Relative and Radiometric Dating
Reconstruct the order of geologic events, calculate ages from half-life evidence, and recognize what the data can—and cannot—prove.
Relative and Radiometric Dating
Not yet verified on this browser.
Sequence first; numerical age second
relative dating → older or youngerradiometric dating → estimated numerical ageRelative dating reads relationships among layers and structures. Radiometric dating uses predictable isotope decay. The two methods strengthen each other when numerical dates bracket a relative sequence.
Superposition
In an undisturbed sedimentary sequence, lower layers are older than those above.
Cross-cutting relationships
A fault or intrusion is younger than any rock it cuts.
Inclusions
Fragments are older than the rock that contains them.
Unconformities
Erosion or nondeposition creates missing time in the rock record.
Half-life
After each half-life, half of the remaining parent isotope decays; the interval itself stays constant.
Use LAYER → CUT → GAP → CLOCK
LAYER: Order undisturbed beds from bottom to top.
CUT: Place faults and intrusions after the rocks they cross.
GAP: Look for tilting, truncation, erosion, or missing deposition.
CLOCK: Count halvings of the parent isotope, then multiply by the half-life.
Why it works
This keeps relative sequence clues separate from numerical calculations and prevents an exact age from being invented when evidence only gives a range.
Five forms you should recognize
Problem: Layers A and B formed before a dike cut them; layer C covers the dike.
Reason: The dike is younger than A and B but older than C.
Order: A → B → dike → C.
Problem: Tilted layers are eroded flat and covered by horizontal beds.
Reason: Deposition was followed by tilting, erosion, and renewed deposition. The erosion surface represents missing time.
Problem: A sample has 12.5% of its original parent isotope.
Work: 100% → 50% → 25% → 12.5%.
Answer: Three half-lives elapsed.
Problem: Three half-lives elapsed and each is 50 million years.
Work: 3 × 50 million years.
Answer: 150 million years.
Problem: A fossil lies above 2.4-million-year ash and below 2.0-million-year ash.
Answer: Its age is between about 2.4 and 2.0 million years—not automatically exactly 2.2 million.
Check before you commit
- Treating relative age as an exact number
- Calling a cutting feature older than the rock it cuts
- Forgetting that inclusions are older than their host
- Skipping erosion or nondeposition at an unconformity
- Halving elapsed time instead of the remaining parent isotope
- Claiming an exact age when dated layers provide only a range
Do you need the lesson-or just practice?
One original question in each form recommends your next step. It does not yet verify mastery.
Work at the level you need.
Foundations
Build the core procedure with immediate explanations.
Core Practice
Use mixed forms with less scaffolding.
UPCAT-Style Transfer
Apply the competency in unfamiliar representations.
Ready to verify this competency?
A score of 5/5 verifies mastery. An unsuccessful attempt loads a different five-form bank.
Relative and Radiometric Dating FAQ
Does a deeper layer always have to be older?
Only if the sequence has not been overturned or otherwise disturbed. Structural evidence must be checked.
Does half-life change as the sample ages?
No. Each isotope has a characteristic half-life under ordinary geologic conditions.
Can every rock be radiometrically dated directly?
No. The rock must contain a suitable isotope-bearing mineral whose clock and later history can be interpreted reliably.
Continue your mathematics review.
Your progress stays on this browser.
Mastery results save to your Teacher Abi study profile.
Return to Student Hub View UPCAT Coverage
No comments:
Post a Comment