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Correlation. What is the history of the Earth?. Unfortunately there is no place on the earth where rocks were formed and preserved for the entire length of the Earth’s history. As we have learned rocks are formed, changed, and disappear.

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what is the history of the earth
What is the history of the Earth?
  • Unfortunately there is no place on the earth where rocks were formed and preserved for the entire length of the Earth’s history.
  • As we have learned rocks are formed, changed, and disappear.
  • The rock record of the earth is incomplete and contains frequent gaps.
  • Geologists combine evidence from outcrops from many locations on Earth. This combining of information relies on the process of correlation.
  • Correlation is the process of matching rocks of similar age and characteristics across great distances.

Correlating a rock type exposed on one side of a valley to a rock exposure on the other side of the valley is a simple correlation.

  • Over a much larger area correlation is a more difficult

Types of correlation;

    • Time
    • Lithology
  • Time correlation means age equivalency. Two rock maybe the same age but not lithology.
    • For example two stones, sandstone and shale may have been deposited at the same time, but in different environments
  • Lithological correlation is the correlation of rock type, which may not necessarily mean age equivalency.
    • For an example here the sandstone may be correlated from one area to another, but this sandstone may have been deposited on a beach environment at different times.

If rock exposures are numerous in a particular area, then the correlation can be a simple trace, such as the Principle of Lateral Continuity.

  • If exposures are not available then other criteria for matching are used. The two most important pieces of information used by geologist in correlation include index fossils and key beds.
  • A fossil is the remains or traces of past life in a sedimentary rock.
  • For a fossil to be formed, a dead organism must be buried rapidly and possess hard parts. Given time, the hard parts modify to become a fossil. The process of fossilization can occur in several ways.
    • Petrification-turning into stone, is the process by which the small internal cavities in the original structure of the organism become filled with precipitated minerals material, like calcite or silica

Replacement- can occur when some material of the dead organism is removed and replaced with mineral matter. A mould is created when a hard part, like a shell is buried in sediment and subsequently dissolved by water, leaving a mould behind. A cast is created if the mould becomes filled with mineral matter.

  • Trace fossils – these clues from animals such as footprints left behind in soft sediment that later became sedimentary rocks.
  • Amber preservation – when insects become trapped in the resin of ancient trees.
index fossil
Index Fossil
  • An index fossil is a very unique fossil that indicates the age of the sedimentary rock in which it lies. In order for an index fossil to be useful it must meet the following criteria:
    • Produced by an organism (plant or animal) that is abundantly preserved in the rock record
    • Was geographically widespread
    • Existed as a specific species for a relatively short time (geographically speaking)
    • Easy to identify in the field
  • Index fossils are very distinct and represent short periods of geologic time. These can be used over great distances to correlate sequences even though the rock exposure may not display lithological similarities
key bed
Key Bed
  • A key bed is a thin widespread sedimentary layer that was deposited quite rapidly.
    • An example would be volcanic ash deposit which gets carried over a wide area by wind. As the deposit occur at the same time.
    • Coal beds are also useful key beds