Is about one-hundred-million years old.
B.
Is more than about one-hundred-million years old.
C.
Is 4.6 billion years old.
D.
Has been here forever.
E.
Radiometric techniques reveal the Earth to be about 4.6 billion years old, but early geologists did not
have the sophisticated instruments to measure the trace radioactive elements and their offspring.
Working from the rocks, the geologists knew that the age must be in the neighborhood of 100 million
years, plus extra time in unconformities and additional extra time in the oldest, metamorphic rocks.
Points Earned:
1/1
Correct Answer:
C
Your Response:
C
One practical radioactive system used to date lava flows involves:
7.
The solid potassium-40, which decays to the solid grahamspanierum-41.
A.
The gas argon-40, which decays to the gas potassium-40.
B.
The solid potassium-40, which decays to solid argon-40.
C.
The solid potassium-40, which decays to the gas argon-40.
D.
The gas argon-40, which decays to solid potassium-40.
E.
Potassium-40 is common in solid minerals, and decays to produce the gas argon-40. And despite his
great contributions to humanity, no one has named an isotope after Penn State’s president.
Points Earned:
1/1
Correct Answer:
D
Your Response:
D
You start with 800 parent atoms of a particular radioactive type, which decays to give stable
offspring. You wait just long enough for two half lives to pass. You should expect to have how
many parent atoms remaining (on average):
8.
400.
A.
200.
B.
100.
C.
50.
D.
25.
E.
After one half-life, you’ve gone from 800 parents to 400 parents; after a second half-life you go from
400 parents to 200. . (Typical studies of radioactive decay use many more atoms, to avoid statistical
fluctuations, but the question says “on average”, so we asked you about 800 rather than
800,000,000,000,000 to make the math easier.)
Points Earned:
1/1
Correct Answer:
B
Your Response:
B
You are asked to assign as accurate a numerical age as possible (how many years old) to a
sedimentary deposit. You would be wise to use:
9.
Uniformitarian techniques if the deposit is old, and counting of annual layers if the deposit is
young.
A.
Uniformitarian techniques.
B.
Either counting of annual layers or radiometric techniques if the deposit is old (more than
about 100,000 years), and radiometric techniques if the deposit is young (less than about
100,000 years).
C.
Either counting of annual layers or radiometric techniques if the deposit is young (less than
about 100,000 years), and radiometric techniques if the deposit is old (more than about
D.

100,000 years).
Counting of annual layers if the deposit is old (more than about 100,000 years), and
radiometric techniques if the deposit is young (less than about 100,000 years).
E.
If you want an absolute date (number of years) rather than older/younger, you can count layers for
young things, or use radiometric techniques for young things or for old ones. Uniformitarian
calculations aren’t very accurate.

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