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Earth Science
Chapter 21
Worksheet 2
Name ____________________________
Block ____________________________
Use the words from the table to the complete the passage
epoch
Mesozoic
geologic time scale
period
eon
era
Geologists have organized the events of Earth’s history and represented them on the
(1) ______________________________________. This record of Earth’s history is divided into units
of time, the longest of which is the (2) _________________, measured in billions of years. The nest
longest unit of time, the (3) ___________________, is measured in hundreds of millions to billions of
years. The name of one such unit of time is the (4) ______________________, which means “middle
life”. The unit of geologic time defined by the abundance or extinction of life-forms during the time that
certain rocks were deposited is the (5) ______________________. An even smaller unit of time, the
(6) _____________________, is usually measured in terms of millions to tens of millions of years.
Complete the unfinished table below
Era
Meaning of Name
7. ________________
8. _________________
end marked by the biggest extinction event in
Earth’s history
9. ________________
middle life
10. ___________________________________
___________________________________
11. _______________
12. ________________
based on fossil records that are relatively
complete and easily accessed
Description
Circle the letter that corresponds to the correct answer
13. You can tell that the oldest rocks are at the bottom of an undisturbed rock sequence by using the
principle of
a. Uniformitarianism
b. Original horizontality
c. Superposition
d. Cross-cutting relationships
14. The geologic principle that states that sedimentary rocks are deposited in horizontal layers is the
principle of
a. Uniformitarianism
b. Original horizontality
c. Superposition
d. Cross-cutting relationships
15. You can tell that a fault is younger than the rock it curs across by applying the principle of
a. Uniformitarianism
b. Original horizontality
c. Superposition
d. Cross-cutting relationships
16. The principle that states that the processes occurring today have occurred since Earth formed is
known as the principle of
a. Uniformitarianism
b. Original horizontality
c. Superposition
d. Cross-cutting relationships
17. What is the matching of outcrops from one geographic region to another?
a. Correlation
b. Unconformity
c. Superposition
d. Uniformitarianism
18. A buried erosional surface in the rock record is a(n)
a. Correlation
b. Unconformity
c. Inclusion
d. Principle
19. In an undisturbed rock sequence, the youngest rock layer is located
a. At the bottom of the sequence
b. At the top of the sequence
c. Below the sedimentary rock layer
d. Below the unconformity
20. What are the particles eroded from a layer of rock that become incorporated in an overlying rock
layer?
a. Fossils
b. Unconformities
c. Sills
d. Inclusions
21. The rock layers beneath an eroded surface are at an angle to that surface in a(n)
a. Nonconformity
b. Disconformity
c. Angular unconformity
d. Cross-cutting relationship
22. The relative age of a rock layer that contains inclusions is
a. Older than the source of the inclusions
b. Older than the layer below it
c. Younger than the source of the inclusions
d. The same as the intrusion that cuts across it
Circle T if the statement is true or F if the statement is false
T
F
23. Absolute age dating determines the relative age of a rock based on its observed position
in the rock record
T
F
T
F
24. Formed by short-duration events, key beds contain distinctive and easy to recognize
material that geologist use as time markers
25. The half-life of C-14 is longer than that of U-238
T
F
T
F
T
F
T
F
26. After one half-life, a pure sample of U-238 atoms decays into a ratio of 25% U-238 and
75% Pb-206 atoms
27. Varves are alternating light- and dark-colored sediment bands that indicate cycles of
seasonal climate change
28. The emission of radioactive particles by isotopes as they change into other elements
over time is known as dendrochronology
29. The width to which tree ring grow varies in spring and winter
Use the diagram to answer the questions below
Time 1 = 0 years
= 0 half-lives
Time 2 = 5730 years
= 1 half-lives
Time 3 = 11560 years
= 2 half-lives
Time 4 = 17090 years
= 3 half-lives
100%
0%
50%
50%
25%
75%
12.5%
87.5%
C-14
N-14
C-14
N-14
C-14
N-14
C-14
N-14
30. Which element is the radioactive isotope? ___________________________________________
31. Which element is the stable nonradioactive element? ___________________________________
32. What is the half-life of C-14? ______________________________________________________
33. How many half-lives will it take for all but 25% of the original C-14 to decay? ________________
34. How many years is that? _________________________________________________________
35. What percentage of C-14 remains after three half-lives? ________________________________
36. What percentage of N-14 forms after 17,090 years? ____________________________________