Radiometric dating Knowledge-Based Practice

Questions

1. 1.

   Why is carbon-14 dating ineffective for determining the age of dinosaur fossils that are over 65 million years old?

   • A) Fossilization replaces all organic carbon with nitrogen gas, which quickly escapes from the bone matrix.
   • B) The exceptionally long half-life of carbon-14 prevents any measurable decay from taking place over millions of years.
   • C) Cosmic radiation in the upper atmosphere during the Mesozoic Era was insufficient to incorporate carbon-14 into living tissue.
   • D) The relatively short half-life of carbon-14 causes the remaining parent isotope to decay to undetectable levels within about 60,000 years.
2. 2.

   Why is direct radiometric dating of detrital mineral grains in a clastic sedimentary rock generally ineffective for determining the age of sediment deposition?

   • A) Detrital grains continuously exchange parent isotopes with groundwater, preventing the mineral from ever maintaining a closed isotopic system.
   • B) The physical pressure of sedimentary compaction significantly alters the nuclear decay constants of long-lived parent radioisotopes.
   • C) Mechanical weathering and surface transport completely reset the isotopic ratio to zero prior to sedimentation.
   • D) The radiometric clock measures when the individual mineral grains crystallized in their source rocks, not when they were deposited as sediment.
3. 3.

   In radiocarbon dating, what event initiates the radiometric "clock" used to determine the age of an organic specimen?

   • A) The death of the organism, which halts metabolic carbon intake and allows $^{14}\text{C}$ to decay without replenishment.
   • B) The fossilization of the specimen, which locks residual carbon atoms into mineralized crystal structures.
   • C) The burial of the specimen beneath sediment, which isolates it from ambient cosmic radiation.
   • D) The chemical decomposition of soft tissues, which triggers the spontaneous nuclear breakdown of carbon atoms.
4. 4.

   In potassium-argon ($^{40}\text{K}$–$^{40}\text{Ar}$) radiometric dating, how is the calculated age of an igneous rock sample affected if thermal metamorphism caused a portion of the accumulated $^{40}\text{Ar}$ gas to escape?

   • A) The calculated age will be younger than the actual age because the measured ratio of daughter to parent isotopes is reduced.
   • B) The calculated age will be older than the actual age because the decay rate of potassium-40 accelerates upon loss of argon.
   • C) The calculated age will be older than the actual age because the concentration of remaining parent nuclide appears higher.
   • D) The calculated age will be unaffected because the age calculation relies exclusively on the absolute amount of potassium-40.
5. 5.

   A mineral sample contains $^{235}\text{U}$ and its stable decay product $^{207}\text{Pb}$ in an atom ratio of $1:3$. Assuming no lead was present when the mineral formed and given that the half-life of $^{235}\text{U}$ is $7.0 \times 10^8$ years, what is the age of the mineral?

   • A) $1.4 \times 10^9$ years
   • B) $2.1 \times 10^9$ years
   • C) $7.0 \times 10^8$ years
   • D) $2.8 \times 10^9$ years

Answers and step-by-step solutions appear after you finish the test.