Earth Science
JANUARY 08, 2024
How are fossils
used in
determining the
age of layers?
FOSSILS
• Fossils are usually found in sedimentary
rocks, which are usually found in multiple
layers.
• These multiple layers allow for the
determination of age through relative
dating.
• Methods in relative dating use the principles
of stratigraphy in order to determine the
age through rock sequences
The two main
methods of
geologic dating
are relative and
absolute dating.
RELATIVE DATING
•Relative dating gives an
estimate of the age of a rock
layer based on surrounding
rock layers.
ABSOLUTE DATING
•Gives a numerical estimate of the age of a
rock layer.
This uses methods involving the following:
1. Isotopes are atoms of the same elements but have a different
number of neutrons.
2. Radioactive decay refers to the spontaneous degradation of
the nuclei in a process called radioactivity. This breaking apart
produces daughter isotopes.
3. The half-life is the time it takes for half the isotope’s amount to
decay.
Group
Activity
15-minutes Activity
1.
2.
Group the students into 2 big groups. One group will be assigned to relative
dating, and the other to absolute dating.
Each group will be further subdivided into the following:
a. process;
3.
4.
5.
b. definition; and
c. Applications.
Write the words “relative dating” on one side of the board, and”absolute
dating” on the opposite side.
Each big group will be given 5 minutes to discuss what to write on the board.
Their answers should be related to what big and small group they belong to. For
example, the small group “applications” under the big group “relative dating”
should think of answers related to the applications of relative dating.
All answers will be written on the board.
Guide Questions:
1. What connections were made? Answers vary.
2. What are the differences between relative and absolute
dating? Relative dating gives an estimate of age based
on sequences, and absolute dating gives a numerical
estimate.
3. What is the importance of using both of them? These
two methods can be used side-by-side since they can
give important insights that one method alone cannot
give.
Geologic Time
• Used to divide Earth’s history
into different periods using
information gathered from rock
layers.
Geologic
Time
• This information is gathered
through the laws of stratigraphy
and the use of fossils as markers
for geologic time.
• Tracking geologic time can give
a glimpse into Earth’s past.
• i. Rock layers usually contain
information about past organisms
and environmental conditions in
the past.
Geologic
Map
• In the late eighteenth century, French paleontologists,
Georges Cuvier, and Alexandre Brongniart together with
an English geologist, William Smith, published the first
geologic maps.
George Cuvier
(1769-1832)
• was a
French naturalist and
zoologist, sometimes
referred to as the
"founding father of
paleontology"
• French chemist, mineralogist, geologist,
paleontologist, and zoologist, who
collaborated with Georges Cuvier on a
study of the geology of the region
around Paris.
• Observing fossil content as well
as lithology in sequences, he
classified Tertiary formations and was
responsible for defining 19th century
geological studies as a subject of
science by assembling observations
and classifications
Alexandre
Brongniart
(1770-1847)
William Smith
• Proposed the principle of fossil
succession.
• Worked as a mine surveyor,
which allowed him to observe
rock layers across the world.
William Smith
(1769-1939)
Geologic Map
• A geologic map is a document that shows illustrations of
geologic features of an area.
• Rocks of the same age may contain similar fossils even
though they are found in different areas.
• This led to the formation of the principle of fossil
succession.
• This states that fossils succeed one another in a definite
order and any time period can be recognized by the
fossils present.
The discovery
of fossil
succession
William Smith
• English geologist
• Proposed the principle of fossil
succession.
• Worked as a mine surveyor,
which allowed him to observe
rock layers across the world.
William Smith
(1769-1939)
William Smith
• Smith’s job allowed him to observe
•
•
•
•
rock layers.
He noticed that rock layers
occurred in a pattern.
He began to suspect that these
patterns extend across England.
As a surveyor traveling 10 000
miles per year, he had an
opportunity to map strata of every
place he visited.
He noticed that rock layers which
have similar fossils may come from
the same geologic time
William Smith
(1769-1939)
The Principle of Fossil Succession
● This principle states that fossils succeed one another in a
definite order and any time period can be recognized by
the fossils present.
Fossil succession diagram
Index Fossil
• Fossils that are distributed in a wide area and exist for
limited time periods are called index fossils.
• It has to be geographically widely distributed.
1. This is so that they can be used for comparing different areas.
• It should only be available in a limited or short span of
geologic time. This is to narrow down geologic time into
specific periods.
• It needs to be distinct and easily recognizable so as to not
confuse it with other fossils. iv. It needs to be abundant so
that it can be found in multiple areas easily.
Development of
the geologic
time scale
using fossils
Index fossils serve as useful
indicators for the age of a rock
layer. This is because their
appearances indicate a specific
period in Earth’s geologic history.
• Since index fossils are short-lived, there are certain conclusions
that can be drawn when they are present in rock layers.
• Seeing them in a certain rock layer would mean that if they
were found in other rock layers in other locations, you could
conclude that these fossils occur in a one particular geologic
time.
• This is the reason why other fossils cannot be used as index
fossils. Other fossils cannot narrow down geologic time
because of being too long-lived.