Geological Time Scale


The earth is believed to have formed ~4.6 billion years ago and the earliest forms of life are thought to date back to ~3.5 billion years ago

To describe the geology and history of life on earth, scientists have developed the geological time scale


Geological Time Scale

The geological time scale measures time on a scale involving four main units:

  • An epoch is the smallest unit of time on the scale and encompasses a period of millions of years
  • Chronologically, epochs are clumped together into larger units called periods
  • Periods are combined to make subdivisions called eras
  • An eon is the largest period of geological time


The division of time units in the geological time scale is usually based on the occurrence of significant geological events (e.g. mass extinctions)

As such, the geological time categories do not usually consist of a uniform length of time


Relative Dating

Relative dating uses geological evidence to assign comparative ages of fossils

  • The earth is arranged into sedimentary layers (stratification), with younger stratum sitting on top of older stratum (law of superposition)
  • The relative age and evolutionary history of fossils can be traced by mapping the relative ages of the strata in which different fossils are found
  • Occasionally, a strata representing a particular time period may be missing or appear different due to environmental conditions at the time (e.g. flooding, erosion, etc.)
  • Index fossils are remains that represent short lived species that only appear in a specific period within the geological time scale
  • The presence of index fossils can be used to synchronise the age of rock layers between two different regions


Relative Dating with Index Fossils


Absolute Dating

Absolute dating uses radiometric data analysis to determine more exact ages

It involves comparing the ratio of radioactive isotopes in rock samples or fossils to that found in the atmosphere

Radioactive isotopes decay at a constant rate and the time taken for half the original radioisotope to decay is known as the half life

Different radioisotopes have different half lives and are thus useful for dating different types of fossilised remains


Absolute Dating Based on the Decay Curve for a Radioisotope


Short Range Dating

  • All living things are built from carbon-based organic matter
  • While alive, this carbon content exists as a mixture of two isotopes - C12 (stable) and C14 (radioactive) - maintained in constant proportions 
  • When an organism dies, the proportion of radioactive C14 begins to decrease as it is no longer being replaced
  • The proportion of C14 remaining can be used to identify the age of a sample
  • Carbon-14 analysis is only an effective means of dating for sample up to ~50,000 years of age as it has a half life of only 5,730 years


Long Range Dating

  • Longer range dating can be accomplished by dating the rocks around a fossil to determine an age range (i.e. relative dating)
  • These dating techniques can only be undertaken on igneous rocks, not on the fossils themselves or the sedimentary rock in which they are found
  • Some examples of possible radioactive isotopes that may be used for long range dating include:
    • Uranium-238 breaks down into Lead-207  (half life = 704 million years)
    • Potassium-40 breaks down into Argon-40  (half life = 1,300 million years)


Electron Spin Resonance 

  • Electron spin resonance (ESR) is a useful way of dating organic samples aged between 50,000 - 500,000 years old
  • When objects are buried they are bombarded by radiation from the soil, causing electrons to move to a higher energy state, where they remain
  • This 'electron clock' can be used to date fossils, as the number of high energy electrons can be used to determine when the sample was buried