Time is an important element in all
geological processes.
Geologists in 19th century recognised that Earth had experience many
episodes of mountain
building and erosion, which required great spans of
geologic time.
Earth is very old. But how old?
Before the advancement of
radiometric dating, geologic events are based on relative dating principles.
The age of the Earth and its
inhabitants are now determined through two complementary lines of evidence:
relative dating (stratigraphic age) and absolute dating (radiometric age)
Relative dating places fossils in a
temporal sequence by noting their positions in layers of
rocks, known as
strata. As shown in the diagram, fossils found in lower strata were typically
deposited first and are deemed to be older.
By studying and comparing strata
from all over the world we can learn which came first and which
came next, but
we need further evidence to ascertain the specific, or numerical, ages of
fossils.
Absolute dating relies on the decay
of radioactive elements that gives the actual number of years
that have passed
since an event occurred. By dating volcanic ash layers both above and below a
fossil-bearing layer, as shown in the diagram, you can determine “older than X,
but younger than Y”
dates for the fossils.
Geologists have assembled a
geological time scale on the basis of numerical dating of rocks from
around the
world.
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