4.2 Relative and Absolute Dating
7 min read•august 14, 2024
Geologists use two main methods to determine the age of rocks and fossils: relative and . arranges events in order without specific dates, while absolute dating provides numerical ages. These techniques are crucial for understanding Earth's history.
Relative dating uses principles like superposition and to sequence events. Absolute dating, including radiometric methods, measures isotope decay to calculate precise ages. Both approaches have strengths and limitations but together paint a comprehensive picture of Earth's past.
Relative vs Absolute Dating
Defining Relative and Absolute Dating
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- Relative dating determines the order of events or the relative age of geologic features without specifying the exact time or date
- Based on the assumption that lower layers in a sequence of sedimentary rocks or materials are older than upper layers, unless the sequence has been overturned
- Absolute dating provides a numerical age or range in contrast with relative dating which places events in order without any measure of the age between events
- Determines the actual ages of rocks, minerals, and fossils in years or millions of years
Methods for Relative and Absolute Dating
- Relative dating methods include , , and cross-cutting relationships
- Stratigraphy examines the order and position of layers of sedimentary rock to determine relative age
- Biostratigraphy uses fossils to determine relative age based on the
- Cross-cutting relationships determine relative age by examining how geologic features cut across or intersect with other features
- Absolute dating methods include , amino acid dating, , and
- Radiometric dating measures the decay of radioactive isotopes in rocks and minerals to determine absolute age
- Amino acid dating measures the breakdown of amino acids in organic materials like shells or bones to determine absolute age
- Dendrochronology uses tree rings to determine absolute age by counting the number of rings and matching patterns between trees
- Thermoluminescence measures the amount of light emitted by crystals in pottery or rocks when heated to determine absolute age
Stratigraphy for Relative Age
Principles of Stratigraphy
- 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
- The oldest layers are at the bottom and the youngest layers are at the top, unless the sequence has been overturned by folding or faulting
- The states that layers of sediment are originally deposited in a nearly horizontal position and parallel to the earth's surface
- If the layers are no longer horizontal, they must have been moved into their present position after they were deposited by processes like folding or tilting
- The states that layers of sediment initially extend laterally in all directions until they thin out or grade into a different sediment type
- A layer can be traced over a wide area, and if a part of it is eroded away, or missing, it can still be inferred by the presence of the surrounding layers
Geologic Features and Stratigraphy
- The states that a geologic feature that cuts across another is younger than the feature it cuts across
- For example, a fault that cuts through layers of rock must be younger than the layers it cuts through
- Igneous intrusions like dikes or sills that cut across sedimentary layers must be younger than the layers they intrude
- The states that fragments of one rock included in another must be older than the host rock
- For example, a xenolith (fragment of foreign rock) in an igneous rock must be older than the rock that contains it
- Clasts (rock fragments) in a sedimentary rock must be older than the sedimentary rock that contains them
- The principle of fossil succession states that fossil organisms succeed one another in a definite, irreversible, and determinable order
- Therefore, a particular fossil of a known age found in a rock layer can determine the age of the entire layer
- Index fossils, which are distinctive, globally-distributed, and short-lived fossil species, are particularly useful for determining the relative age of rock layers
Radiometric Dating for Absolute Age
Radioactive Decay and Half-Life
- Radiometric dating is based on the decay of radioactive isotopes, which are forms of elements with unstable atomic nuclei that emit radiation as they decay into more stable atomic nuclei
- For example, decays into nitrogen-14, and decays into lead-207
- The rate of decay is conveniently expressed in terms of an isotope's half-life, or the time it takes for one-half of a particular radioactive isotope in a sample to decay
- Most radioactive isotopes have rapid rates of decay that can be measured in days, years, or millions of years
- For example, carbon-14 has a half-life of 5,730 years, and uranium-235 has a half-life of 704 million years
Calculating Absolute Age with Radiometric Dating
- To determine the age of a rock, geologists measure the amount of parent and daughter isotopes, and from this measurement, they can calculate how many half-lives have passed since the sample formed
- The more half-lives that have passed, the older the rock is
- For example, if a rock contains equal amounts of parent and daughter isotopes, then one half-life has passed and the rock is the age of one half-life of the isotope
- The most widely used radiometric dating method is , which archaeologists prefer to use
- Radiocarbon dating measures the decay of carbon-14 in organic material and can be best used to date items between 100 to 50,000 years old
- It is based on the assumption that the ratio of carbon-14 to carbon-12 in the atmosphere has remained constant over time
- Other commonly used radiometric dating methods include , , and
- Each of these methods is suitable for different applications due to the types of atoms present in the mineral or rock and its age
- Potassium-argon dating is used for rocks that are millions to billions of years old and contain potassium-bearing minerals
- Uranium-lead dating is used for rocks that are billions of years old and contain uranium-bearing minerals
- Fission track dating is used for rocks that are millions to billions of years old and contain uranium-bearing minerals
Advantages and Limitations of Dating Methods
Relative Dating Advantages and Limitations
- Relative dating is useful for determining the relative order of events in Earth's history and for determining whether one object is older or younger than another
- It can be used to establish the sequence of events in a geologic setting, such as the order of deposition of sedimentary layers or the order of intrusion of igneous rocks
- It can also be used to correlate rock layers across wide geographic areas based on their fossil content or other characteristics
- However, relative dating does not provide the absolute age of an object in years
- It cannot tell us exactly how old a rock or fossil is, only that it is older or younger than other rocks or fossils
- It also requires the assumption that the processes that formed the rocks or fossils have not changed over time, which may not always be the case
Radiometric Dating Advantages and Limitations
- Radiometric dating provides numerical ages in years for rocks, minerals, and fossils
- It can give us the actual age of a rock or fossil, rather than just its relative age
- It can be used to date rocks and fossils that are too old for other methods like radiocarbon dating
- However, radiometric dating requires the presence of certain elements in the rock, mineral, or fossil being dated, and the rock must not have been altered or weathered since its formation
- If the rock has been heated or deformed, the isotopes may have escaped or been redistributed, making the age determination inaccurate
- Some minerals like quartz do not contain any isotopes that can be used for radiometric dating
- Radiocarbon dating is widely used in archaeology to date organic materials, but it has a relatively short range of about 50,000 years
- It cannot be used to date older materials like rocks or fossils
- It also requires the assumption that the ratio of carbon-14 to carbon-12 in the atmosphere has remained constant over time, which may not always be the case due to changes in the earth's magnetic field or the burning of fossil fuels
- Potassium-argon dating is useful for dating rocks that are millions to billions of years old, but it requires the presence of potassium-bearing minerals in the rock
- It also assumes that no argon has escaped from the rock since its formation, which may not always be the case if the rock has been heated or deformed
- Uranium-lead dating is useful for dating rocks that are billions of years old, but it requires the presence of uranium-bearing minerals in the rock
- It also assumes that no lead has escaped from the rock since its formation, which may not always be the case if the rock has been heated or deformed
- Fission track dating is useful for dating rocks that are millions to billions of years old, but it requires the presence of uranium-bearing minerals in the rock
- It also assumes that the rock has not been heated to temperatures high enough to erase the fission tracks, which may not always be the case in geologic settings with high heat flow
Key Terms to Review (33)
Absolute dating: Absolute dating is a method used to determine the specific age of a rock, fossil, or archaeological artifact, often expressed in years. This technique relies on the decay of radioactive isotopes and other measurable physical and chemical processes, allowing scientists to provide a concrete age rather than a sequence or relative timeframe. Understanding absolute dating is essential for constructing the geologic time scale and offers insight into the timing of significant events in Earth's history.
Biostratigraphy: Biostratigraphy is the branch of stratigraphy that uses fossil organisms to establish the relative ages of rock layers and correlate them across different regions. By analyzing the distribution and abundance of fossils within sedimentary strata, biostratigraphy helps scientists understand geological history, including changes in environments and biological evolution over time.
Carbon dating: Carbon dating is a scientific method used to determine the age of organic materials by measuring the amount of carbon-14 they contain. This technique is vital for understanding historical timelines and archaeological contexts, as it provides an absolute dating framework that complements relative dating methods.
Carbon-14: Carbon-14 is a radioactive isotope of carbon that is used in radiocarbon dating to determine the age of organic materials. It forms when cosmic rays interact with nitrogen in the atmosphere and becomes incorporated into living organisms. When these organisms die, the carbon-14 begins to decay at a known rate, allowing scientists to calculate the time since death based on the remaining amount of carbon-14 present.
Charles Lyell: Charles Lyell was a British geologist, best known for his principles of geology, which laid the groundwork for modern geology and introduced the concept of uniformitarianism. His ideas emphasized that the Earth was shaped by the same processes still in operation today, connecting the past with the present and influencing how geologists understand Earth's history and time scales.
Cross-cutting relationships: Cross-cutting relationships is a principle in geology that states that if a geological feature cuts through another, the feature that has been cut is older than the feature that does the cutting. This principle helps in understanding the chronological order of geological events and is crucial for both relative and absolute dating of rocks, as well as for constructing the geologic time scale.
Dendrochronology: Dendrochronology is the scientific method of dating tree rings to determine the age of trees and the historical climate conditions in which they grew. By examining the growth patterns in tree rings, scientists can create chronological records that provide insights into environmental changes over time, making it a key technique for understanding both relative and absolute dating in geology and archaeology.
Eon: An eon is the largest division of geologic time, spanning hundreds of millions to billions of years. Eons are typically divided into smaller units called eras, which help to organize Earth's history and its significant geological and biological events. Understanding eons is crucial for interpreting the geologic time scale and aids in both relative and absolute dating methods by placing events and formations within a broader context.
Era: An era is a significant period in geological time characterized by distinct developments in Earth's history, marked by changes in climate, life forms, and geological features. These periods are essential for understanding the evolution of the planet and its inhabitants, as they help to organize the vast timeline of Earth's past into manageable segments for analysis. Eras are used in conjunction with relative and absolute dating methods to determine the age of rocks and fossils, and they are integral to the geologic time scale that divides Earth's history into intervals based on major events.
Faunal Succession: Faunal succession is the principle that certain fossilized organisms appear in a specific, recognizable order in the geological record. This concept allows geologists and paleontologists to correlate strata and determine relative ages of rock layers based on the presence of particular fossils, aiding in the understanding of Earth's history and the evolution of life.
Fission Track Dating: Fission track dating is a radiometric dating method that analyzes the paths left by the spontaneous fission of uranium-238 within minerals and glass. This technique allows scientists to determine the age of geological materials and provides insights into thermal history, as the number of fission tracks accumulated over time relates to the age of the sample.
Fossilization: Fossilization is the process through which organic material is preserved as a fossil, typically occurring over millions of years. This process allows remnants of ancient life forms, such as plants and animals, to be transformed into rock-like structures, providing crucial insights into the Earth's biological history. Understanding fossilization helps us relate the age of these fossils to geological timeframes, linking them to both relative and absolute dating methods.
Geochronology: Geochronology is the science of determining the age of rocks, sediments, and fossils through the use of various dating techniques. This field combines both relative and absolute dating methods to provide a comprehensive timeline of Earth's history and the timing of geological events. By understanding the age of geological materials, geochronology helps to reconstruct past environments and informs us about the processes that have shaped the planet over time.
Law of superposition: The law of superposition is a fundamental principle in geology stating that in any undisturbed sequence of sedimentary rocks, the oldest layers are at the bottom and the youngest layers are at the top. This concept helps geologists and paleontologists understand the relative ages of rock layers and the fossils contained within them, making it essential for establishing the history of the Earth over time.
Mass spectrometer: A mass spectrometer is an analytical instrument used to measure the mass-to-charge ratio of ions. It plays a crucial role in identifying the composition and structure of substances by providing precise measurements that are essential for both relative and absolute dating methods in Earth science.
Period: In geology, a period is a unit of geological time that represents a specific span of time during which certain rock layers were formed and specific events occurred. Periods are subdivisions of eras and help in organizing Earth's history into manageable segments for understanding the evolution of life, climate changes, and geological transformations.
Potassium-argon dating: Potassium-argon dating is a radiometric dating method used to determine the age of rocks and minerals by measuring the ratio of radioactive potassium-40 to argon-40. This technique is particularly useful for dating volcanic rock and ash layers, which can provide important chronological context for geological and archaeological studies.
Principle of cross-cutting relationships: The principle of cross-cutting relationships states that geologic features, such as faults or igneous intrusions, that cut across other rocks are younger than the rocks they affect. This principle helps establish the relative ages of rock layers and structures by indicating that any disturbance must have occurred after the formation of the affected layers. Understanding this principle is crucial for reconstructing the geological history of an area and is a key element in both relative and absolute dating.
Principle of fossil succession: The principle of fossil succession states that fossil organisms follow one another in a definite and recognizable order in the geological record. This concept is fundamental in understanding how layers of rock can be correlated based on their fossil content, allowing geologists to determine relative ages of rock layers and the sequence of events in Earth's history.
Principle of inclusions: The principle of inclusions states that if one rock body contains fragments of another rock, the fragments are older than the rock that contains them. This principle is crucial for understanding the relative ages of rock layers and helps geologists interpret the geological history of an area by establishing a timeline based on the relationships between different rock materials.
Principle of lateral continuity: The principle of lateral continuity states that sedimentary layers are deposited in continuous sheets across a landscape, extending in all directions until they thin out or encounter a barrier. This principle is crucial for understanding the spatial relationships between rock layers and for reconstructing past geological events, especially in the context of determining relative ages of rock formations.
Principle of original horizontality: The principle of original horizontality states that layers of sedimentary rock are originally deposited in horizontal or nearly horizontal layers due to the influence of gravity. This principle is fundamental in understanding how sedimentary rocks form and is essential for interpreting geological history, particularly in the context of relative and absolute dating as well as the geologic time scale.
Radiocarbon Dating: Radiocarbon dating is a scientific method used to determine the age of an artifact or fossil by measuring the amount of carbon-14 it contains. This technique is vital for understanding the timeline of archaeological and geological events, allowing researchers to place materials in chronological order based on their carbon content. As living organisms take in carbon from their environment, the ratio of carbon-14 to carbon-12 changes after death, making this method effective for dating organic materials up to about 50,000 years old.
Radiometric Dating: Radiometric dating is a method used to determine the age of rocks and fossils based on the decay of radioactive isotopes. This technique relies on the principle that certain isotopes are unstable and will break down into stable forms at a predictable rate, allowing scientists to calculate the time elapsed since the rock or fossil formed. It plays a crucial role in understanding Earth's history, providing precise numerical ages for geological events and helping to establish a timeline of Earth's formation and evolution.
Relative dating: Relative dating is a method used by geologists and archaeologists to determine the age of rocks, fossils, or artifacts in relation to one another rather than providing an exact numerical age. This technique helps establish the chronological order of geological events and the succession of life forms over time, allowing scientists to piece together the history of the Earth and its life.
Stratigraphy: Stratigraphy is the branch of geology concerned with the study of rock layers (strata) and layering (stratification). It helps scientists understand the chronological order of geological events and the history of the Earth by analyzing the sequences of sedimentary layers and their characteristics. This foundational concept is crucial in determining relative ages of rocks and fossils, as well as providing a framework for the geologic time scale.
Stratum: A stratum is a distinct layer of rock or sediment that has formed over time, often representing a specific period in geological history. Strata can vary in thickness, composition, and fossil content, which can provide important clues about the Earth's past environments and biological evolution. Understanding strata is crucial for interpreting geological timelines and establishing relative and absolute dating methods.
Thermoluminescence: Thermoluminescence is a dating method used to determine the last time a mineral, usually quartz or feldspar, was heated to a temperature sufficient to release trapped electrons. This technique is particularly valuable for dating archaeological materials and sediments that are beyond the range of carbon dating, allowing scientists to gain insights into past human activity and environmental changes.
Uranium-235: Uranium-235 is a naturally occurring isotope of uranium, notable for its ability to sustain a nuclear chain reaction, making it essential for nuclear power generation and atomic weapons. It constitutes about 0.7% of natural uranium and is key in the process of radioactive decay, which is utilized in absolute dating techniques to determine the age of rocks and fossils.
Uranium-238: Uranium-238 is a naturally occurring isotope of uranium that makes up about 99.3% of natural uranium. It is significant in the fields of geology and radiometric dating because it undergoes radioactive decay to form lead-206, allowing scientists to determine the age of rocks and minerals, thereby contributing to our understanding of Earth's formation and early history.
Uranium-lead dating: Uranium-lead dating is a radiometric dating method that uses the decay of uranium isotopes into lead isotopes to determine the age of rocks and minerals. This technique is one of the most reliable forms of absolute dating, allowing scientists to accurately date geological materials that are millions to billions of years old. By measuring the ratio of uranium to lead in a sample, researchers can calculate the time that has elapsed since the rock or mineral formed, making it essential for understanding the history of the Earth and its geological events.
Volcanic eruptions: Volcanic eruptions are natural events where magma from beneath the Earth's crust escapes to the surface, often accompanied by the ejection of ash, gas, and volcanic rocks. These eruptions play a significant role in shaping the Earth’s landscape and can influence climate patterns, making them critical in understanding geological time scales and significant historical events.
William Smith: William Smith was an English geologist known as the 'Father of English Geology' for his pioneering work in the field of stratigraphy and the development of the principle of faunal succession. His contributions laid the groundwork for understanding the relative dating of rock layers and the construction of the geologic time scale, which are essential for interpreting Earth's history and the ages of geological formations.