Geology Online Subchapter
Superposition of rock units is a very simple and straightforward method of relative The principle states that in a sequence of undeformed sedimentary rocks the. Using relative dating principles and the position of layers within rock, it is possible This principle states that in a sequence of undisturbed sedimentary layers or. Relative dating is used to arrange geological events, and the rocks they leave behind, in a sequence. This is the principle of 'superposition'.
At location C, layers 1 through 5 were deposited and remained intact. The rock record is complete. At location A, layers 1 and 2 were deposited. However, during times 3 and 4, no layers were deposited. During time 5, deposition resumed, and layer 5 was deposited. At location B, layers 1 through 3 were deposited.
During time 4, all of layer 3 plus the upper part of layer 2 were removed by erosion. During time 5, deposition resumed, with layer 5 being deposited on top of what remained of layer 2. Unconformities caused by erosion are commonly represented diagrammatically by an irregular or jagged line, such as is seen between layers 2 and 5 at location B.
If the layers are indeed sedimentary or volcanic, then the assumption that the layers formed one after the other, from bottom to top, is justified. But if the layers are made of metamorphic or intrusive igneous rocks, then the age relationships may be quite different. In metamorphic rocks, layering may develop in response to application of pressure. In that case, the layers may all form at the same time. The position of a layer within the series, above or below another layer, will not be indicative of whether it is younger or older.
For the rocks in cross-section A, the order of events, from oldest to youngest was: Note that the sill is younger than both the layers above and beneath it. Lava flows and sills strongly resemble each other: If sills and lava flows are wrongly identified, age relationships will be wrongly interpreted.
In cross-section C, layer 30 had not yet been deposited when the sill was emplaced. Only after the sill was emplaced was layer 30 deposited cross-section D.
An important question, therefore, is how may cross-section C in which the sill is younger than layer 30 be distinguished from cross-section D in which the sill is older than layer 30?
Finding an answer to that question will be discussed in subsequent sections. How may a lava flow be distinguished from a sill?
In cross-section B, if the sill was misidentified as a lava flow, what would its relative age be compared to layers 28 and 29? If it was identified correctly, what would its relative age be compared to layers 28 and 29? In cross-section B, if lava flow B was misidentified as a sill, what would its relative age be compared to layer 30?
If it was identified correctly, what would its relative age be compared to layers 30?
The Principle of Superposition
My answer to Question 1: My answer to Question 2: My answer to Question 3: This observation is expressed as the Law of Original Horizontality.
There are exceptions to the law for example, layers deposited on a steeply inclined surfacebut they are relatively few and will not be considered. At location A, three layers are present. They have not been deformed and remain as originally deposited. The layers are covered except for the area within the circle. Looking at the exposed layers and applying the Law of Superposition, an observer concludes correctly that the bottommost layer dark brown is oldest and the topmost layer orange-tan is youngest.
At location B, the layers are slightly folded. A second observer, who has not been to location A, sees slightly inclined layers and concludes correctly that the layers have been somewhat deformed, but that the topmost layer is the youngest and the bottommost the oldest.
At location C, the layers have been tightly folded. In the exposed circled area, the layers are vertical. A third observer, who has not been to locations A or B, sees the vertical layers and cannot decide which layer was originally 'topmost' and which 'bottommost' and draws no conclusion about their relative ages.
At location D the layers have undergone extreme deformation. The layers within the circled area have actually been inverted. What now appears to be the 'topmost' layer was originally the 'bottommost' compare with the order of the layers in Diagram A.
A fourth observer, who has not been to locations A, B or C, sees the almost horizontal layers and assumes incorrectly that the layers have not been significantly deformed. Applying the Law of Superposition to determine the relative ages of the layers, the observer gets the relative ages of the layers reversed.
Fortunately, many depositional layers both sedimentary layers and lava flows contain features that indicate original orientation. There are hundreds of such features called primary structures. Here are some examples of primary structures: The points of the ripples point upward. The crater basins are convex down; the crater rims point up. The branches of tree roots point downward. Another primary structure that may be used to determine 'tops' and 'bottoms' of layers is the tilt or lack of tilt of the layers.
If the layers are horizontal and traceable over considerable distances, the geologist will conclude unless evidence to the contrary turns up that there is a very high probability that the layers are right-side-up.
Justification for this conclusion is that where obviously deformed rock layers can be observed, the places where complete overturning has been achieved are quite local. This not surprising since it is harder takes more energy for lengthy portions of layers to be 'turned over' than for local portions. Diagram A illustrates an extensive outcrop of horizontal layers exposed over a great distance. The layers have a high probability of being 'right-side-up'.
Diagram B illustrates several separated local outcrops in which horizontal layers are exposed. The layers in the separate outcrops 'line up' with one another. The geologist assumes dashed lines that if the grass and soil were removed, the layers would be continuous over the whole area. Diagram C illustrates a single local outcrop of horizontal layers. Because completely inverted layers are rare layers turned right over to become horizontal againthe geologist assumes, in the absence of contrary evidence, that the layers are probably 'right-side-up'.
That is, the geologist infers that graded bedding, ripple marks, vesicles, etc. Sedimentary rocks frequently contain objects that have been interpreted as evidence that life existed at the time the sediment accumulated. These 'objects in rocks' are exceedingly diverse, including many whose shapes resemble organisms alive today.
Shells and bones or their imprints, or impressions such as tracks or burrows are amongst the most common objects. Others are quite different from any life form that exists today, but seem to have an organization or shape that seems somehow suggestive of life. These life-related objects in rocks have come to be called fossils.
The modern interpretation of fossils is that they actually are remains or artifacts of once living organisms.
Normally, after living organisms die, their remains are quickly scattered and decayed and the record of their existence is rapidly obliterated.
On rare occasions, quick burial of the remains by mud, sand or volcanic ash prevents their destruction and they become preserved as the loose material in which they are embedded is lithified. The preservation of soft parts of organisms is extremely rare. Preserved hard parts are commonly mineralized turned into rocky substances. By the early 19th century, through observation of fossils in rocks, it was accepted that through time, the nature of life on Earth has changed.
That is, individual species appear in the rock record, exist for a certain period of time, and then disappear forever from the rock record.
Consider the diagram opposite. A sequence of rock layers numbered 52 to 63 exposed at location 'X'. One of the rock layers, 55exhibits graded bedding, indicating the layers are 'right-side-up'. Hence, layer 52 is oldest, layer 63 is youngest.
Each layer formed during a certain period of time and represents what happened at location 'X' during that time. A series of colored dots that represent the levels within the rocks where specimens of fossil species A, B, C, and D have been found at location 'X'.
Each level represents a moment in time. A series of colored double-headed arrows indicating the range of time spanned between the lowest and highest levels of the occurrence of each fossil species at location 'X'.
A series of black double-headed arrows indicating the range of time spanned between the lowest and highest levels of the occurrence of each fossil species found in rocks throughout the world.
It may be seen that the ranges of the different fossils species overlap, so that in some layers, more than one fossil species may occur. That is not surprising since more than one type of organism lives at the same time.
Different fossils species that occur together constitute a fossil assemblage. The time interval between the first and last appearance anywhere in the world of a fossil species is known as its 'geologic range'. With continued investigation, The geologic ranges of individual species are subject to revision as investigation of rocks continues. Newly discovered occurrences may place the introduction and extinction of species respectively earlier and later in time.
How do you study it? How can you make any conclusions about rock layers that make such a crazy arrangement?Law of Superposition
Geologists establish the age of rocks in two ways: Numerical dating determines the actual ages of rocks through the study of radioactive decay. Relative dating cannot establish absolute age, but it can establish whether one rock is older or younger than another. Relative dating requires an extensive knowledge of stratigraphic succession, a fancy term for the way rock strata are built up and changed by geologic processes.
In this lesson, we'll learn a few basic principles of stratigraphic succession and see whether we can find relative dates for those strange strata we found in the Grand Canyon. Original Horizontality In order to establish relative dates, geologists must make an initial assumption about the way rock strata are formed. 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. Law of Superposition 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.