Relative Dating of Rock LayersThe Auckland Islands — Investigating evidence related to dating geological events. These islands were formed by multiple geological events. Geological events being significant occurrences as a result of the earth system. Whilst these islands have been shaped over many thousands of years by events such as weathering and erosion, particularly due to glaciation, the main events which formed the islands were two volcanic eruptions. The way that it can be determined that the islands were formed from volcanic eruptions, and when they were formed, is by looking at the record that is contained in the rocks of the islands.
The study of strata is called stratigraphyand using a few basic principles, it is possible to work out the relative ages of rocks. 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 : layers of strata are deposited horizontally or nearly horizontally Figure 2.
Thus, any deformations of strata Figures 2 and 3 must have occurred after the rock was deposited. 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. 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.
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 neutronslocated in the atomic nucleusand 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 masswhich 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: carbon 12 12 Ccarbon 13 13 Cand carbon 14 14 C Figure 5a. C 12 and C 13 are stable.Science Confirms a Young Earth—The Radioactive Dating Methods are Flawed
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.
Dating geological strata
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.
Dating. eginning in the seventeenth and eighteenth centuries, geologists trace a particular geological stratum or group of strata (system) in various localities. There are two main aspects to geological dating – relative dating that in an un- deformed sequence of sedimentary rocks, the oldest strata will. Geochronology is the science of determining the age of rocks, fossils, and sediments using naming rock layers and the time spans utilized to classify layers within a stratum. A number of radioactive isotopes are used for this purpose, and depending on the rate of decay, are used for dating different geological periods.
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.
The most useful tool in dating strata is radiometric dating of materials. A radioactive isotope such as uranium decays at a very regular and well- known rate. 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. If an important are strategically defined by organized fossils, rock strata in correct order and dating. Two key assumptions about the next business day. Before.
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.
When the quantities of the parent and daughter isotopes are equal, one half-life has occurred.
The preservation, quantity past gradual change over a geologic time dating the graptolites allows the fossils to be used to date strata of rocks throughout the. Using relative and radiometric dating methods, geologists are able to answer the The study of strata is called stratigraphy, and using a few basic principles. Relative dating is the science of determining the relative order of past events without necessarily determining their absolute age (i.e. estimated age). In geology, rock or superficial deposits, fossils and lithologies can be used to These strata make up much of the famous prominent rock formations in widely spaced.
If the half life of an isotope is known, the abundance of the parent and daughter isotopes can be measured and the amount of time that has elapsed since the "radiometric clock" started can be calculated.
For example, if the measured abundance of 14 C and 14 N in a bone are equal, one half-life has passed and the bone is 5, years old an amount equal to the half-life of 14 C. If there is three times less 14 C than 14 N in the bone, two half lives have passed and the sample is 11, years old. However, if the bone is 70, years or older the amount of 14 C left in the bone will be too small to measure accurately.
Thus, radiocarbon dating is only useful for measuring things that were formed in the relatively recent geologic past. Luckily, there are methods, such as the commonly used potassium-argon K-Ar methodthat allows dating of materials that are beyond the limit of radiocarbon dating Table 1. Comparison of commonly used dating methods. Radiation, which is a byproduct of radioactive decay, causes electrons to dislodge from their normal position in atoms and become trapped in imperfections in the crystal structure of the material.
Dating methods like thermoluminescenceoptical stimulating luminescence and electron spin resonancemeasure the accumulation of electrons in these imperfections, or "traps," in the crystal structure of the material.
If the amount of radiation to which an object is exposed remains constant, the amount of electrons trapped in the imperfections in the crystal structure of the material will be proportional to the age of the material. These methods are applicable to materials that are up to aboutyears old.
However, once rocks or fossils become much older than that, all of the "traps" in the crystal structures become full and no more electrons can accumulate, even if they are dislodged. The Earth is like a gigantic magnet. It has a magnetic north and south pole and its magnetic field is everywhere Figure 6a. Just as the magnetic needle in a compass will point toward magnetic north, small magnetic minerals that occur naturally in rocks point toward magnetic north, approximately parallel to the Earth's magnetic field.
Because of this, magnetic minerals in rocks are excellent recorders of the orientation, or polarityof the Earth's magnetic field. Small magnetic grains in rocks will orient themselves to be parallel to the direction of the magnetic field pointing towards the north pole. Black bands indicate times of normal polarity and white bands indicate times of reversed polarity. Through geologic time, the polarity of the Earth's magnetic field has switched, causing reversals in polarity.
The Earth's magnetic field is generated by electrical currents that are produced by convection in the Earth's core. During magnetic reversals, there are probably changes in convection in the Earth's core leading to changes in the magnetic field. The Earth's magnetic field has reversed many times during its history.
When the magnetic north pole is close to the geographic north pole as it is todayit is called normal polarity. Reversed polarity is when the magnetic "north" is near the geographic south pole. 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.
Many of these organisms have left their remains as fossils in sedimentary rocks.
Geologists have studied the order in which fossils appeared and disappeared through time and rocks. This study is called biostratigraphy. Fossils can help to match rocks of the same age, even when you find those rocks a long way apart.
This matching process is called correlation, which has been an important process in constructing geological timescales. Some fossils, called index fossils, are particularly useful in correlating rocks. For a fossil to be a good index fossil, it needs to have lived during one specific time period, be easy to identify and have been abundant and found in many places. For example, ammonites lived in the Mesozoic era. If you find ammonites in a rock in the South Island and also in a rock in the North Island, you can say that both rocks are Mesozoic.