2. Dating of Rocks
and Geologic
Time
Cadavedo – Cagalitan – Lopez, Carl – Masungab – Avelino –
Calimbas – Damigan – Jamilo – Matonilla – Montalban –
Segovia
3. Rocks are composed of layers or
features called strata (singular,
stratum). These layers are used to
determine the age of rocks. There are
two ways to determine the age of
rocks.
5. Relative Rock Dating
This is used to determine the comparative or relative
age of the rock to each other. However, this technique
cannot give the exact age of the rock. It is only used to
arrange geological events based on rock strata. Relative
rock dating is guided by the three basic laws which is
known as Steno’s Laws, authored by Nicolaus Steno.
These laws are the bases in determining the relative ages
of rock layers or features.
7. Law of Crosscutting
This states that any
feature such as fault
that cuts across a
sequence of rocks is
younger that
everything it cuts.
8. Law of Inclusion
This states that any
rock, fragment, clast or
intrusion that is
included inside of
another rock is older
than the rock in which
it is included.
10. Absolute Rock Dating/Radiometric Rock Dating
This technique is used to determine the
absolute age of the rock. In this technique, the
decay rate of radioactive isotopes which are
embedded in the rock are measured. Isotopes are
elements with equal number of protons but
different masses. There are some types of
absolute dating.
11. Carbon Dating
Carbon-14 dating or radiocarbon dating is a good technique in
determining the absolute age of fossils found in rocks. Fossils are
remains of organisms preserved in rocks. For example, a tree which is a
living thing takes in carbon-14 and carbon-12 at the same time. When the
tree dies, the carbon-14 starts to decay and become less while the carbon-
12 is unchanged. Then the ratio of carbon-14 left to the unchanged
carbon-12 is measured. With this, the absolute age of the fossil is
determined. Carbon-14 has a half-life of 5,730 years meaning, every
5730 years, the parent isotope will produce two daughter isotopes and
these daughter isotopes will produce another daughter isotopes and so on.
12. Potassium Dating
Another example of a radioactive
isotope used in absolute rock dating
is potassium-40. It has a half-life of
1.25 x10 E9 years when it undergoes
a beta decay.
13. Uranium Dating
The oldest known crystal on earth is the zircon
crystal from Australia. It remained intact for an
incredible 4.4 billion years which is of almost
same age of the planet earth. The radioactive
isotope used to determine the absolute age of
zircon crystal was uranium which decayed into
lead atoms.
14. Younger rocks can be dated using carbon-14.
Igneous rocks contain the radioactive isotopes
which make them good for radiometric dating. In
sedimentary and metamorphic rocks, scientists
will look for the presence of igneous rocks
imbedded in them to determine their absolute
ages.
15. How Relative and
Absolute Dating
Were Used to
Determine the
Subdivisions of
Geologic Time
Dating of Rocks and
Geologic Time
16. Chrono-stratigraphy is the branch of stratigraphy that
studies the ages of rock layers or features in relation to
geologic time. Scientists use Geologic Time Scale or GTS
to explain which event on earth occurred first and which
event occurred last. This GTS is a chart that mark as a
guide on the earth’s major events in its history. The
discovery of the fossils and the oldest rocks are the bases
of these markings in the GTS.
17. Geologic Time Scale
Today, the geologic time scale is divided into
major chunks of time called eons. Eons may be
further divided into smaller chunks called eras,
and each era is divided into periods. Eon is the
largest time span and there are only two major
eons in the GTS and these are Precambrian and
Phanerozoic.
19. Precambrian Eon
Precambrian is divided by Hadean, Archean and Proterozoic
events in the GTS. The Earth’s history before the formation of
the first and oldest rock belong in this time. This time span lasted
from 4.6 billion to 541 million years ago. The rocks that belong
in this time contain half of the earth’s deposits of minerals. The
appearance of the multicellular organism also appeared in this
eon. In this eon also, the first soft bodied invertebrates appeared
and the first bacteria and blue green algae begin to free oxygen to
the atmosphere.
22. Phanerozoic Eon
An eon is subdivided into eras. The
Phanerozoic eon for example, is subdivided
into 3 major eras. These 3 eras are Paleozoic
era which means ancient life; Mesozoic era
which means middle life and Cenozoic era
which means present life.
23. An era is subdivided into Periods. For example, in
the Paleozoic era, there are 6 periods, namely: Permian,
Carboniferous, Devonian, Silurian, Ordovician and
Cambrian periods. Permian period is known as the “age
of amphibians”; Silurian period is where the first insects
appeared; jawed fish and vascular plants also appeared
in this period; corals first appeared in Ordovician period
while the Cambrian period is the age of trilobites.
24. The Mesozoic era has 3 periods and these
are Triassic wherein dinosaurs are dominant in
this period; Jurassic period wherein birds first
appeared in this time and Cretaceous period
which is the mark of the extinction of
dinosaurs.
25. In the Cenozoic era of the Phanerozoic eon, the ice age occurred.
A Period is further subdivided into Epochs. An epoch is a time scale
which is longer than an age but shorter than the period. In the
Carboniferous period, there are two epochs, namely: Pennsylvanian
and Mississippian. An epoch is subdivided into Ages. The
Mississippian epoch has 3 ages. These are Tournaisian, Visean and
Serpukhovian. Triassic, Jurassic and cretaceous periods of Mesozoic
era is the “age of Reptiles”; Devonian period part of the Paleozoic era
is the “age of fishes” while Cenozoic era is the “age of mammals”.
29. Geologic Time Scale
One of the first scientists to understand geologic time was
James Hutton. In the late 1700s, he traveled around Great Britain
and studied sedimentary rocks and their fossils. He believed that
the same processes that work on Earth today formed the rocks
and fossils from the past. Before Hutton, most people believed
the Earth was only several thousand years old. His work helped
us understand that the laws of nature never change and that the
Earth is very old. He is sometimes called the “father of geology”.
30.
31.
32. How Relative and
Absolute Dating were
Used to Determine
the Subdivisions of
Geologic Time
Dating of Rocks and
Geologic Time
33. In order to identify the ages of the rocks and
fossils found in our planet through exhumation,
scientists use radiometric dating which uses the
radioactive isotopes present in the exhumed
artefacts. After determining the age of the artefact,
the age is compared to the geologic time scale
above.
34. Example
For example, in the fossil
record, the trilobites were
dated to some 540 to 488
million years ago and they
belong to the Cambrian
period. Trilobites are the
relatives of the present day
crabs and lobsters.
35. Example
The absolute age of this young
rock could be identified by
Carbon-14 dating. Once the
absolute age has been computed
using the radioactive isotope
present in the rock, the scientists
will then compare its age with
the Geologic Time Scale to tell
which period it belong.
36. Summary
• The Earth is very old, and the study of Earth’s past requires us to think about times
that were millions or even billions of years ago. Scientists use the geologic time
scale to illustrate the order in which events on Earth have happened.
• The geologic time scale was developed after scientists observed changes in the
fossils going from oldest to youngest sedimentary rocks. They used relative dating
to divide Earth’s past in several chunks of time when similar organisms were on
Earth.
• Later, scientists used absolute dating to determine the actual number of years ago
that events happened. The geologic time scale is divided into eons, eras, periods,
and epochs.
37. Summary
• Relative dating is a technique used to tell the geologic events in rock.
• Absolute dating uses Carbon-14, Potassium-40 and Uranium- 238 and
235.
• The absolute age of the rock is determined only using the radiometric
dating.
• Radiometric dating makes use the radioactive isotopes present in rocks.
• Once the absolute age of rock is measured, it is then compared to a scale
called Geologic Time Scale to identify the time the rock was formed.
