Geological Time
The Earth is approximately 4.6 billion years old. Scientists have
divided this geological time scale into eons, eras, periods, and
epochs. Eons represent the greatest time. Eons are divided into
eras. Each era is subdivided into periods. Finally periods are
divided into still smaller units called epochs.
Below is the Geological Time Scale.
The geologic time scale is a visual record or Earth’s history, with
the individual units based on changes in the rocks and fossils.
Notice that the geological time scale before 650 million years
doesn’t have epochs but has a super eon called the Precambrian
which includes 3 eons. This is because Precambrian time, there
were fewer life forms. These life forms are more difficult to
identify and the rocks have been disturbed often.
Measuring The Age Of Earth And The Events That Shaped It
How understanding the principles of rock layers help to
determine the age of rocks.
In geology the principal laws are:
 Principle of Uniformitarianism: Processes that are
happening today also happened in the
past__________________.
 Principle of Superposition – The rock on the bottom are
older than the rock on the top
 Principle of Cross-Cutting Relations: A rock is
__________ younger than any rock it cuts across.
In the above picture a vertical column of magma called a
dike cooled into igneous rock. The magma that cooled into
igneous rock that cut across the other three layers of rock is
younger than the three other layers.
Looking at the picture to the right
arrange rock layers A, B, C, E, F, G, H, J, K in order from the
oldest to the youngest. Ignore letters D and I.
Rock layers from the oldest to the youngest:
_J,K,F,H,B,G,E,C,A________________________________
Rock layer D (red) going into layers A and B is younger than
A and B but older then ____C_, ___F___, ___G and
____H___. Rock layer D represents a pocket of magma that
traveled up from beneath layers A and B which hardened into
igneous rock.
Order the rock layers in order from the oldest to youngest.
_________________________________________________
_________________________________________________
__________________________________
 Unconformity: Is a place where rock layers are missing.
After a rock layer is formed, the area can be uplifted to the
Earth’s surface (uplifting can be caused by earthquakes and
plate tectonics). After, the process of weathering and erosion
can wear the rock layer away. Rock layers often occur above
the unconformity, but they are not the kind of rock that
would have formed in the same way as the rock layer beneath
the unconformity.
Example:
Order the rock layers in order from the oldest to youngest.
_______________________________________________
Order the rock layers in order
from the oldest to youngest.
_______________________________________________
Two Types of Dating
Relative Age Dating: Is a way to describe the age of one
event compared to another object or event. Relative age
dating is based on comparisons of the age of objects. Thus,
this method of dating objects always includes words such as
before, after, earlier, later, older, and younger.
Example using the principal of superposition:
If you found an arrow head in one sedimentary layer of rock
and a saber tooth tiger skull in a lower layer of rock, you can
use the law of superposition to conclude that the skull is older
than the arrow head.
Absolute Age: This describes the actual age of an object or
event. The absolute ages of objects from long ago are found
by analyzing the chemicals in the object or the rock layers in
which they were found.
Radiometric Dating – This is the most accurate form of
dating. This method measures the decay (decrease in size
and/or energy) of naturally occurring radioactive isotopes.
Example: Carbon-14 Dating (The 14 is the total number of
protons and neutrons in the nucleus of the atom)
Most atoms of carbon have 6 protons and 6 neutrons in the
nucleus of atom. A very few carbon atoms have a different
number of neutrons and are called isotopes. In conclusion,
an isotope is an atom of the same element with a different
amount of neutrons in the nucleus. Carbon-14 is an isotope
of carbon and is radioactive. Radioactive describes an
element that gives off tiny particles and energy from
inside its atom.
This means that it tends to give off particles from its nucleus.
Above picture show the decay of cacbon-14 to nitrogen-14.
The antineutrino will destroy itself and give off energy.
The carbon-14 atom changes into an atom of nitrogen-14,
which is not radioactive but stable. This change is called
radioactive decay. Only about 1 in a 1,000,000,000,000
(trillion) carbon atoms are carbon-14, the majority of the
atoms are carbon-12. The dating process compares the
ratio of carbon-14 to carbon-12 in an object.
All living things contain very small amounts of carbon-14.
This carbon is recycled as we live our lives (carbon in food is
released as carbon dioxide as we burn food for energy). The
amount of carbon-14 remains about the same as we live our
lives. When an organism dies no more carbon-14 is obtained
and the carbon-14 decays to the stable element nitrogen14. As the carbon-14 decays, the ratio of carbon-14 to
carbon-12 in the organic object decreases at a steady rate
because the amount of carbon-14 decreases. Scientists
have figured out that it takes about 5730 years for half of
the carbon-14 to change to nitrogen-14.
Picture of how carbon-14 is renewed or produced on the Earth.
Another important radioactive isotope is Uranium which
decays into the stable element lead. This method has let
scientists to determine the approximate age of the Earth as
4.6 billion years old.
Below are 3 common radioactive isotopes that are used to
date objects. The parent or mother isotope is the
radioactive unstable isotope and the daughter isotope is the
stable element that forms from decay.