliths are older than the rock which contains them Relative dating is used to determine the order of events on Solar System objects other than Earth; for decades, planetary scientists have used it to decipher the development of bodies in the Solar System , particularly in the vast majority of cases for which we have no surface samples.
Many of the same principles are applied. For example, if a valley is formed inside an impact crater , the valley must be younger than the crater. Craters are very useful in relative dating; as a general rule, the younger a planetary surface is, the fewer craters it has. If long-term cratering rates are known to enough precision, crude absolute dates can be applied based on craters alone; however, cratering rates outside the Earth-Moon system are poorly known.
Relative dating methods in archaeology are similar to some of those applied in geology.