correlation using relative ages

matching up rock layers

Superposition and cross-cutting are helpful when rocks are touching one another and lateral continuity helps match up rock layers that are nearby. To match up rocks that are further apart we need the process of correlation. How do geologists correlate rock layers that are separated by greater distances? There are three kinds of clues:

key beds

  1. A key bed can be used like an index fossil since a key bed is a distinctive layer of rock that can be recognized across a large area. A volcanic ash unit could be a good key bed. One famous key bed is the clay layer at the boundary between the Cretaceous Period and the Tertiary Period, the time that the dinosaurs went extinct (Figure in asteroids. In 1980, the father-son team of Luis and Walter Alvarez proposed that a huge asteroid struck Earth 66 million years ago and caused the mass extinction. Click image to the left or use the URL below. URL: Click image to the left or use the URL below. URL:


index fossils

  1. Two separated rock units with the same index fossil are of very similar age. What traits do you think an index fossil should have? To become an index fossil the organism must have (1) been widespread so that it is useful for identifying rock layers over large areas and (2) existed for a relatively brief period of time so that the approximate age of the rock layer is immediately known. Many fossils may qualify as index fossils (Figure below). Ammonites, trilobites, and graptolites are often used as index fossils. Microfossils, which are fossils of microscopic organisms, are also useful index fossils. Fossils of animals that drifted in the upper layers of the ocean are particularly useful as index fossils, since they may be distributed over very large areas. A biostratigraphic unit, or biozone, is a geological rock layer that is defined by a single index fossil or a fossil assemblage. A biozone can also be used to identify rock layers across distances. The famous White Cliffs of Dover in southwest England can be matched to similar white cliffs in Denmark and Germany.


distinctive rock formations

  1. Distinctive rock formations may be recognizable across large regions (Figure 1.1).

instructional diagrams

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a thin rock unit with a high concentration of a very rare element in it found all over the globe could be the same unit.

-->  a) true

b) false

an index fossil should

a) be widespread.

b) have existed for only a brief period of time.

c) be identifiable.

-->  d) all of the above.

to determine that similar rock layers spread over a large area are actually the same rock unit, try to identify

-->  a) an index fossil.

b) the rock type.

c) the rocks relative age.

d) how the rock formed.

the only useful index fossils are shells and skeletons of marine organisms.

a) true

-->  b) false

a key bed must

a) contain one or more index fossils.

-->  b) be very distinctive.

c) must be found only in a limited area.

d) all of these.

which of the following is good for correlation?

a) a volcanic ash because it may be spread around the globe.

b) a microfossil that can be spread over the ocean surface and then in the seafloor sediments.

c) a distinctive fossil assemblage.

-->  d) all of these.

a good example of a key bed is the thin clay with high iridium that indicates that a huge asteroid struck earth 66 million years ago.

-->  a) true

b) false

the thin clay with a high concentration of iridium correlates with

a) the extinction of 95% of all earths life at the end of the permian.

b) a massive volcanic eruption that was triggered by the asteroid impact.

-->  c) the extinction of the dinosaurs and other organisms at the end of the cretaceous.

d) the beginning of life on earth, which was triggered by the asteroid impact.

diagram questions

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