Geochronometry

Key takeaways

Geochronometry belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Geochronometry to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Geochronometry from memory before moving on to harder problems.

Reference excerpt

Geochronometry is a branch of stratigraphy aimed at the quantitative measurement of geologic time. It is considered a branch of geochronology.

Brief history The measurement of geologic time is a long-standing problem of geology. When geology was at its beginnings, a major problem for stratigraphers was to find a reliable method for the measurement of time. In the eighteenth century, and during most of the nineteenth century, the ideas on the geologic time were indeed so controversial that the estimates for the age of the Earth encompassed the whole range from ca. 6000 years to 300 million years. The longer estimate came from Charles Darwin, who probably went closer to the truth because he had clear in mind that the evolution of life must have required a lot of time to take place. The current estimate of the age of the Earth is ca. 4500 million years. The solution of the dating problem arrived only with the discovery that some natural elements undergo a continuous decay. This led to the first radiometric datings by Boltwood and Strutt. Today, the determination of the age of the Earth is not a primary scope of geochronometry anymore, and most efforts are rather aimed at obtaining increasingly precise radiometric datings. At the same time, other methods for the measurement of time were developed, so the quantification of geologic time can now be endeavored with a variety of approaches.

Radiometric dating All methods based on the radioactive decay belong to this category. The principle at the base of radiometric dating is that natural unstable isotopes, called 'parent isotopes', decay to some isotope which is instead stable, called the 'daughter isotope'. Under the assumptions that: (1) the initial amount of parent and daughter isotopes can be estimated, and (2) after the geologic material formed, parent and daughter isotopes did not escape the system, the age of the material can be obtained from the measurement of isotope concentrations, through the laws of radioactive decay. Methods of this kind are usually identified with the names of the parent/daughter elements. The radiometric methods under this category are:

U/Pb U/Th K-Ar (and Ar-Ar) Rb/Sr Sm/Nd Re/Os Lu/Hf Each of these methods perform better in different time ranges and has different limitations. However, uranium–lead dating on zircon and Argon-argon dating on sanidine and hornblende are the two single methods that achieve today the best results. Other methods of radiometric dating are also available, that are based on slightly or largely different principles, but always rely on the phenomenon of radioactive decay. These alternative radiometric methods are:

14C, or radiocarbon Fission track dating Optical luminescence dating and Thermoluminescence dating Cosmic ray exposure dating These methods, especially radiocarbon, are particularly reliable for recent samples, but are much less accurate for deep geologic time. More specifically, radiocarbon becomes unreliable already for samples >50000 years old.

Incremental dating These methods are based on the building of incremental chronologies from a point of known age, which is usually the present. When a chronology is not tied to such a known age point, it is called a floating chronology. Incremental dating methods include:

Dendrochronology Lichenometry Sclerochronology in mollusc shells or coral skeletons Varve chronology (e.g., in lake sediments or ice cores) Lamina counting in speleothems Cyclostratigraphy based on Milankovitch cycles

Geologic time scale A major achievement of geochronometry is the documentation of geologic time, as represented in geologic time scales. A geologic time scale is a scheme that integrates the geochronologic subdivisions of geologic time and their absolute ages and durations. The latest version of the geologic time scale was published in 2004, and includes a comparison of present and past time scales. The greater efforts of geochronometry today are aimed at retrieving accurate ages of major events in the Earth's history and of stage/age boundaries.

See also Biostratigraphy Chronostratigraphy Hydrology Palynology

References

External links International Commission on Stratigraphy EarthTime project This article incorporates material from the Citizendium article "Geochronometry", which is licensed under the Creative Commons Attribution-ShareAlike 3.0 Unported License but not under the GFDL.

Worked examples

Example 1 — a first encounter with Geochronometry

Start with the simplest possible case. Write down what Geochronometry claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Geochronometry before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Geochronometry ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Geochronometry

In research: Geochronometry appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Geochronometry in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Geochronometry is common in secondary-school and first-year university syllabi. It links to neighbouring topics Geochronological dating methods, Stratigraphy, so understanding it makes those chapters shorter.
In everyday life: Look for Geochronometry outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Geochronometry in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Geochronometry means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Geochronometry out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Geochronometry in simple terms?

Geochronometry is a branch of stratigraphy aimed at the quantitative measurement of geologic time. It is considered a branch of geochronology.

Why does Geochronometry matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Geochronometry?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Geochronometry.