68. absolute/radionetric dating can tell us the ______ of a rock, fossil, or landform.
69. isotopes are atoms that have the same number of ______ but a different number of ______.
70. radioactive isotopes decay from ______ to ______.
71. define half-life.
72. what kinds of rocks can be dated using radioactive isotopes? why?

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Accepted Answer

### Question 68
# Brief Explanations:
Absolute/radioisotopic dating is a method used to determine the age of a rock, fossil, or landform. It relies on the decay of radioactive isotopes, where the ratio of parent to daughter isotopes is measured to calculate the time elapsed since the formation or existence of the sample.
# Answer:
age

### Question 69
# Brief Explanations:
Isotopes are atoms of the same element that have the same number of protons (which defines the element) but a different number of neutrons. This difference in neutrons leads to different atomic masses for the isotopes.
# Answer:
protons; neutrons

### Question 70
# Brief Explanations:
Radioactive isotopes undergo radioactive decay, where an unstable parent isotope (with a specific number of protons and neutrons) decays into a more stable daughter isotope (which can be a different element or a more stable isotope of the same element) through processes like alpha decay, beta decay, or gamma decay.
# Answer:
parent isotope; daughter isotope

### Question 71
# Brief Explanations:
Half - life is defined as the time it takes for half of the atoms of a radioactive isotope in a sample to decay. It is a characteristic property of each radioactive isotope and is used in radiometric dating to calculate the age of materials.
# Answer:
The time required for half of the radioactive atoms in a sample to decay.

### Question 72
# Brief Explanations:
1. **Igneous Rocks**:
   - Igneous rocks (both intrusive like granite and extrusive like basalt) are formed from the cooling and solidification of magma or lava. When they form, they trap radioactive isotopes (e.g., uranium - 238, potassium - 40) within their mineral structures. Since there is no exchange of isotopes with the external environment (as they are a closed system once solidified), the decay of these isotopes can be used to date the rock. For example, potassium - argon dating is often used for potassium - rich igneous rocks. The potassium - 40 in the rock decays to argon - 40, and by measuring the ratio of potassium - 40 to argon - 40, the age of the igneous rock can be determined.
2. **Metamorphic Rocks (sometimes)**:
   - Some metamorphic rocks can be dated if the metamorphic process did not completely reset the isotopic clock. If the metamorphism was not too intense and the original radioactive isotopes and their decay products were retained, radiometric dating can be applied. However, if the metamorphism involved significant melting or fluid flow that allowed isotopes to escape or be added, the dating may not be accurate.
3. **Sedimentary Rocks (indirectly)**:
   - Sedimentary rocks themselves are usually not directly dated using radioactive isotopes because they are composed of sediments (fragments of other rocks, organic matter, etc.) that have been transported and deposited. However, the fossils within sedimentary rocks can be dated using radiometric methods if the sedimentary rock contains volcanic ash layers (which are igneous in origin) interbedded with the sedimentary layers. The volcanic ash layers can be dated using radiometric dating of the radioactive isotopes in the ash, and this age can be used to constrain the age of the associated sedimentary rock and its fossils. For example, if a sedimentary rock with fossils has a volcanic ash layer above and below it, the age of the ash layers can be used to determine the time period during which the sedimentary rock and its fossils were formed.

In general, igneous rocks are the most commonly and reliably dated using radioactive isotopes because they form as closed systems with respect to the radioactive isotopes, while metamorphic and sedimentary rocks have more complex scenarios regarding the retention of isotopes for dating.
# Answer:
- **Igneous Rocks**: Can be dated as they trap radioactive isotopes during formation (closed system). E.g., granite (intrusive) and basalt (extrusive) using methods like potassium - argon dating.
- **Metamorphic Rocks (some)**: Can be dated if metamorphism didn't reset isotopic clock; inaccurate if isotopes were lost/gained.
- **Sedimentary Rocks (indirectly)**: Usually not directly dated, but volcanic ash layers within them (igneous in origin) can be dated to constrain the age of the sedimentary rock and its fossils.

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