Relative geologic dating
Relative dating is used to arrange geological events, and the rocks they leave behind, in a sequence. The method of reading the order is called stratigraphy layers of rock are called strata. Relative dating does not provide actual numerical dates for the rocks. Next time you find a cliff or road cutting with lots of rock strata, try working out the age order using some simple principles:. Fossils are important for working out the relative ages of sedimentary rocks. Throughout the history of life, different organisms have appeared, flourished and become extinct.
Despite seeming like a relatively stable place, the Earth's surface has changed dramatically over the past 4. Mountains have been built and eroded, continents and oceans have moved great distances, and the Earth has fluctuated from being extremely cold and almost completely covered with ice to being very warm and ice-free. These changes typically occur so slowly that they are barely detectable over the span of a human life, yet even at this instant, the Earth's surface is moving and changing.
As these changes have occurred, organisms have evolved, and remnants of some have been preserved as fossils. A fossil can be studied to determine what kind of organism it represents, how the organism lived, and how it was preserved. However, by itself a fossil has little meaning unless it is placed within some context. The age of the fossil must be determined so it can be compared to other fossil species from the same time period. Understanding the ages of related fossil species helps scientists piece together the evolutionary history of a group of organisms.
For example, based on the primate fossil record, scientists know that living primates evolved from fossil primates and that this evolutionary history took tens of millions of years. By comparing fossils of different primate species, scientists can examine how features changed and how primates evolved through time. However, the age of each fossil primate needs to be determined so that fossils of the same age found in different parts of the world and fossils of different ages can be compared.
There are three general approaches that allow scientists to date geological materials and answer the question: Relative dating puts geologic events in chronological order without requiring that a specific numerical age be assigned to each event. Second, it is possible to determine the numerical age for fossils or earth materials.
Numerical ages estimate the date of a geological event and can sometimes reveal quite precisely when a fossil species existed in time. Third, magnetism in rocks can be used to estimate the age of a fossil site. This method uses the orientation of the Earth's magnetic field, which has changed through time, to determine ages for fossils and rocks. Geologists have established a set of principles that can be applied to sedimentary and volcanic rocks that are exposed at the Earth's surface to determine the relative ages of geological events preserved in the rock record.
For example, in the rocks exposed in the walls of the Grand Canyon Figure 1 there are many horizontal layers, which are called strata. The study of strata is called stratigraphy , and using a few basic principles, it is possible to work out the relative ages of rocks. Figure 1: Just as when they were deposited, the strata are mostly horizontal principle of original horizontality. The layers of rock at the base of the canyon were deposited first, and are thus older than the layers of rock exposed at the top principle of superposition.
All rights reserved. In the Grand Canyon, the layers of strata are nearly horizontal. Most sediment is either laid down horizontally in bodies of water like the oceans, or on land on the margins of streams and rivers. Each time a new layer of sediment is deposited it is laid down horizontally on top of an older layer. This is the principle of original horizontality: Thus, any deformations of strata Figures 2 and 3 must have occurred after the rock was deposited. Figure 2: The principles of stratigraphy help us understand the relative age of rock layers.
Layers of rock are deposited horizontally at the bottom of a lake principle of original horizontality. Younger layers are deposited on top of older layers principle of superposition. Layers that cut across other layers are younger than the layers they cut through principle of cross-cutting relationships. The principle of superposition builds on the principle of original horizontality.
The principle of superposition states that in an undeformed sequence of sedimentary rocks, each layer of rock is older than the one above it and younger than the one below it Figures 1 and 2. Accordingly, the oldest rocks in a sequence are at the bottom and the youngest rocks are at the top. Sometimes sedimentary rocks are disturbed by events, such as fault movements, that cut across layers after the rocks were deposited. This is the principle of cross-cutting relationships.
The principle states that any geologic features that cut across strata must have formed after the rocks they cut through Figures 2 and 3. Figure 3: The sedimentary rock layers exposed in the cliffs at Zumaia, Spain, are now tilted close to vertical. According to the principle of original horizontality, these strata must have been deposited horizontally and then titled vertically after they were deposited. In addition to being tilted horizontally, the layers have been faulted dashed lines on figure.
Applying the principle of cross-cutting relationships, this fault that offsets the layers of rock must have occurred after the strata were deposited. The principles of original horizontality, superposition, and cross-cutting relationships allow events to be ordered at a single location. However, they do not reveal the relative ages of rocks preserved in two different areas. In this case, fossils can be useful tools for understanding the relative ages of rocks.
Each fossil species reflects a unique period of time in Earth's history. The principle of faunal succession states that different fossil species always appear and disappear in the same order, and that once a fossil species goes extinct, it disappears and cannot reappear in younger rocks Figure 4. Figure 4: The principle of faunal succession allows scientists to use the fossils to understand the relative age of rocks and fossils. Fossils occur for a distinct, limited interval of time.
In the figure, that distinct age range for each fossil species is indicated by the grey arrows underlying the picture of each fossil. The position of the lower arrowhead indicates the first occurrence of the fossil and the upper arrowhead indicates its last occurrence — when it went extinct. Using the overlapping age ranges of multiple fossils, it is possible to determine the relative age of the fossil species i. For example, there is a specific interval of time, indicated by the red box, during which both the blue ammonite and orange ammonite co-existed.
If both the blue and orange ammonites are found together, the rock must have been deposited during the time interval indicated by the red box, which represents the time during which both fossil species co-existed. In this figure, the unknown fossil, a red sponge, occurs with five other fossils in fossil assemblage B. Fossil assemblage B includes the index fossils the orange ammonite and the blue ammonite, meaning that assemblage B must have been deposited during the interval of time indicated by the red box.
Because, the unknown fossil, the red sponge, was found with the fossils in fossil assemblage B it also must have existed during the interval of time indicated by the red box. Fossil species that are used to distinguish one layer from another are called index fossils. Index fossils occur for a limited interval of time. Usually index fossils are fossil organisms that are common, easily identified, and found across a large area.
Because they are often rare, primate fossils are not usually good index fossils. Organisms like pigs and rodents are more typically used because they are more common, widely distributed, and evolve relatively rapidly. Using the principle of faunal succession, if an unidentified fossil is found in the same rock layer as an index fossil, the two species must have existed during the same period of time Figure 4. If the same index fossil is found in different areas, the strata in each area were likely deposited at the same time.
Thus, the principle of faunal succession makes it possible to determine the relative age of unknown fossils and correlate fossil sites across large discontinuous areas. All elements contain protons and neutrons , located in the atomic nucleus , and electrons that orbit around the nucleus Figure 5a. In each element, the number of protons is constant while the number of neutrons and electrons can vary.
Atoms of the same element but with different number of neutrons are called isotopes of that element. Each isotope is identified by its atomic mass , which is the number of protons plus neutrons. For example, the element carbon has six protons, but can have six, seven, or eight neutrons. Thus, carbon has three isotopes: Figure 5: Radioactive isotopes and how they decay through time. C 12 and C 13 are stable.
The atomic nucleus in C 14 is unstable making the isotope radioactive. Because it is unstable, occasionally C 14 undergoes radioactive decay to become stable nitrogen N The amount of time it takes for half of the parent isotopes to decay into daughter isotopes is known as the half-life of the radioactive isotope. Most isotopes found on Earth are generally stable and do not change. However some isotopes, like 14 C, have an unstable nucleus and are radioactive. This means that occasionally the unstable isotope will change its number of protons, neutrons, or both.
This change is called radioactive decay. For example, unstable 14 C transforms to stable nitrogen 14 N. The atomic nucleus that decays is called the parent isotope. The product of the decay is called the daughter isotope. In the example, 14 C is the parent and 14 N is the daughter. Some minerals in rocks and organic matter e. The abundances of parent and daughter isotopes in a sample can be measured and used to determine their age. This method is known as radiometric dating.
Some commonly used dating methods are summarized in Table 1. The rate of decay for many radioactive isotopes has been measured and does not change over time. Thus, each radioactive isotope has been decaying at the same rate since it was formed, ticking along regularly like a clock. For example, when potassium is incorporated into a mineral that forms when lava cools, there is no argon from previous decay argon, a gas, escapes into the atmosphere while the lava is still molten.
When that mineral forms and the rock cools enough that argon can no longer escape, the "radiometric clock" starts. Over time, the radioactive isotope of potassium decays slowly into stable argon, which accumulates in the mineral. The amount of time that it takes for half of the parent isotope to decay into daughter isotopes is called the half-life of an isotope Figure 5b.
Relative dating is used to arrange geological events, and the rocks they leave behind, in a sequence. The method of reading the order is called stratigraphy (layers of rock are called strata). Relative dating does not provide actual numerical dates for the rocks. Using relative and radiometric dating methods, geologists are able to answer the question: how old is this fossil?.
The Principle of Superposition tells us that deeper layers of rock are older than shallower layers Relative dating utilizes six fundamental principles to determine the relative age of a formation or event. 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.
Relative dating is the science of determining the relative order of past events i. In geology, rock or superficial deposits , fossils and lithologies can be used to correlate one stratigraphic column with another.
Despite seeming like a relatively stable place, the Earth's surface has changed dramatically over the past 4. Mountains have been built and eroded, continents and oceans have moved great distances, and the Earth has fluctuated from being extremely cold and almost completely covered with ice to being very warm and ice-free.
Dating Rocks and Fossils Using Geologic Methods
The simplest and most intuitive way of dating geological features is to look at the relationships between them. For example, the principle of superposition states that sedimentary layers are deposited in sequence, and, unless the entire sequence has been turned over by tectonic processes or disrupted by faulting, the layers at the bottom are older than those at the top. The principle of inclusions states that any rock fragments that are included in rock must be older than the rock in which they are included. For example, a xenolith in an igneous rock or a clast in sedimentary rock must be older than the rock that includes it Figure 8. The principle of cross-cutting relationships states that any geological feature that cuts across, or disrupts another feature must be younger than the feature that is disrupted. An example of this is given in Figure 8.
8.2 Relative Dating Methods
8.2 Relative Dating Methods