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Unit 3 Study Sheet: Recognizing Change over Time

2. 3.2.1 Study: Recognizing Change over Time
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3. Determining Earth’s Age
Scientists look for similar clues to figure out how old Earth is. Take a look at the picture on the right. You can see many mountain ranges. Mountain ranges take thousands of years to form

4. Earth is about 4.5 billion years old

5. Determining Earth’s age
Relative Dating: Using the position of rock layers to determine the ages of rocks relative to one another.
Sediment: Bits of rock, dirt and sand that can be deposited in layers.
Layers of sediment , or bits of rock, dirt, and sand, deposit in the same way. These layers eventually harden into rock. Similar to the stacked books, the top layers are younger than the bottom layers.

6. Determining Earth’s Age
Radiometric Dating: Determining the age of rocks by looking at radioactive elements they contain.
Half-Life: The time it takes for half the original number of radioactive atoms in a sample to change into some other kind of atom.
APEX page 4
Just because you can tell that one rock layer is older than another rock layer doesn't mean you know the rock's age. So how can scientists determine the exact age of a rock? They use a method called radiometric dating .
To understand how radiometric dating works, you need to know a little about the structure of an atom. An atom of a certain element has a nucleus with a set number of particles called protons. For example, helium has two protons.
If the number of protons changes, the atom becomes a different element. For example, if helium loses a proton, it becomes hydrogen.
Most elements are stable, which means that their atoms stay the same forever. Some elements, on the other hand, are radioactive . The atoms of a radioactive element will eventually change into atoms of another element. This process is called decay.
There are many kinds of radioactive elements. Each kind of radioactive element takes a predictable amount of time to change called a half-life . A half-life is the time it takes for half the original number of radioactive atoms in a sample to change into some other kind of atom.
The half-life of uranium-235 is 700 million years. Suppose a rock forms with 16 uranium atoms in it. In 700 million years, half of these atoms will have changed to lead, leaving 8 uranium atoms. After another 700 million years, a second half-life will have passed, so only 4 uranium atoms will be left.

7. Suppose scientists found a rock that had 8 uranium-235 atoms and 8 lead atoms. They know the half-life of uranium-235 is 700 million years.
Remember, uranium-235 decays into lead. Since there are 8 uranium-235 atoms and 8 lead atoms, you know that half of the uranium-235 has decayed. This means that one half-life, or 700 million years, has passed since the rock formed. Therefore, the rock is 700 million years old

8. Sample Question: Using Half-Lives to Date Rocks
The table below shows the number of atoms in a sample. A radioactive form of the element carbon, carbon-14 breaks down into nitrogen.
How old is this rock sample if it originally contained 16 atoms of carbon-14? Use the table above and the fact that carbon-14 has a half-life of 5,700 years to determine the rock's age.
Element
Number of atoms
Carbon-14 4
Nitrogen 12
After one half-life (5,700 years), you would lose half of the original carbon-14 atoms. That would give you 8 carbon-14 atoms and 8 nitrogen atoms. After another half-life, you would lose half of the remaining carbon atoms. This would give you 4 carbon-14 atoms and 12 nitrogen atoms, which is what we have.

9. Geological Time Scale
A timeline that describes the timing and relationships of events in Earth’s history

10. APEX page 7
APEX page 9

11. Unit 1 Study Sheet: 1.2.1 Meiosis
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Unit 1 Study Sheet: Meiosis