Our planet inherits a large number of artifacts and monuments bestowed upon us by older historic civilizations. These remains are subjected to dating techniques in order to predict their ages and trace their history. This ScienceStruck post enlists the differences between the absolute and relative dating methods. Although both relative and absolute dating methods are used to estimate the age of historical remains, the results produced by both these techniques for the same sample may be ambiguous. Geological specimens that are unearthed need to be assigned an appropriate age.
Cross dating: This method compares the age of remains or fossils found in a layer with the ones found in other layers.
The comparison helps establish the relative age of these remains. Fluorine dating: Bones from fossils absorb fluorine from the groundwater. The amount of fluorine absorbed indicates how long the fossil has been buried in the sediments. Radiometric dating: This technique solely depends on the traces of radioactive isotopes found in fossils.
The rate of decay of these elements helps determine their age, and in turn the age of the rocks. Amino acid dating: Physical structure of living beings depends on the protein content in their bodies. The changes in this content help determine the relative age of these fossils. Dendrochronology: Each tree has growth rings in its trunk. This technique dates the time period during which these rings were formed. Thermoluminescence: It determines the period during which certain object was last subjected to heat.
It is based on the concept that heated objects absorb light, and emit electrons. The emissions are measured to compute the age. A Venn diagram depicts both dating methods as two individual sets. The area of intersection of both sets depicts the functions common to both.
Take a look at the diagram to understand their common functions.
When we observe the intersection in this diagram depicting these two dating techniques, we can conclude that they both have two things in common:. Provide an idea of the sequence in which events have occurred. Determine the age of fossils, rocks, or ancient monuments. Although absolute dating methods determine the accurate age compared to the relative methods, both are good in their own ways. Force applied per unit area of any surface is called pressure.
The number of tracks corresponds to the age of the grains. Fission track dating has also been used as a second clock to confirm dates obtained by other methods [ 23 ; 7 ]. Geyh, M. Spring-er-Verlag, New York Wilde, S.
Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4. Nature- Dickin, A. Radiogenic isotope geology. Cambridge University Press, Jaffey, A. Precision measurement of half-lives and specific activities of U and U C Nucl.
Dass, C. Basics of mass spectrometry. Precambrian Res. Burleigh, R. Libby and the development of radiocarbon dating. Antiquity 55 MacDougall, Doug.
Brent Dalrymple, G. The Age of the Earth. Stanford University Press, Stacey, F. Patterson, C. Age of meteorites and the earth. Acta 10- Ireland, T. New tools for isotopic analysis. Science- Valley, J. A cool early Earth. Geology 30- Bell, E. Potentially biogenic carbon preserved in a 4. Murray, A. Precision and accuracy in the optically stimulated luminescence dating of sedimentary quartz: a status review.
Geochronometria 21 Optically stimulated luminescence dating of sediments over the pastyears. Earth Planet. Christopher B.
DuRoss, Stephen F. Personius, Anthony J. Crone, Susan S. Bulletin of the Seismological Society of America- Advances in Fission-Track Geochronology. Figure: Simulation of half-life.
On the left, 4 simulations with only a few atoms. On the right, 4 simulations with many atoms. Figure: Granite left and gneiss right. Dating a mineral within the granite would give the crystallization age of the rock while dating the gneiss might reflect the timing of metamorphism. Figure: An alpha decay: Two protons and two neutrons leave the nucleus. Periodic Table of the Elements The loss of four particles, in this case, two neutrons and two protons, also lowers the mass of the atom by four.
Figure: Decay chain of U to stable Pb through a series of alpha and beta decays.
Geology absolute dating
Figure: The two paths of electron capture Electron capture is when a proton in the nucleus captures an electron from one of the electron shells and becomes a neutron. Figure: Graph of the number of half-lives vs. Figure: Schematic of carbon going through a mass spectrometer.
Figure: Carbon dioxide concentrations over the lastyears. Figure: Apatite from Mexico. References 5. James Hutton see Chapter 1 realized geologic processes are slow and his ideas on uniformitarianism i.
Stratigraphy is the study of layered sedimentary rocks. This section discusses principles of relative time used in all of geology, but are especially useful in stratigraphy.
Lower strata are older than those lying on top of them. Principle of Superposition : In an otherwise undisturbed sequence of sedimentary strataor rock layers, the layers on the bottom are the oldest and layers above them are younger. Principle of Original Horizontality : Layers of rocks deposited from above, such as sediments and lava Liquid rock on the surface of the Earth. The exception to this principle is at the margins of basins, where the strata can slope slightly downward into the basin.
Principle of Lateral Continuity : Within the depositional basinstrata are continuous in all directions until they thin out at the edge of that basin. Of course, all strata eventually end, either by hitting a geographic barrier, such as a ridge, or when the depositional process extends too far from its source, either a sediment source or a volcano.
Strata that are cut by a canyon later remain continuous on either side of the canyon. Dark dike cutting across older rocks, the lighter of which is younger than the grey rock. Principle of I nclusions: When one rock formation contains pieces or inclusions of another rock, the included rock is older than the host rock.
Principle of Fossil Succession: Evolution has produced a succession of unique fossils that correlate to the units of the geologic time scale. Assemblages of fossils contained in strata are unique to the time they lived, and can be used to correlate rocks of the same age across a wide geographic distribution. Assemblages of fossils refers to groups of several unique fossils occurring together.
The Grand Canyon of Arizona illustrates the stratigraphic principles. The photo shows layers of rock on top of one another in order, from the oldest at the bottom to the youngest at the top, based on the principle of superposition.
The predominant white layer just below the canyon rim is the Coconino Sandstone.
7 Geologic Time
This layer is laterally continuous, even though the intervening canyon separates its outcrops. The rock layers exhibit the principle of lateral continuityas they are found on both sides of the Grand Canyon which has been carved by the Colorado River.
Relative and Absolute Dating PP
In the lowest parts of the Grand Canyon are the oldest sedimentary formationswith igneous and metamorphic rocks at the bottom. The principle of cross-cutting relationships shows the sequence of these events. The metamorphic schist 16 is the oldest rock formation and the cross-cutting granite intrusion 17 is younger. As seen in the figure, the other layers on the walls of the Grand Canyon are numbered in reverse order with 15 being the oldest and 1 the youngest.
This illustrates the principle of superposition. The Grand Canyon region lies in Colorado Plateau, which is characterized by horizontal or nearly horizontal stratawhich follows the principle of original horizontality.
These rock strata have been barely disturbed from their original depositionexcept by a broad regional uplift. The red, layered rocks of the Grand Canyon Supergroup overlying the dark-colored rocks of the Vishnu schist represents a type of unconformity called a nonconformity. Because the formation of the basement rocks and the deposition of the overlying strata is not continuous but broken by events of metamorphismintrusion, and erosionthe contact between the strata and the older basement is termed an unconformity.
An unconformity represents a period during which deposition did not occur or erosion removed rock that had been deposited, so there are no rocks that represent events of Earth history during that span of time at that place. Unconformities appear in cross sections and stratigraphic columns as wavy lines between formations.
Unconformities are discussed in the next section. There are three types of unconformitiesnonconformitydisconformityand angular unconformity. A nonconformity occurs when sedimentary rock is deposited on top of igneous and metamorphic rocks as is the case with the contact between the strata and basement rocks at the bottom of the Grand Canyon.
The strata in the Grand Canyon represent alternating marine transgressions and regressions where sea level rose and fell over millions of years. When the sea level was high marine strata formed.
When sea-level fell, the land was exposed to erosion creating an unconformity. In the Grand Canyon cross-section, this erosion is shown as heavy wavy lines between the various numbered strata. This is a type of unconformity called a disconformitywhere either non- deposition or erosion took place.
In other words, layers of rock that could have been present, are absent. The time that could have been represented by such layers is instead represented by the disconformity.
Disconformities are unconformities that occur between parallel layers of strata indicating either a period of no deposition or erosion. In the lower part of the picture is an angular unconformity in the Grand Canyon known as the Great Unconformity. Notice flat lying strata over dipping strata Source: Doug Dolde. The Phanerozoic strata in most of the Grand Canyon are horizontal. However, near the bottom horizontal strata overlie tilted strata.
This is known as the Great Unconformity and is an example of an angular unconformity.
The lower strata were tilted by tectonic processes that disturbed their original horizontality and caused the strata to be eroded. Later, horizontal strata were deposited on top of the tilted strata creating the angular unconformity. Here are three graphical illustrations of the three types of unconformity.
Absolute dating, also called numerical dating, arranges the historical remains in order of their ages. Whereas, relative dating arranges them in the geological order of their formation. The relative dating techniques are very effective when it comes to radioactive isotope or radiocarbon dating. There are several types of absolute dating discussed in this section but radioisotopic dating is the most common and therefore is the focus on this section. Radioactive Decay Figure: Three isotopes of hydrogen. Geology 30, Bell, E. A., Boehnke, P., Harrison, T. M. & Mao, W. L. Potentially biogenic carbon. With a combination of relative and absolute dating, the history of geological events, age of Earth, and a geologic time scale have been determined with considerable accuracy. Stratigraphic correlation is additional tool used for understanding how depositional environments change geographically.
Disconformitywhere is a break or stratigraphic absence between strata in an otherwise parallel sequence of strata. Block diagram to apply relative dating principles. The wavy rock is a old metamorphic gneiss, A and F are faults, B is an igneous granite, D is a basaltic dike, and C and E are sedimentary strata.
In the block diagram, the sequence of geological events can be determined by using the relative-dating principles and known properties of igneoussedimentary, metamorphic rock see Chapter 4Chapter 5and Chapter 6. The sequence begins with the folded metamorphic gneiss on the bottom.
Next, the gneiss is cut and displaced by the fault labeled A. Both the gneiss and fault A are cut by the igneous granitic intrusion called batholith B; its irregular outline suggests it is an igneous granitic intrusion emplaced as magma into the gneiss.
Since batholith B cuts both the gneiss and fault A, batholith B is younger than the other two rock formations. Next, the gneissfault A, and batholith B were eroded forming a nonconformity as shown with the wavy line. This unconformity was actually an ancient landscape surface on which sedimentary rock C was subsequently deposited perhaps by a marine transgression.
Next, igneous basaltic dike A narrow igneous intrusion that cuts through existing rock, not along bedding planes. This shows that there is a disconformity between sedimentary rocks C and E.
The top of dike A narrow igneous intrusion that cuts through existing rock, not along bedding planes. Fault F cuts across all of the older rocks B, C and E, producing a fault scarpwhich is the low ridge on the upper-left side of the diagram. The final events affecting this area are current erosion processes working on the land surface, rounding off the edge of the fault scarpand producing the modern landscape at the top of the diagram.
Relative time allows scientists to tell the story of Earth events, but does not provide specific numeric ages, and thus, the rate at which geologic processes operate.
Relative dating principles was how scientists interpreted Earth history until the end of the 19th Century. Because science advances as technology advances, the discovery of radioactivity in the late s provided scientists with a new scientific tool called radioisotopic dating. Using this new technology, they could assign specific time units, in this case years, to mineral grains within a rock. These numerical values are not dependent on comparisons with other rocks such as with relative datingso this dating method is called absolute dating.
There are several types of absolute dating discussed in this section but radioisotopic dating is the most common and therefore is the focus on this section. All elements on the Periodic Table of Elements see Chapter 3 contain isotopes.
An isotope is an atom of an element with a different number of neutrons. For example, hydrogen H always has 1 proton in its nucleus the atomic numberbut the number of neutrons can vary among the isotopes 0, 1, 2. Recall that the number of neutrons added to the atomic number gives the atomic mass. When hydrogen has 1 proton and 0 neutrons it is sometimes called protium 1 Hwhen hydrogen has 1 proton and 1 neutron it is called deuterium 2 Hand when hydrogen has 1 proton and 2 neutrons it is called tritium 3 H.
Many elements have both stable and unstable isotopes. For the hydrogen example, 1 H and 2 H are stable, but 3 H is unstable.
Dating, in geology, determining a chronology or calendar of events in the history of Earth, using to a large degree the evidence of organic evolution in the sedimentary rocks accumulated through geologic time in marine and continental simplybeyondexpectations.com date past events, processes, formations, and fossil organisms, geologists employ a variety of techniques. Geologists often need to know the age of material that they find. They use absolute dating methods, sometimes called numerical dating, to give rocks an actual date, or date range, in number of years. This is different to relative dating, which only puts geological events in time order. Start studying Geology Relative and Absolute Dating. Learn vocabulary, terms, and more with flashcards, games, and other study tools.
Unstable isotopescalled radioactive isotopesspontaneously decay over time releasing subatomic particles or energy in a process called radioactive decay.
When this occurs, an unstable isotope becomes a more stable isotope of another element. For example, carbon 14 C decays to nitrogen 14 N. Simulation of half-life. On the left, 4 simulations with only a few atoms. On the right, 4 simulations with many atoms. The radioactive decay of any individual atom is a completely usimplybeyondexpectations.comedictable and random event. However, some rock specimens have an enormous number of radioactive isotopesperhaps trillions of atoms, and this large group of radioactive isotopes does have a predictable pattern of radioactive decay.
The radioactive decay of half of the radioactive isotopes in this group takes a specific amount of time. The time is takes for half of the atoms in a substance to decay is called the half-life. In other words, the half-life of an isotope is the amount of time it takes for half of a group of unstable isotopes to decay to a stable isotope.
The half-life is constant and measurable for a given radioactive isotopeso it can be used to calculate the age of a rock. For example, the half-life uranium U is 4. The principles behind this dating method require two key assumptions.
First, the mineral grains containing the isotope formed at the same time as the rock, such as minerals in an igneous rock that crystallized from magma. Second, the mineral crystals remain a closed systemmeaning they are not subsequently altered by elements moving in or out of them. These requirements place some constraints on the kinds of rock suitable for dating, with igneous rock being the best. Metamorphic rocks are crystalline, but the processes of metamorphism may reset the clock and derived ages may represent a smear of different metamorphic events rather than the age of original crystallization.
Detrital sedimentary rocks contain clasts from separate parent rocks from unknown locations and derived ages are thus meaningless. However, sedimentary rocks with precipitated mineralssuch as evaporitesmay contain elements suitable for radioisotopic dating. Igneous pyroclastic layers and lava Liquid rock on the surface of the Earth. Cross-cutting igneous rocks and sill A type of dike that is parallel to bedding planes within the bedrock.
There are several ways radioactive atoms decay. We will consider three of them here- alpha decaybeta decayand electron capture.
Alpha decay is when an alpha particle, which consists of two protons and two neutrons, is emitted from the nucleus of an atom. This also happens to be the nucleus of a helium atom; helium gas may get trapped in the crystal lattice of a mineral in which alpha decay has taken place.
When an atom loses two protons from its nucleus, lowering its atomic number, it is transformed into an element that is two atomic numbers lower on the Periodic Table of the Elements. Periodic Table of the Elements The loss of four particles, in this case two neutrons and two protons, also lowers the mass of the atom by four.
For example alpha decay takes place in the unstable isotope U, which has an atomic number of 92 92 protons and mass number of total of all protons and neutrons.
When U spontaneously emits an alpha particle, it becomes thorium Th. The radioactive decay product of an element is called its daughter isotope and the original element is called the parent isotope. In this case, U is the parent isotope and Th is the daughter isotope. The half-life of U is 4. This isotope of uranium, U, can be used for absolute dating the oldest materials found on Earth, and even meteorites and materials from the earliest events in our solar system. Beta decay is when a neutron in its nucleus splits into an electron and a proton.
The electron is emitted from the nucleus as a beta ray. For example, Th is unstable and undergoes beta decay to form protactinium Pawhich also undergoes beta decay to form uranium U. Notice these are all isotopes of different elements but they have the same atomic mass of The decay process of radioactive elements like uranium keeps producing radioactive parents and daughters until a stable, or non- radioactivedaughter is formed.
Such a series is called a decay chain. The decay chain of the radioactive parent isotope U progresses through a series of alpha red arrows on the adjacent figure and beta decays blue arrowsuntil it forms the stable daughter isotopelead Pb. The two paths of electron capture Electron capture is when a proton in the nucleus captures an electron from one of the electron shells and becomes a neutron.
This produces one of two different effects: 1 an electron jumps in to fill the missing spot of the departed electron and emits an X-ray, or 2 in what is called the Auger process, another electron is released and changes the atom into an ion An atom or molecule that has a charge positive or negative due to the loss or gain of electrons.
The atomic number is reduced by one and mass number remains the same. An example of an element that decays by electron capture is potassium 40 K. Radioactive 40 K makes up a tiny percentage 0. Below is a table of some of the more commonly-used radioactive dating isotopes and their half-lives. Some common isotopes used for radioisotopic dating. For a given a sample of rock, how is the dating procedure carried out? The parent and daughter isotopes are separated out of the mineral using chemical extraction.
In the case of uranium, U and U isotopes are separated out together, as are the Pb and Pb with an instrument called a mass spectrometer. Graph of number of half-lives vs. This can be further calculated for a series of half-lives as shown in the table. The table does not show more than 10 half-lives because after about 10 half-lives, the amount of remaining parent is so small it becomes too difficult to accurately measure via chemical analysis.
Modern applications of this method have achieved remarkable accuracies of plus or minus two million years in 2. The existence of these two clocks in the same sample gives a cross-check between the two. Ratio of parent to daughter in terms of half-life.
Schematic of carbon going through a mass spectrometer. Another radioisotopic dating method involves carbon and is useful for dating archaeologically important samples containing organic substances like wood or bone. Radiocarbon datingalso called carbon dating, uses the unstable isotope carbon 14 C and the stable isotope carbon 12 C. Carbon is constantly being created in the atmosphere by the interaction of cosmic particles with atmospheric nitrogen 14 N.