Lab Manual Exercise 6: Determining Geologic Age (pp.98 110)

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Name: ____________________________________ Date: _______________________ Period: ______

Lab Manual


Exercise 6: Determining Geologic Age

(pp.98


110)

Relative Dating

Relative dating, the placing of geologic events in their proper sequence or order,

does not tell how long
ago something occurred, only that it _______________________ one event and followed another.
Several logical doctrines, laws, and principles govern the techniques used to establish the relative age of
an object or event.

Doctrine o
f Uniformitarianism

First proposed by James _________________ in the late 1700s, this doctrine states that the physical,
chemical, and biological laws that operate today have operated throughout Earth’s ______________.
Although geologic processes such as
____________________, deposition, and volcanism are governed
by these unchanging laws, their rates and intensities may vary. The doctrine is often summarized in the
statement, “________________________________________________________”.

Principle of Origin
al Horizontality

Sediment, when deposited, forms nearly horizontal ______________________. Therefore, if we
observe beds of sedimentary rocks that are folded or inclined at a steep angle, the implication is that
some deforming force took place ___________
_______ the sediment was deposited.

Law of Superposition

In any sequence of undeformed sedimentary rocks (or surface deposited igneous rocks such as lava
flows and layers of volcanic ash)
, the ___________________ rock is always at the __________________
and the ________________________ is at the _____________.
Therefore, each layer of rock represents
an interval of time that is more recent than the underlying rocks.

Answer questions:

1.


Oldest (first) _____, _____, _____, _____, _____, _____, _____, _
____, _____ Youngest (last)

2.

______________________________________________________________________________

3.

______________________________________________________________________________

4.

Oldest ____________________________________________________________
____ Youngest

Inclusions

Inclusions are pieces of one rock unit that are contained within another unit, for example, pieces of
granite embedded in a shale. The rock mass adjacent to the one containing the inclusions must have
been there _________________
___ in order to provide the rock fragments. Therefore, the rock
containing the inclusions is the ___________________________ of the two.

5.


6.

______________________

Unconformities

As long as continuous sedimentation occurs at a particular place, there will
be an uninterrupted record
of the material and life forms. However, if the sedimentation process is suspended by an emergence of
the area from below sea level, then no sediment will be _________________________ and an erosion
surface will develop. The r
esult is that no rock record will ____________________ for a part of geologic
time. Such a ____________ in the rock record is termed an ______________________________. An
unconformity is typically shown on a cross
-
sectional (side view) diagram by a wavy
line (____________).

7.


Principle of Cross
-
Cutting Relationships

Whenever a fault or intrusive igneous rock cuts through an existing feature, it is __________________
than the structure it cuts.

8.

______________________________________

9.

______________________________________

10.

______________________________________

11.

______________________________________________________________________________

12.

_____________________________, _______________________________

13.

____________________________________
__________________________________________

Fossils and the Principle of Fossil Succession

Fossils are among the most imports tools used to interpret Earth’s history. They are used to define the
ages of rocks, correlate one rock unit with another, and dete
rmine past __________________________
on Earth. Earth has been inhabited by different assemblages of plants and animals at different times.
As rocks form, they often incorporate the preserves remains of these organisms as fossils. According to
the princ
iple of _______________________________, fossil organisms succeed each other in a definite
and determinable order. Therefore, the time that a rock originated can frequently be determined by
noting the _______________________________________ that are found

within it.

14.

OMIT

15.

OMIT

16.



Petrification
: The small internal cavities and pores of the original organism are filled with
precipitated mineral matter. PHOTO ______________

Cast
: The space one occupied by a dissolved shell or other structure is subsequently
filled with
mineral matter. PHOTO ______________

Impression
: A replica of a former fossil left in fine
-
grained sediment after the fossilizing
material, often carbon, is removed. PHOTO ______________

In
direct evidence
: Traces of pre
historic life, but no
t the
organism itself. PHOTO ____________

17.


Applying Relative Dating Techniques

Geologists often apply several of the techniques of relative dating when investigating the geologic
history of an area.

18.


19.

_________________________; _______________________
_____________________________

20.

_________________________; ____________________________________________________

21.

_________________________; ______________________ and __________________________

22.

_________________________

23.

Answer on right of the diagram

24.

______________________________________________________________________________

Radiometric Dating

The discovery

of radioactivity and its subsequent understanding has provided a reliable means for
calculating the ________________________________ in years of

many Earth materials. Radioactive
atoms, such as the isotope __________________________, emit particles from their nuclei that we
detect as radiation. Ultimately, this process of decay produces an atom that is stable and no longer
radioactive. For exam
ple, eventually the stable atom lead
-
206 is produced from the radioactive decay of
____________________________.

Determining Radiometric Ages

The radioactive isotope used to determine a radiometric date is referred to as the _________________.
The amount
of time it takes for __________________ of the radioactive nuclei in a sample to change to
their stable end product is referred to as the ______________________ of the isotope. The isotopes
resulting from the decay of the parent are termed the ___________
______________________. For
example, if we begin with one gram of radioactive material, half a gram would decay and become a
daughter product after one half
-
life. After the second half
-
life, one half of the remaining radioactive
isotope, 0.25 grams or ¼
of the original amount (1/2 or ½) would exist. With each successive half
-
life,
the remaining parent isotope would be reduced by _______________.



25.





Fraction of Parent Isotope Remaining

One half
-
life


Two half
-
lives


Three half
-
lives


Four
half
-
lives



26.




Remaining

Parent Isotope (in grams)

One half
-
life


Four half
-
lives



27.

_________________________________

28.

_________________________________

29.

½ parent and ½ daughter: Age = __________

1/8 parent and 7/8 daughter: Age = __________

1/32
parent and 31/32 daughter: Age = _________

Applying Radiometric Dates

When used in conjunction with relative dates, radiometric dates help Earth scientists refine teir
interpretation of the geologic history of an area.

30.

A. ____________________________


B. _____________________________


C. _____________________________



31.

Answer on figure 6.11

32.

____________________________________________________; ________________________;
______________________________________________________________________________

33.

______
_________________________; ______________________________________________

34.

_______________________________; ______________________________________________

35.

_______________________________; ______________________________________________

The Geologic Time Scal
e

Applying the techniques of geologic dating, the history of Earth has been subdivided into several
different units which provide a meaningful time frame within which the events of the geologic past
are arranged. Since the span of a human life is but a “b
link of an eye” compared to the age of Earth,
it is often difficult to comprehend the magnitude of geologic time.

36.

OMIT

37.

OMIT

38.

OMIT

39.

_____________________________________________________________________________

40.

OMIT

41.

OMIT