Radiometric Dating & Absolute Age

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...

WRITE AFTER HOOK FORM Reflect before continuing

I learned that...

WRITE Half-Life Simulation

Station 1 — Half-Life Simulation

My observation of the decay curve:

After 3 half-lives, % parent remaining =

In my own words, "half-life" means:

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?

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:

A dinosaur fossil is dated by the volcanic ash above and below because:

25% parent remaining → ___ half-lives → age =

WRITE Constructing Earth's Timeline

The main idea of the to-scale timeline is...

This connects to Week 2 (rock layers) because...

I can apply absolute dating by...

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

≥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...

Remaining Question

I still wonder...

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