Geological Time
The Earth is approximately 4.6 ___________________ years old.
Scientists have divided this geological time scale into eons, eras,
periods, and epochs. Eons represent the _______________ time.
Eons are divided into __________. Each era is subdivided
into_________________. Finally periods are divided into still
smaller units called ____________________.
The geologic time scale is a visual record or Earth’s history, with
the individual units based on changes in the _______ and_______.
Notice that the geological time scale doesn’t have periods or
epochs and only one era the Precambrian. This is because
Precambrian time, there were fewer _______________ 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
__________________.
 Principle of Superposition – The rock on the
__________________ are older than the rock on the
_____________.
 Principle of Cross-Cutting Relations: A rock is
_________________________ than any rock it cuts across.
Rock layer D (red) going into layers A and B is
_____________________ than A and B but
____________________ then _____, ______, _____and
_______. 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 _____________ layers are
missing. After a rock layer is formed, the area can be uplifted
to the Earth’s surface (uplifting can be caused by
______________________ and plate tectonics). After, the
process of __________________ and _____________ 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 _______________ 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_____________.
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
________________ than the arrow head.
Absolute Age: This describes the _______________ age of
an object or event. The absolute ages of objects from long
ago are found by analyzing the ____________________ 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 ____________ (decrease
in size and/or energy) of naturally occurring
___________________ isotopes.
Example: Carbon-14 Dating (The 14 is the total number of
________________ and _____________ in the nucleus of
the atom)
Most atoms of carbon have 6 _____________ and 6
_______________ in the nucleus of atom. A very few carbon
atoms have a different number of ___________________ and
are called isotopes. Carbon-14 is an _____________ of
carbon and is radioactive. This means that it tends to give off
_____________________ and energy from its nucleus.
The carbon-14 atom changes into an
atom of nitrogen-14, which is not ___________________
but stable. This change is called radioactive decay. The
________________ process compares the ratio of carbon-14
to nitrogen-14 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 ________________ __________________ as we
burn food for___________________).
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 __________________ to a
stable element. Since scientists have figured out that it takes
about 5730 ___________________ for half of the carbon-14
to change to nitrogen-14.
Another important radioactive isotope is Uranium which
decays into the stable element______________. This
method has let scientists to determine the approximate age
of the Earth as ______ _______________ years old.
Below are 3 common radioactive isotopes that are used to
date objects. The parent or mother isotope is the
_________________________ isotope and the daughter
isotope is the _____________________ element that forms
from decay.