Science Focus Topic 9 Notes: Geologic Time
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The principle of superposition states that in undisturbed layers of rock, the oldest
layers are always on the bottom and the youngest
layers are always on the top.
As new layers of sediment form sedimentary rock, the layers can be identified. This layering is called strata.
Geologists use a technique called relative dating, to find the order in which events occurred. The relative age of the rock is determined by its position
within the strata. Fossils found in a layer can help to identify the age of the rock. If the fossil was on the
Earth for a short time and widespread then it is called an index fossil.
Clues from Technology
- Over billions of years, some elements will change into other elements - uranium
is such an element - in 4.5 billion years, half of the uranium will change into lead (which will not change). The
uranium is called the parent element.
This time period is called the half-life
- By measuring the amounts of change in a sample, scientist can clculate the absolute age of the rock. This is
called Radiometric Dating
- Scientists also use a process called radiocarbon dating (which uses carbon-14, a rare form of carbon, as its
All organisms take in carbon-14 to build cells and tissue. The carbon-14 changes to nitrogen gas (when the animal
dies) in a half-life of 5730 years. The amount of carbon-14 left in the tissue allows scientists to determine the
age of the remains
Geological Time Scale
The geological time scale is a division of Earth's history into smaller units based on the appearances of different
(see Figure 5.87, p. 426)
The largest divisions are called eons,
which are divided into eras and then
further divided into periods.
Rodinia (Figure 5.85) was the
first supercontinent and Pangaea
(Figure 5.86) was the second supercontinent (Ref. p. 425)