What is the process of determining a specific age of an object?

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When a piece of bone, spearhead or rock is dug from the ground, a long process begins to work out where it came from and how it got there.

An important part of this investigation is determining the age of the specimen, for which there is a wide array of dating methods available. 

Absolute dating

If you want to know the precise age of something, absolute dating techniques are the only option. They work by analysing the activity of elements and their decay over time.

An element is defined by the number of protons in an atom’s nucleus. Neutrons can also be added or removed from the nucleus; these change the mass of the atom and produce different isotopes.

This neutron addition or subtraction can also make the isotope unstable. If this is the case, a proton or a neutron can be released as the atom rearranges itself into a more stable isotope.

This produces radiation and is particularly prominent with larger atoms that are easily able to fall apart spontaneously, leading to new elements or a lighter form of the original element.

The time it takes for half of the atoms of an element in a sample to decay is known has its half-life. Depending on the isotope, this can range from milliseconds to billions of years. But the rate is constant, so as long as the original isotope is not replenished, the ratio of isotopes in a sample can tell you its age.

Carbon-14, an isotope of the common carbon-12, has a half-life of around 5,730 years.

It’s found throughout the food chain – it’s taken up by plants for photosynthesis, then eaten by herbivores which are, in turn, eaten by carnivores – so is usually used to date samples which were once alive, from woolly mammoths to Egyptian mummies.

The more carbon-14, the younger the specimen, while a higher proportion of carbon-12 indicates an older specimen.

Carbon-14 atoms are counted using a method called Accelerator Mass Spectrometry. Credit: PAWEL SYTNIEWSKI / OXFORD UNIVERSITY IMAGES / SCIENCE PHOTO LIBRARY / Getty Images

It starts to fail for objects around  50,000 years old, because by then 99.8% carbon-14 will have decayed with the remaining fraction too small to reliably detect. But other isotopes with a longer half-life can be used. 

Moving into 100,000-year timescales and beyond, argon and potassium are often used to uncover the age of ancient bones and geological formations.

Potassium-40 is an isotope with a half-life of 1.28 billion years that decays into argon-40.

Traditionally scientists compared the ratio of argon and potassium but samples had to be split to measure each, increasing the chance of an error.

So, using a nuclear reactor, a proton is knocked out of stable potassium-39 atoms, turning them into radioactive argon-39. Then the argon-40/ argon-39 ratio is measured from the one sample.

Other isotope combinations used in dating include samarium-neodymium, rubidium-strontium and uranium-lead.

Where isotope analysis is not suitable, scientists can also use optically stimulated luminescence dating. This technique measures when the sediments surrounding the specimen were last exposed to light. 

A laser is used to detect energy released from a slight electrical charge accumulated in a crystal, formed as thousands of years of local radiation knocked electrons in the crystal out of place.

For samples billions of years old, fission tracks can be analysed. These are microscopic formations often found within extremely hardy zircon minerals, created by the spontaneous decay of uranium-238 – the same material found in nuclear reactors.

When the uranium decays it shoots off particles that punch a hole of sorts in the zircon and leave a small trail.

These are preserved in the mineral and can be seen through a high-power optical microscope. The density of these marks provides an estimate of the sample’s age.

Ammonite fossils can give palaeontologists a rough idea of how old those rock layers are. Credit: Chris Howes / Getty Images

Relative dating

This type of dating works on the principle that an object’s age can be determined by analysing the surrounding geology.

These processes are not super precise but act as an easy first-line dating method that can later be confirmed by absolute methods.

Stratigraphy relies on the fact that below you are multitudes of geological layers, each made from different rock types that tell a detailed story of the history of the region.

If the age of the layers above and below a rock layer are known, it’s possible to “relatively” date the rock. Any formations and fossils found within the middle layer are then, in theory, is younger than the layer below and older than the layer above.

Biostratigraphy takes this one step further, analysing the fossils found within each layer.

If a particular fossil found in a rock layer comes from a known period, it can give an indication of the age of the rock layer and other fossils found nearby. These fossils are known as “index fossils” and include trilobites and ammonites.

Looking at the type of rock or organisms found within a formation is not the only way a relative age can be deduced. The earth’s magnetic field is also used in palaeomagnetic stratigraphy.

Iron is commonly found in geological formations. As hot rock pours onto the Earth’s surface or underwater, iron minerals align with the Earth’s magnetic field at the time. They lock in place as the rock cools.

According to NASA, the Earth’s magnetic field has reversed on average once every 250,000 years over the past 20 million years. Comparing data on known alignments from other sites with newly found formations can give an indication of a rock’s age.

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The age of a rock is determined by stratigraphy, a branch in geology which studies the chronology of events and changes, along with the development of organisms, which have determined the development of the Earth from when it became an independent spatial body until today. The age, or the chronology of geological creations and events is determined using relative and absolute age.

In determining the relative age of a rock, the data from sedimentary rocks are generally used. Relative age of magmatic and metamorphic rocks is determined according to their relation with sedimentary rocks.

Determining the relative age of a rock

The determination of the relative age of a rock is based on the principle of original horizontality of the sediments, principle of superposition, principle of original lateral continuity, principle of cross-cutting relationships, principle of inclusions, principle of biological succession and the lithology of a rock.

• The principle of original horizontality departs from the assumption that most of the sedimentary rocks are deposited under the action of gravity, in approximately horizontal layers, i.e. parallel to the surface to which they deposit.
• The principle of superposition is based on the assumption that, in a regular sequence of layers, the oldest layer will be on the bottom of the sequence, while all the other layers are successively more recent.

Photo 1. The principle of superposition

• The principle of original lateral continuity states that layers of sediment extend in all directions until they become thinner or until they are interrupted by an obstacle.

Photo 2. The principle of lateral continuity

• The principle of cross-cutting relationships states that a geological object (magmatic intrusion) cutting other rocks must be younger of the two features. Moreover, the faults (cracks along which a shift of rocks from one side to the other is visible) are younger than the rocks they cut (fault).

Photo 3. The principle of cross-cutting relations

• The principle of inclusion states that each rock containing inclusions of a neighbouring rock must be younger than that rock

The lithology, such as colour, cut, chemical composition, degree of diagenesis, can be used for comparing and determining relative age of a rock, but with the assumption that equal or similar rocks are also contemporary.

By determining the age of a rock, only its chronology is defined. What does this data say about the time of its origin? In order to answer this question, fossils are of crucial importance.

The paleontological method is based on the study and interpretation of animal and vegetal organisms’ evolution during geological history, and in order to determine the relative age of a rock, the principle of superposition is used as a starting point, as well as fossil deposits preserved within a rock. This method has been developed as part of paleontology – science concerned with fossils and development of life through geological history.

Photo 4. The paleontological method

In defining the order of deposits by relative dating, only the order of deposits has been determined, i.e. the events in Earth’s history (what came first), but not the exact time when those events took place. For the determination of the “exact” time when certain rocks appeared, it was the beginning of the 20th century, i.e. the discovery of radioactivity that gave to the geologists a “clock” which helped them to define it.

Determining the absolute age of a rock

The determination of absolute (radiometric) age of a rock is based on the radioactive decay of isotopes. Radioactive elements emit α and β particles, as well as γ rays, thus causing their mass to reduce over time, shifting eventually to stabile isotopes. The final stabile product (isotope) can be compared in quantity to the original radioactive element. This is used for evaluation of the age of a rock using the so called radiometric methods

In determining the absolute age of a rock different methods are used Uranium – Lead, Uranium – Thorium, Potassium – Argon, Rubidium – Strontium, Carbon “Fission Tracks” and Thermoluminescence.

Table 1.Time of half-decay radioactive elements

In determining the chronology of rocks and facies of all marine, mixed and continental sediments, the fluctuations and changes in the order of land and sea in Earth’s past are followed. The stratigraphist obtains exceptional information by “reading” the facies and the data regarding its chronology; he reconstructs spatial and chronological events (dynamics) on Earth during its history, i.e. from its beginnings to nowadays.