Relative dating
Encyclopedia
Relative dating is the science determining the relative order of past events, without necessarily determining their absolute age.
In geology rock
or superficial deposits
, fossil
s and lithologies
can be used to correlate one stratigraphic column
with another. Prior to the discovery of radiometric dating
which provided a means of absolute dating
in the early 20th century, archaeologists and geologist
s were largely limited to the use of relative dating techniques to determine the geological
events.
Though relative dating can only determine the sequential order in which a series of events occurred, not when they occur, it remains a useful technique especially in materials lacking radioactive isotopes. Relative dating by biostratigraphy
is the preferred method in paleontology
, and is in some respects more accurate (Stanley, 167–69). The Law of Superposition
was the summary outcome of 'relative dating' as observed in geology from the 17th century to the early 20th century.
The regular order of occurrence of fossils in rock layers was discovered around 1800 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 surveyor, he 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.
are similar to some of those applied in geology. The principles of typology
can be compared to the biostratigraphic approach described above.
, particularly in the vast majority of cases for which we have no surface samples. Many of the same principles are applied. For example, if a valley is formed inside an impact crater
, the valley must be younger than the crater.
Craters themselves are highly useful in relative dating; as a general rule, the younger a planetary surface is, the fewer craters it has. If long-term cratering rates are known to enough precision, crude absolute dates can be applied based on craters alone; however, cratering rates outside the Earth-Moon system are poorly known.(Hartmann, 258)
In geology rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
or superficial deposits
Superficial deposits
Superficial deposits refer to geological deposits typically of Quaternary age. These recent unconsolidated sediments may include stream channel and floodplain deposits, beach sands, talus gravels and glacial drift and moraine...
, fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s and lithologies
Lithology
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of...
can be used to correlate one stratigraphic column
Stratigraphic column
A stratigraphic column is a representation used in geology and its subfield of stratigraphy to describe the vertical location of rock units in a particular area....
with another. Prior to the discovery of radiometric dating
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
which provided a means of absolute dating
Absolute dating
Absolute dating is the process of determining an approximate computed age in archaeology and geology. Some scientists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies an unwarranted certainty and precision...
in the early 20th century, archaeologists and geologist
Geologist
A geologist is a scientist who studies the solid and liquid matter that constitutes the Earth as well as the processes and history that has shaped it. Geologists usually engage in studying geology. Geologists, studying more of an applied science than a theoretical one, must approach Geology using...
s were largely limited to the use of relative dating techniques to determine the geological
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
events.
Though relative dating can only determine the sequential order in which a series of events occurred, not when they occur, it remains a useful technique especially in materials lacking radioactive isotopes. Relative dating by biostratigraphy
Biostratigraphy
Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them. Usually the aim is correlation, demonstrating that a particular horizon in one geological section represents the same period...
is the preferred method in paleontology
Paleontology
Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...
, and is in some respects more accurate (Stanley, 167–69). The Law of Superposition
Law of superposition
The law of superposition is a key axiom based on observations of natural history that is a foundational principle of sedimentary stratigraphy and so of other geology dependent natural sciences:...
was the summary outcome of 'relative dating' as observed in geology from the 17th century to the early 20th century.
The regular order of occurrence of fossils in rock layers was discovered around 1800 by William Smith
William Smith (geologist)
William 'Strata' Smith was an English geologist, credited with creating the first nationwide geological map. He is known as the "Father of English Geology" for collating the geological history of England and Wales into a single record, although recognition was very slow in coming...
. While digging the Somerset Coal Canal
Somerset Coal Canal
The Somerset Coal Canal was a narrow canal in England, built around 1800 from basins at Paulton and Timsbury via Camerton, an aqueduct at Dunkerton, Combe Hay, Midford and Monkton Combe to Limpley Stoke where it joined the Kennet and Avon 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 surveyor, he 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 chronology
- UniformitarianismUniformitarianismIn the philosophy of naturalism, the uniformitarianism assumption is that the same natural laws and processes that operate in the universe now, have always operated in the universe in the past and apply everywhere in the universe. It has included the gradualistic concept that "the present is the...
(Hutton) Popularized by Charles Lyell - Law of superpositionLaw of superpositionThe law of superposition is a key axiom based on observations of natural history that is a foundational principle of sedimentary stratigraphy and so of other geology dependent natural sciences:...
(Steno) - Original Horizontality (Steno)
- Lateral continuityPrinciple of lateral continuityThe principle of lateral continuity states that layers of sediment initially extend laterally in all directions; in other words, they are laterally continuous...
(Steno) - Cross cutting relationships (Hutton)
- Inclusions of Igneous rocksMelt InclusionsMelt inclusions are small parcels or "blobs" of molten rock that are trapped within crystals that grow in the magmas that form igneous rocks. In many respects they are analogous to fluid inclusions. Melt inclusions are generally small - most are less than 100 micrometres across...
(Hutton) - Principle of faunal succession (Smith)
- Law of included fragmentsLaw of included fragmentsThe law of included fragments is a method of relative dating in geology. Essentially, this law states that clasts in a rock are older than the rock itself. One example of this is a xenolith, which is a fragment of country rock that fell into passing magma as a result of stoping...
Archaeology
Relative dating methods in archaeologyArchaeology
Archaeology, or archeology , is the study of human society, primarily through the recovery and analysis of the material culture and environmental data that they have left behind, which includes artifacts, architecture, biofacts and cultural landscapes...
are similar to some of those applied in geology. The principles of typology
Typology (archaeology)
In archaeology a typology is the result of the classification of things according to their characteristics. The products of the classification, i.e. the classes are also called types. Most archaeological typologies organize artifacts into types, but typologies of houses or roads belonging to a...
can be compared to the biostratigraphic approach described above.
Planetology
Relative dating is used to determine the order of events on objects other than Earth; for decades, planetary scientists have used it to decipher the evolution of bodies in the Solar SystemSolar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
, particularly in the vast majority of cases for which we have no surface samples. Many of the same principles are applied. For example, if a valley is formed inside an impact crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
, the valley must be younger than the crater.
Craters themselves are highly useful in relative dating; as a general rule, the younger a planetary surface is, the fewer craters it has. If long-term cratering rates are known to enough precision, crude absolute dates can be applied based on craters alone; however, cratering rates outside the Earth-Moon system are poorly known.(Hartmann, 258)