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GEOLOGIC TIME 

Learning Objectives


Related sites:

Information about the Geologic Time Scale and info about each geologic era.

Information about geologic time.

USGS publication about geologic time.
 


The Earth is 4.6 billion years old.  It is difficult to imagine the length of geologic time but geologists have tried to date events and order them chronologically.  Geologic time can be referred to as absolute and relative.

Absolute time is the determination of an absolute age in years before present through the use of radiometric dating techniques.

Absolute Dating:
 
Absolute dating (radiometric dating) uses the decay ratio of radioactive isotopes found in rocks.  Radioactive isotopes spontaneously decay to a more stable form.  These isotopes decay at a constant rate and are not affected by chemical changes or weathering.

Half-life: the time required for one half of the isotope to decay into the "daughter" product.  If the rate of decay is known then by measuring the ratio of daughter product to original isotope the age of a rock can be determined.

Radiometric dates have been assigned to the Geologic Time Scale but the time scale was developed using the appearance and disappearance of fossil assemblages.
 

Relative Dating

Relative time is the determination of age through the ordering of events.  There is no age in years before present but rather the idea of "this is older than that."  Relative time is based on the Laws of Geology.


Geochronology: the study of time in Geology.

Geochronologic units: units of geologic time includes eons, eras, periods and epochs.  Divisions
are based on the fossil record and each finer division of the geologic record  is based on more
subtle differences in the fossil record.
 

EONS: longest unit of geologic time, not commonly used
 

    Precambrian: term used for time between 4.6 billion and 544 million years ago,
    comprises 87% of all geologic time.  Divided into 3 eons:

    Hadean: no rocks from this time.   4.6 - 3.9 billion years ago
    Archean: oldest rocks from this time.  3.9 - 2.5 billion years ago
    Proterozoic: 2.5 billion - 544 million years ago

     Phanerozoic: (visible life) from 544 million years ago - present

 

Eras: divisions based on profound differences in fossil assemblages, each era is divided into
Periods: based on more subtle differences in fossil life, Cenozoic is further divided into
Epochs

 Paleozoic Era: a period of rapid evolutionary change and development.  544 - 66 mya,
 divided into 7 periods:

      Cambrian: 544-505 mya
      Ordovician: 505 - 441 mya
      Silurian: 441 - 418 mya
      Devonian: 418 - 362 mya
      Mississippian: 362 - 320 mya
      Pennsylvanian: 320 - 286 mya
      Permian: 286 - 245 mya
     
 Mesozoic Era: reptiles are dominant biologic group, “Age of Dinosaurs”, 245 - 66 mya,
 divided into 3 periods:
      Triassic: 245 - 208 mya
      Jurassic: 208 - 144 mya
      Cretaceous: 144 - 66 mya
     
 Cenozoic Era: mammals are dominant biologic group, “Age of Mammals”,
  66mya - present, divided into two periods:
      Tertiary: 66 - 2 mya
      Quarternary: 2 mya - present
     
Chronostratigraphic Units: actual rock units that formed during a particular time and
correspond to a geostratigraphic unit.
 

Geologists ordered time based on the fossil record.  A type section refers to a standard section of
rock units containing the characteristic fossils for a particular geologic time.  Period names are
derived from the geologic locale that contains the type section.

The Geologic Time Scale was developed by comparing fossil assemblages and ordering them
based on relative dates.  NO ACTUAL DATES were applied to the Geologic Time Scale until
radiometric dating techniques were developed many years after the time scale was established.