Radioactive Dating Lab — Recording Sheet

Every living thing on Earth — every plant, animal, and person — is built around the element carbon, which is why scientists call us “carbon-based life forms.” While we are alive, we take in a tiny amount of a radioactive version of carbon called Carbon-14. When we die, no more Carbon-14 gets in, and the atoms we already have start breaking down at a steady, predictable rate.

The “half-life” is how long it takes for half of those atoms to disappear. After one half-life, fifty percent of the original amount is left. After two half-lives, half of a half remains — only twenty-five percent of the original. The amount keeps shrinking, but the time between each halving stays the same — like cutting a piece of cake in half over and over every 5 minutes: the slices keep getting smaller, but the time between each cut never changes. Because that rate is so reliable, archeologists use math to turn “how much is left” back into “how long ago was this being alive?”

Rocks have a different clock: when magma cools deep underground, tiny crystals called zircon form, and they trap microscopic bits of uranium inside themselves. Over millions and even billions of years, that uranium slowly turns into lead. Before scientists figured out radioactive dating in the 1940s and 50s, archeologists had to guess at ages using how deep things were buried, the style of pottery nearby, or how thick tree rings were — methods that often resulted in uncertain estimates of the age of artifacts, with possible ranges in the thousands or even millions of years. Radiometric dating changed everything.

Reading Check · Fill in each blank using the word bank

Living cells are built using the element . While alive, plants and animals absorb a radioactive version of this element called . After something dies, no more enters its body, and the radioactive atoms inside slowly break apart. Scientists call the time it takes for half of those atoms to break down a . Very old rocks sometimes contain crystals of , and these crystals contain a different radioactive element, , that breaks down over a much longer timeline — over and even of years. Scientists can measure the amount of radioactive elements present and estimate how much they would originally contain. From the percentage still left, archeologists can estimate the age of artifacts with improved accuracy. Before this kind of dating was figured out in the , the age of artifacts was estimated only from clues like buried depth, pottery style, or tree rings.

Word Bank

carbon Carbon-14 half-life zircon uranium millions billions 1940s and 50s atoms nuclear

Methods of Estimating Age

Circle the methods that were used in the past to age artifacts and are still used today to confirm radioactive dating with multiple sources of data.

🔭Telescope

🌳Tree rings

🌡️Thermometer

🏺Pottery style

🧭Compass

⛰️Buried depth (soil layers)

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Label the age of each artifact and rock

Phrase / Word Bank

60,000 years 1 million years 5,730 years Carbon-14 Uranium-238 carbon zircon neither carbon-14 nor uranium-238 is 5,730 years is 4.468 billion years is long enough to detect a tiny remaining amount

Try Writing

To date an organic artifact (like the Ishango Bone or mammoth tusk), I use the probe, because the cells and tissues of living organisms are built with atoms.

To date a rock (like the granite or volcanic tuff), I use the probe, because the rock has crystals that contain that radioactive element.

A fossil — like the T. rex skull (where minerals replaced the bone) or the trilobite (an imprint pressed into sediment that became rock) — gives no signal because it contains .

Try Writing How did you estimate the age of the fossils?

How certain are you that your estimate is the exact age of the fossil? Why would this certainty be different from the others?

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Tier 2 · General Academic Words

amount — how much there is of something.
buried — covered up under dirt, sand, or rock.
element — a basic kind of matter that cannot be broken into smaller parts (like oxygen, iron, or carbon).
estimates — best guesses based on what you know.
methods — ways of doing something.
microscopic — too small to see without a microscope.
original — the first one, before any changes.
pottery — bowls, pots, and other objects made out of baked clay.
predictable — easy to guess what will happen next, because it always works the same way.
ranges — the spread between the smallest and biggest numbers.
rate — how fast or slow something happens.
reliable — works the same way every time and can be trusted.
remains — what is still left after most is gone.
resulted — ended up; what something led to.
shrinking — getting smaller.
uncertain — not sure.
version — a different form of the same thing.

Tier 3 · Science Words

archeologists — scientists who study people and things from a long time ago.
artifacts — objects made by people in the past.
atoms — the tiniest building blocks that make up everything in the universe.
carbon — an element found in every living thing.
Carbon-14 — a special form of carbon that is radioactive and slowly breaks down over thousands of years.
crystals — solid shapes that grow with flat sides and sharp edges (like ice, salt, or zircon).
half-life — the time it takes for half of a radioactive material to break down.
halving — cutting something into two equal pieces.
lead — a heavy gray metal element. (When uranium breaks down, it slowly turns into lead.)
magma — hot melted rock deep inside the Earth.
radioactive — gives off tiny invisible energy as it slowly breaks down into something else.
radiometric — a way to measure age by how much radioactive material is left in something.
tree rings — the rings inside a tree trunk that show how old the tree is. One ring grows each year.
uranium — a heavy radioactive metal element used to date very old rocks.
zircon — a tiny crystal that forms inside cooling magma and traps uranium inside it.

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