Question

directions: use the diagram below to answer the following questions.

1. what is older, a or d? ____
2. what is older, e or a? ____
3. what is older, f or d? ____
4. what is older, the tilting/folding of the bottom layers or layer g?
5. on the diagram, there are several unconformities. identify an unconformity and make a dark line to show its location on the diagram.

use the chart and graph below to answer questions 15 - 20.

% of parent atoms	% of daughter atoms	half - lives elapsed	to calculate age:
50%	50%	1	1.0 x time of half - life
25%	75%	2	2.0 x time of half - life

the mineral zircon contains uranium. uranium - 238 decays to form lead - 206. the half - life of uranium - 238 is 4.5 billion years.

1. how old would a sample with 50% uranium - 238 and 50% lead - 206 be?
2. how old would a sample with 70.7% uranium - 238 and 29.3% lead - 206 be?
3. how old would a sample with 25% uranium - 238 and 75% lead - 206 be?

the mineral mica contains potassium. potassium - 40 decays to form argon - 40. the half - life of potassium - 40 is 1.3 billion years.

1. how old would a sample with 50% potassium - 40 and 50% argon - 40 be?
2. how old would a sample with 70.7% potassium - 40 and 29.3% argon - 40 be?
3. how old would a sample with 25% potassium - 40 and 75% argon - 40 be?

Explanation:

Step1: Apply principle of superposition for rock - layer age

In an undisturbed sequence of sedimentary rocks, the older layer is at the bottom. For questions 10 - 13, we assume the rock - layers were initially horizontal. Rock layer D is below layer A, so D is older. Layer E is below layer A, so E is older. Layer D is below layer F, so D is older. The tilting/folding of the bottom layers must have occurred before the deposition of layer G, so the tilting/folding of the bottom layers is older.

Step2: Identify unconformity

An unconformity is a gap in the geologic record. A possible unconformity is between the tilted layers at the bottom and the horizontal layers above them. You can draw a dark line separating the two sets of layers.

Step3: Calculate age using half - life

For questions 15 - 20, we use the formula in the table. For uranium - 238 with a half - life of 4.5 billion years:

• When 50% uranium - 238 and 50% lead - 206, 1 half - life has elapsed, so the age is $1\times4.5$ billion years = 4.5 billion years.
• When 70.7% uranium - 238 and 29.3% lead - 206, 0.5 half - lives have elapsed, so the age is $0.5\times4.5$ billion years = 2.25 billion years.
• When 25% uranium - 238 and 75% lead - 206, 2 half - lives have elapsed, so the age is $2\times4.5$ billion years = 9 billion years.

For potassium - 40 with a half - life of 1.3 billion years:

• When 50% potassium - 40 and 50% argon - 40, 1 half - life has elapsed, so the age is $1\times1.3$ billion years = 1.3 billion years.
• When 70.7% potassium - 40 and 29.3% argon - 40, 0.5 half - lives have elapsed, so the age is $0.5\times1.3$ billion years = 0.65 billion years.
• When 25% potassium - 40 and 75% argon - 40, 2 half - lives have elapsed, so the age is $2\times1.3$ billion years = 2.6 billion years.

Answer:

1. D
2. E
3. D
4. The tilting/folding of the bottom layers
5. Draw a dark line between the tilted layers at the bottom and the horizontal layers above them (location may vary depending on exact interpretation of unconformity)
6. 4.5 billion years
7. 2.25 billion years
8. 9 billion years
9. 1.3 billion years
10. 0.65 billion years
11. 2.6 billion years