Radiometric Dating & Absolute Age - Study Guide

REF STUDY GUIDE: Big Ideas Use to answer form questions

1.Radioactive isotopes decay at a fixed, predictable rate called the half-life — the time it takes for half the parent atoms in a sample to convert into daughter atoms. After one half-life 50% of the original parent remains; after two, 25%; after three, 12.5%. Because every U-238, K-40, or C-14 atom shares the same half-life regardless of temperature or pressure, the parent-to-daughter ratio inside a rock or fossil acts like a stopwatch that started the moment the rock solidified.

2.Different isotopes match different age ranges, so geologists pick the right tool for the sample. Carbon-14 (half-life 5,730 yr) dates organic material up to about 50,000 years old. For ancient rocks and fossils — including dinosaur-era samples millions of years old — scientists use potassium-argon or uranium-lead dating, applied to igneous rock like volcanic ash above and below the fossil. The fossil itself is dated indirectly by bracketing it between two dateable rock layers.

3.Radiometric (absolute) dating and the relative dating tools from Week 2 are complementary, not competing. Superposition and index fossils tell us the order of events; radiometric dating assigns numeric ages in years. Combined, they produce the geologic time scale: a calendar of Earth's 4.6-billion-year history divided into eons, eras, periods, and epochs that is consistent with independent evidence from meteorites (~4.56 Ga) for the age of the solar system.

DRAW KEY TERMS Google each → Images tab → sketch what you see

radiometric dating

Dating rocks using radioactive decay rates

half-life

Time for half of radioactive isotope to decay

trace fossil

Evidence of organism activity like footprints or burrows

absolute dating

Determining actual age in years using radiometric methods

OTHER VOCAB: intrusive igneous · extrusive igneous · body fossil · parent isotope · daughter isotope · geologic time scale

WRITE FOCUS QUESTION Your initial thinking

A museum placard under a Tyrannosaurus rex skeleton reads "67 million years old." Nobody was there with a stopwatch. Yet geologists agree on that number across hundreds of independent labs, using radioactive isotopes inside the volcanic ash layers above and below the fossil. The bone itself is dated indirectly — by bracketing it between two precisely-dated layers of rock.

How can scientists determine the age of a dinosaur bone?

I think...scientists can't put a stopwatch on a dinosaur bone directly, but they can date the volcanic ash layers ABOVE and BELOW the fossil using radioactive isotopes — and that tells them the fossil falls between those two ages.

WRITE AFTER HOOK FORM Reflect before continuing

I learned that... every radioactive isotope decays at a fixed rate called its half-life — the time it takes for half the parent atoms to turn into daughter atoms. That clock works the same in every rock, so it lets us measure age in years.

WRITE Half-Life Simulation

Station 1 — Half-Life Simulation

My observation of the decay curve: the number of parent atoms gets cut in HALF after every half-life — first 50%, then 25%, then 12.5%. The curve drops steeply at first and then flattens out, but it never quite reaches zero.

After 3 half-lives, % parent remaining = After 3 half-lives, 12.5% of the original parent atoms remain (100 → 50 → 25 → 12.5).

In my own words, "half-life" means: Half-life is the time it takes for half of the radioactive parent atoms in a sample to decay into daughter atoms. It's a CONSTANT for each isotope — it doesn't change with temperature, pressure, or how much of the sample you have.

CHECK DAY 1 CHECK-IN Before you leave

Q1: Based on today's investigation of radiometric dating & absolute age, explain one key finding using evidence from your data. Connect your answer to this week's focus question: How can scientists determine the age of a dinosaur bone?

Radioactive isotopes decay at a constant rate called the half-life. After one half-life, 50% of the original parent atoms remain; after two, 25%; after three, 12.5%. Because the rate is constant, the parent-to-daughter ratio in a rock acts like a clock that started when the rock solidified. That's how scientists assign numeric ages to volcanic ash layers — and through bracketing, to the dinosaur bones between them.

Confidence:

Still building Getting clearer Ready to explain

WRITE Isotope Selection & Age Calculation

Station 2 — Isotope Selection & Age

For a 1,200-year-old wooden tool I would choose ___ because: Carbon-14 (half-life ≈ 5,730 yr) — it works for organic material younger than ~50,000 years, and a 1,200-year-old wooden tool fits inside that window.

A dinosaur fossil is dated by the volcanic ash above and below because: the fossil itself doesn't contain enough usable isotope to date directly, but the volcanic ash layers above and below it solidified at known moments — so the fossil must be YOUNGER than the layer below and OLDER than the layer above (bracketing).

25% parent remaining → ___ half-lives → age = 25% parent remaining → 2 half-lives → age = 2 × (the isotope's half-life).

WRITE Constructing Earth's Timeline

The main idea of the to-scale timeline is...Earth's 4.6-billion-year history is so long that human civilization (~10,000 years) is barely a thin line at the very end of a to-scale timeline — most of Earth's history happened before complex life appeared.

This connects to Week 2 (rock layers) because...Week 2 used SUPERPOSITION (younger rocks on top) and index fossils to put events in ORDER. Radiometric dating now adds the NUMBER of years between those events — together they build the geologic time scale.

I can apply absolute dating by...looking at a rock cross-section, identifying datable volcanic-ash layers, and using radiometric ages from those layers to bracket the age of any fossils sandwiched between them.

DRAW CONCEPT MODEL Half-life decay curve (parent vs. daughter)

Plot the half-life decay curve: % parent (y) vs. half-lives (x). Mark 100/50/25/12.5/6.25%, add the daughter mirror curve, annotate the 25%-parent point with its age, and label ≥3 word-bank terms.

Word Bank: parent isotope · daughter isotope · half-life · radiometric dating · absolute dating

Curve 1 (parent isotope): starts at 100% on the y-axis, drops to 50% at 1 half-life, 25% at 2 half-lives, 12.5% at 3, 6.25% at 4. Curve 2 (daughter isotope): mirror image — starts at 0%, rises to 50%, 75%, 87.5%, 93.75%. Both curves cross at 50% / 1 half-life. Annotate the 25% parent point: age = 2 half-lives. Labeled terms: parent isotope, daughter isotope, half-life, radiometric dating, absolute dating.

≥3 word bank terms labeled

Parent curve hits 50%, 25%, 12.5%

25%-remaining point annotated with age

CHECK TEST YOUR THINKING Rate each: FALSE ← 1-2-3-4-5 → TRUE · 4 new + 1 last week + 1 spiral

1. Carbon-14 dating can be used directly on a 65-million-year-old dinosaur bone.

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2. Radiometric dating measures the age of the fossil bone itself, not the surrounding rock.

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3. "Half-life" means half the age of the rock.

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4. Earth is only a few million years old.

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5. [Last week] In any stack of rock layers, the older rocks are ALWAYS deeper than the younger rocks — folding and faulting can never reverse this order.

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6. [Spiral · C6 Plate Tectonics] The continents have always been in their current positions — landmasses cannot move across the surface of Earth.

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Pick any TWO statements -- especially any you rated 2, 3, or 4. Justify your rating with evidence:

Statement #___ -- I rated it ___ because:

THINK SCIENTIST CIRCLE DEBRIEF Reflect after your Scientist Circle discussion

AHA! Moment

The one I changed my thinking most about is... the rock around a fossil is what gets dated, not the fossil itself — and that bracketing is why we can give a dinosaur bone a NUMERIC age even though no one was there with a stopwatch.

Remaining Question

I still wonder... If half-life is constant for an isotope, what happens at the very smallest scale — does each individual atom 'know' when it's supposed to decay, or is it pure randomness across the whole sample?

WRITE FOCUS QUESTION REVISIT Final written response

How can scientists determine the age of a dinosaur bone?

Word Bank: intrusive igneous · extrusive igneous · trace fossil · body fossil · radiometric dating · half-life · parent isotope · daughter isotope · absolute dating · geologic time scale

Scientists determine a dinosaur bone's age by radiometrically dating the volcanic ash (igneous rock) layers ABOVE and BELOW the fossil. The fossil itself doesn't usually hold enough usable isotope, so its age is bracketed by the surrounding rocks.

Inside those rocks, isotopes like potassium-40 or uranium-238 decay into daughter atoms at a fixed half-life. By measuring the parent-to-daughter ratio, scientists calculate when the rock solidified — giving the fossil a numeric age (absolute dating).

Combined with superposition and index fossils from Week 2, radiometric dating turns rock layers into a calibrated geologic time scale that's consistent with independent evidence from meteorites (~4.56 billion years for the solar system).