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Geological Time

| Geologic Time Scale | Plate Tectonics | Radiometric Dating | Deep Time | Geological History of New Zealand |

Deep Time

Geologic history is often referred to as "deep time," and it's a concept perhaps as difficult to conceive as "deep space".

Time in geological terms has been described in two different ways: relative time and absolute time.

Relative Time

Relative time is the sequence of events without consideration of the amount of time. Relative time looks at the succession of layers of rock to attribute them to certain geological events. Relative time was determined long before absolute time. Index fossils are often used to determine a specific era. Sedimentary rocks naturally form horizontal layers (strata, singular stratum). These strata allows geologists to determine relative time (that is, sequence of deposition of each layer, and thus the relative age of the fossils in each layer).

There are 5 important principles or laws which govern relative time:
  • Law of Superposition: in any sequence of strata that has not been overturned the topmost layer is always the youngest and the lowermost layer the oldest.
  • Law of Original Horizontality: all sedimentary rocks were originally laid down in a more or less horizontal attitude.
  • Law of Faunal and Floral Succession: Fossil organisms succeed one another in a definite recognisable order so that the same fossil assemblages are similar in age.
  • Law of Cross-cutting Relationships: any structure (fold, fault, weathering surface, igneous rock intrusion, etc.) which cuts across or otherwise deforms strata must be younger than the rocks and structures it cuts across or deforms.
  • Principle of included fragments: In principle particles are older than rock masses in which they are included.

Absolute Time

Absolute time is sometimes also called "numerical time". It dates durations of events in terms of seconds, years, millions of years, etc. Although the Geologic Column was developed as a relative time scale, geologists wanted to figure out the numerical age dates for Era-Era boundaries and other events.

Various techniques are applied to determine absolute time.

  • Radiometric dating: Through the discovery of radioactivity the rate of decay (breakdown of radioactive elements) can be used to date minerals and rocks.
  • Marker Beds: Volcanic eruptions like the eruptions of the Taupo Caldera disperse layers of ash over vast areas and can be used to pinpoint geological events.
  • Magnetostratigraphy: This is the use of magnetized rocks to determine the history of events in record of changes in earth's magnetic field during the past geologic ages.

The most effective approach in getting age dates is to combine multiple techniques. First to get relative age relationships between local units, then to find index fossil ages for the sedimentary rocks and radiometric dates where possible.

Index Fossils

Fossils are called index fossils if they allow a correlation from continent to continent. Only certain types of fossils are useful for correlation. To be a good index fossil, the species should:

  • Have been very common, so chances of individual specimens being buried is good
  • Have hard parts, so chances of fossilization are good
  • Have a wide geographic range, so that correlation over a wide region is possible
  • Lived in (or could be deposited in) different environments, so can be found in different formations
  • Have some distinctive features, so it can be distinguished from closely related forms
  • Have a short geological duration (a few million years at most), so finding a fossil of the species in a rock means it had to be deposited in those few million years

Using index fossils, geologists were able to correlate across Europe, and then to other continents.

Reference Websites  Reference Websites

For more information about relative geological time check out the USGS website:

More about relative and absolute time on University of Calgary Department of Geology and Geophysic's website:

Reference Books  Reference Books

Nelson, C.S., Balks, M. R. and Chapman, R. (1999). Discovering Planet Earth: Study Guide for 0772.101. Hamilton, New Zealand: Department of Earth Sciences, The University of Waikato

Press, F. and Siever, R. (1994). Understanding Earth. W.H. Freeman and Company.

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