Law of superposition - Wikipedia
The law of superposition is that the youngest rock is always on top and the oldest rock is always on the bottom. so the relative ages came be. The Law of Superposition. In any series of undisturbed sedimentary rocks, THE OLDEST ROCKS ARE ON BOTTOM AND THE YOUNGEST ROCKS ARE ON. The law of superposition is an axiom that forms one of the bases of the sciences of geology, archaeology, and other fields dealing with geological stratigraphy. It is a form of relative dating.
Geology[ edit ] The regular order of the occurrence of fossils in rock layers was discovered around by William Smith. While digging the Somerset Coal Canal in southwest England, he found that fossils were always in the same order in the rock layers.
As he continued his job as a surveyorhe found the same patterns across England. He also found that certain animals were in only certain layers and that they were in the same layers all across England. Due to that discovery, Smith was able to recognize the order that the rocks were formed. Sixteen years after his discovery, he published a geological map of England showing the rocks of different geologic time eras.
Principles of relative dating[ edit ] Methods for relative dating were developed when geology first emerged as a natural science in the 18th century. Geologists still use the following principles today as a means to provide information about geologic history and the timing of geologic events. Uniformitarianism[ edit ] The principle of Uniformitarianism states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time.
In geology, when an igneous intrusion cuts across a formation of sedimentary rockit can be determined that the igneous intrusion is younger than the sedimentary rock. There are a number of different types of intrusions, including stocks, laccolithsbatholithssills and dikes. Cross-cutting relationships[ edit ] Cross-cutting relations can be used to determine the relative ages of rock strata and other geological structures.
The principle of cross-cutting relationships pertains to the formation of faults and the age of the sequences through which they cut. Faults are younger than the rocks they cut; accordingly, if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault. Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault.
For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer.
Law of superposition
A similar situation with igneous rocks occurs when xenoliths are found. 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. Original horizontality[ edit ] The principle of original horizontality states that the deposition of sediments occurs as essentially horizontal beds. Observation of modern marine and non-marine sediments in a wide variety of environments supports this generalization although cross-bedding is inclined, the overall orientation of cross-bedded units is horizontal. This is because it is not possible for a younger layer to slip beneath a layer previously deposited.
This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed. As organisms exist at the same time period throughout the world, their presence or sometimes absence may be used to provide a relative age of the formations in which they are found. Based on principles laid out by William Smith almost a hundred years before the publication of Charles Darwin 's theory of evolutionthe principles of succession were developed independently of evolutionary thought.
The principle becomes quite complex, however, given the uncertainties of fossilization, the localization of fossil types due to lateral changes in habitat facies change in sedimentary strataand that not all fossils may be found globally at the same time. As a result, rocks that are otherwise similar, but are now separated by a valley or other erosional feature, can be assumed to be originally continuous.
Layers of sediment do not extend indefinitely; rather, the limits can be recognized and are controlled by the amount and type of sediment available and the size and shape of the sedimentary basin. Sediment will continue to be transported to an area and it will eventually be deposited. However, the layer of that material will become thinner as the amount of material lessens away from the source.
What is the law of superposition and how can it be used to relatively date rocks?
During a certain period of time, while layers of sediment were being deposited elsewhere, no layers were deposited at the location in question. Or Layers were deposited at the location in question, but were subsequently removed by erosion. 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? 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.Laws of Relative Rock Dating
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.
- Relative dating Law of superposition Law of horizontality Original
- Relative dating
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.
Relative dating Law of superposition Law of horizontality Original
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'.