The age of rocks on earth or the samples of rocks and dust brought back from the moon can be found by determining the proportion of radioactive………..in the rock of dust.

Options

A. Potassium and stable calcium
B. Uranium and stable lead
C. Carbon and stable carbon
D. Radium and stable lead

Answer

To determine the age of rocks on Earth or samples of rocks and dust brought back from the moon, we can use the concept of radioactive dating. The process involves measuring the proportion of a radioactive isotope present in the rock or dust sample. Here’s a step-by-step solution to the question:

### Step 1: Identify the Radioactive Isotope
We need to determine which radioactive isotope can be used for dating rocks. The options given are potassium, uranium, carbon, and radium.

### Step 2: Analyze the Isotopes
- **Potassium**: Potassium-40 is a radioactive isotope, but it has a very low radioactivity and is not suitable for dating ancient rocks.
- **Uranium**: Uranium-238 and Uranium-235 have very long half-lives (4.5 billion years and 700 million years respectively), making them suitable for dating geological samples that are millions to billions of years old.
- **Carbon**: Carbon-14 is used for dating organic materials and has a half-life of about 5,730 years, which is too short for dating rocks that are millions of years old.
- **Radium**: Radium-226 has a half-life of about 1,600 years, which is also too short for dating ancient rocks.

### Step 3: Conclusion
Based on the analysis, the most appropriate radioactive isotope for determining the age of rocks is **Uranium**. The process of dating using uranium is known as **Uranium-Lead dating**.

### Final Answer
The age of rocks on Earth or the samples of rocks and dust brought back from the moon can be found by determining the proportion of radioactive **Uranium** in the rock or dust.

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