Geologic Dating

Regents Earth Science

• Relative Dating: Determining the order of past events without determining their absolute age.
• Absolute Dating: Determining the numerical age of rocks and fossils.

Part 2: Absolute Dating

Absolute Dating Basics

• Absolute dating determines the numerical age of rocks and fossils.
• Common methods: radiometric dating (e.g., radiocarbon, uranium–lead, potassium–argon).
• Used to build a timeline of Earth’s history.

Radioactive Decay Basics

• Unstable parent isotopes decay into stable daughter isotopes.
• Decay follows an exponential law characterized by half-life.
• Half-life: time for half of the parent atoms to decay.

Visualizing Half-Life

• As time passes, the amount of parent isotope decreases by half for each half-life.
• The curve never truly reaches zero.

Counting Half-Lives

• 0 Half-Lives: 100% Parent
• 1 Half-Life: 50% Parent, 50% Daughter
• 2 Half-Lives: 25% Parent, 75% Daughter
• 3 Half-Lives: 12.5% Parent, 87.5% Daughter

Radiocarbon (Carbon-14) Dating

• Carbon-14 is produced in the atmosphere and incorporated into living organisms.
• After death, C-14 decays; no new C-14 is added.
• Effective dating range: up to ~50,000 years.
• Measured via ratio of C-14 to stable C-12/C-13.

Example

Carbon has a half life of years

If Carbon-14 goes through 2 half lives...

1. How many years will go by?
   • 5,700 x 2 = 11,400 years
2. What % of the original mass will remain?
   • 25%
3. What % of daughter product will you have?
   • 75%
4. What is the daughter product?
   • Nitrogen-14

Limits to Radiometric Dating

• After 10 half-lives, there is so little original substance remaining that it is hard to trace
• Carbon-14 is great to use because it is found in many places
• However, it is only effective in dating up to 50,000 years ago. Beyond that it is useless.
• For the oldest rocks, we use the other radioactive isotopes when we can find it in the rock.
• Problem is these other isotopes are not as abundant
• If we cannot determine absolute ages, we then will determine relative ages.

Other Radio Active Elements

Page 15 on reference table...

Part 2: Relative Dating

Relative vs. Absolute Dating

• Relative Dating

  • "This rock is older than that rock."
  • Uses laws of stratigraphy.
  • No specific numbers.

• Absolute Dating

  • "This rock is 1.2 million years old."
  • Uses radiometric dating.
  • Specific numbers.

Principles of Relative Dating

1. Law of Superposition
2. Principle of Original Horizontality
3. Principle of Cross-Cutting Relationships
4. Inclusions
5. Unconformities

Law of Superposition

• In an undeformed sequence of sedimentary rocks, each layer is older than the one above it and younger than the one below it.
• Bottom = Oldest
• Top = Youngest

Principle of Original Horizontality

• Layers of sediment are generally deposited in a horizontal position.
• If rock layers are flat, it means they have not been disturbed.
• If they are folded or tilted, the disturbance happened after deposition.

Principle of Cross-Cutting Relationships

• An igneous intrusion or a fault that cuts through rock layers is younger than the rocks it cuts.
• The "cutter" is younger than the "cuttee".

Inclusions

• Pieces of one rock unit that are contained within another.
• The rock containing the inclusion is younger than the inclusion itself.
• The inclusion had to exist before it could get stuck in the new rock.

Unconformities

• A break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time.
• Represents a "gap" in time.
• Angular Unconformity: Tilted or folded sedimentary rocks are overlain by younger, more flat-lying strata.

Index Fossils

• Fossils used to define and identify geologic periods.
• Characteristics of a good index fossil:
  1. Widespread geographically (found everywhere).
  2. Existed for a short period of time (short geologic range).
  3. Abundant (easy to find).
  4. Easy to identify.

Quiz

2. The half-life of carbon-14 is approximately:

• A) 573 years
• B) 5,730 years
• C) 57,300 years
• D) 730 years

Quiz

3. A sample has one-quarter of its original C-14 remaining. About how old is it?

• A) 2,865 years
• B) 5,730 years
• C) 11,460 years
• D) 17,190 years

Quiz

4. Which statement about half-life is correct?

• A) It varies with temperature.
• B) It is the time for all parent atoms to decay.
• C) It is constant for a given isotope.
• D) It increases as more atoms decay.

Quiz

5. Why is calibration needed for radiocarbon dates?

• A) To account for changes in atmospheric C-14 over time
• B) To convert BP to CE only
• C) To adjust for sample mass
• D) To remove contamination automatically

Exit Ticket

• Define half-life in your own words.
• Explain one limitation of radiocarbon dating.
• Identify one context where uranium–lead is preferred.

References

• USGS and NOAA materials on radiometric dating.
• Standard Earth Science Regents references.
• Course notes and lab activities.

Next: Lab Prep

• Plan: measure simulated decay and fit an exponential model.
• Materials: counters, graphing tools, sample scenarios.
• Safety: handling and data integrity (no actual radioisotopes).