Click any term to reveal its definition. Cards close automatically after 8 seconds, but you can click them again anytime. Only one card opens at a time.
Click a term on the left, then click the matching definition on the right. Worth 1 point per correct match.
In the summer of 2000, a homeowner in Hyde Park, New York, was digging out a small pond in his backyard when his backhoe struck what he thought was a rock. It was not a rock. It was a tooth the size of a fist. Over the next several months, paleontologists from the Paleontological Research Institution carefully excavated one of the most complete mastodon skeletons ever found in the eastern United States — over 95% of the animal's bones, lying in soft pond sediment that had preserved them for thousands of years.
But how thousands? Were these bones 5,000 years old? 15,000? 50,000? To answer that, scientists used radioactive decay — specifically, the slow, predictable breakdown of Carbon-14 in the bone collagen and in nearby plant material.
Every living thing contains a tiny, constant amount of Carbon-14 in its tissues. The moment an organism dies, no new Carbon-14 enters the body, and what remains begins to decay into Nitrogen-14 at a known rate. The half-life of Carbon-14 is about 5,700 years, meaning that every 5,700 years, half of the Carbon-14 atoms in a sample have decayed into nitrogen. By measuring the percentage of Carbon-14 remaining in the Hyde Park bone, scientists determined the mastodon died approximately 11,480 years ago — right at the end of the last Ice Age, when massive glaciers were retreating from New York State.
Carbon-14 is a powerful tool for dating organic material from the recent past, but it cannot date rocks that are millions or billions of years old. After about 60,000 years, so little Carbon-14 remains in a sample that it can no longer be measured reliably. For older materials, scientists turn to other radioactive "clocks." Potassium-40 (half-life 1.3 billion years) is used to date volcanic rocks. Uranium-238 (half-life 4.5 billion years) is used to date the oldest rocks on Earth — including the ancient gneiss bedrock of the Adirondack Mountains, which formed over one billion years ago during the Grenville Orogeny.
Each radioactive isotope is a different kind of clock, and choosing the right one depends on how old the sample is suspected to be. A scientist studying a fossil shell from Long Island would reach for Carbon-14. A geologist studying a zircon crystal from the Adirondacks would use Uranium-238. The principle, however, is always the same: parent atoms decay into daughter atoms at a constant, predictable rate, and that rate is the heartbeat of the geologic time scale.
From the ESRT (Page 1): The Radioactive Decay Data table lists the half-lives of common isotopes used in geology. You will use this table in the activities and quiz below.
Work through the activities below. Drag words into the correct order, expand bare-bone sentences, and complete the passage using the word bank. Each activity is worth 1 point. Correctly placed words will turn green.
After two half-lives, only percent of the parent isotope remains. Carbon-14 has a half-life of years and is used to date materials. To date the very old rocks of the Mountains, scientists instead use .
Use the reading and the ESRT (Page 1) to answer these Regents-style questions. Each is worth 1 point.
| Row | Isotope | Half-Life | Best Used For Dating |
|---|---|---|---|
| (1) | Carbon-14 | 4.5 billion years | Adirondack gneiss |
| (2) | Uranium-238 | 5,700 years | Mastodon bone |
| (3) | Carbon-14 | 5,700 years | Hyde Park mastodon bone |
| (4) | Potassium-40 | 5,700 years | Volcanic rock |
You have 100 parent atoms below. Each click of the button advances one half-life: each remaining parent atom has a 50% random chance of decaying into a daughter atom. Run the simulation through several half-lives and fill in the data table below. The completed data table is worth 4 points.
After each half-life, record what you observe. The simulator counts for you — copy the numbers in.
| Half-Lives Elapsed | Time (if C-14) | Parent Atoms | Daughter Atoms | % Parent Remaining |
|---|---|---|---|---|
| 0 | 0 years | |||
| 1 | 5,700 years | |||
| 2 | 11,400 years | |||
| 3 | 17,100 years | |||
| 4 | 22,800 years |
The graph below plots % parent isotope remaining against number of half-lives. This is the universal shape of every radioactive decay curve — no matter which isotope you use, the math is the same. Notice that the curve never reaches zero.
You have 6 mystery samples. For each sample, you'll be told the percentage of parent isotope remaining. Your job: (1) choose the correct isotope to date it (using the ESRT or the half-lives shown below), and (2) calculate the age. Each correct sample is worth 2 points (1 for isotope, 1 for age).
5 questions drawn randomly from a 20-question bank (1 from each of 5 style categories). You need 60% mastery (3 of 5) to pass. If you don't pass, you'll get a fresh set of 5 different questions to retry.