Relative and absolute dating of geologic materials

1. 
   Use the attached graph paper to plot your data. The horizontal axis will represent time in years. Assume that time starts at 0 and that each shake of the container represents 2000 years.
   
2. 
   On the vertical axis you will plot the number of parent isotopes remaining after each shake of the container. At time 0 there will be fifty (50) parent atoms.
   
3. 
   Be sure to label your axes and title your graph.
   
4. 
   After plotting the data, draw a smooth, best-fit curve through the data points.
   

1. 
   Use your graph (decay curve) to determine the half-life for Statium.
   

2. 
   Imagine that you have found an unusual igneous rock and want to know its age. You take the rock to an isotope lab for analysis and are told that the rock contains only 16% of the original amount of Statium. The original amount was 50 atoms.
   

1. 
   Using your decay curve, find the age of this unusual rock.
   

2. 
   How many atoms of Carolinium (the daughter isotope) are in the rock at the time of the analysis?
   

3. 
   Analysis of a different rock shows that the ratio of parent to daughter isotope in the rock is 1/8 Statium to 7/8 Carolinium. Calculate the age of this rock sample using the half life that you determined from your decay curve.
   

4. 
   If the rock in question 3 originally contained 14,000 atoms of Statium, how many are remaining after 4 half-lives have passed? How many Carolinium atoms are there after 4 half-lives?
   

Where ln is the natural logarithm function (found on most calculators)

5 grams parent + 17 grams daughter= 22 grams total

ln (0.23) x (-1.4426) = 2.12 half-lives

2.12 half-lives x 300 million years = 636 million years old

1. 
   Determine the total amount of parent and daughter product and the ratio of parent element in the following rock samples:
   

2. 
   Determine the number of half-lives that have elapsed in the following rock samples given the ratios of parent to total parent plus daughter:
   

Half-lives elapsed

Rock D: 0.65 parent _______________

Rock E: 0.27 parent _______________

Rock F: 0.064 parent _______________

3. 
   Determine the age of the following rock samples if the radioactive element Potassium-40 is used (see Table 1):
   

1. 
   2 half lives: ____________ years old b) 0.2 half lives: ____________ years old
   

c) 0.5 half lives: ____________ years old d) 0.04 half lives: ___________ years old

4. 
   A rock sample has 1.36 g of Uranium-238 and 0.31 g of Lead-206. How old is the rock sample?
   

5. 
   A rock sample has 0.22 g of Uranium-235 and 4.12 g of Lead-207. How old is the rock sample?
   

Table 1: Some common radioactive elements used for absolute dating, their daughter products, and half-lives

Radioactve Parent	Daughter Product	Half-life
Uranium-238	Lead-206	4.5 billion years
Uranium-235	Lead-207	713 million years
Thorium-232	Lead-208	14.1 billion years
Rubidium-87	Strontium-87	47.0 billion years
Potassium-40	Argon-40	1.3 billion years
Carbon-14	Nitrogen-14	5730 years

Part 4. Combining relative and absolute dating to study faults and predict earthquake recurrence

One very practical application of relative and absolute dating is the estimation of earthquake recurrence intervals by excavating at the location of known faults. Trenches dug across faults exposed the sedimentary layers that have accumulated at these locations over time, and by carefully documenting which layers are offset, geologists can estimate the amount of fault motion during individual earthquakes and therefore also estimate the earthquake magnitude. If there is material that can be dated within the layers (for example plant fragments that can be dated by the carbon-14 method), they can also estimate the frequency of faulting events. This frequency is expressed as recurrence interval, i.e., the interval between faulting (earthquake) events.
One are that has been studied extensively by fault trenching is the Pallett Creek area along the San Andreas Fault System. Multiple trenches indicate that 10 earthquakes have occurred along this part of the fault system since the year 671 AD, the last in 1857.


The trench log below shows several closely spaced strands of the San Andreas Fault at the Pallett Creek site, designated by letters, which have moved at different times. The layers exposed in the trench are numbered, and ¹⁴C dates obtained for those layers are show in the table below.

For you to do:

1. 
   Examine the trench record and work out the relative order in which the fault segments have moved (from most recent to oldest). Then fill out the table below, listing the more recently active faults strands at the top, and the oldest at the bottom (the most recent strand has already been filled in for you). You might find it helpful to write the ages of the layers directly on the figure, including where they cross and are offset by the different fault segments.
   

Fault strand	Youngest layer offset	Oldest layer not offset	Age range of fault movement
D	81	93	1812-post 1857

2. 
   Study of multiple trenches in the Pallet Creek area indicates that earthquakes have occurred in the years listed below. Based on these ages, fill in the last column with the time between these events.
   

earthquake	Year (A.D.)	Time since previous earthquake (years)
1	1857
2	1812
3	1480
4	1346
5	1100
6	1048
7	997
8	797
9	734
10	671

Questions to answer:

1. 
   Which of the earthquakes on the previous table are represented in the Trench 10 record?
   

2. 
   What is the shortest time interval between earthquakes along the San Andreas Fault at Pallett Creek since A.D. 671?
   

3. 
   What is the longest time interval between earthquakes along the San Andreas Fault at Pallett Creek since A.D. 671?
   

4. 
   What is the average recurrence interval for earthquakes in this area?
   

5. 
   Given that the last earthquake on this part of the fault occurred in 1867, what does the geologic record predict will be the time range in which the next big event will occur?
   

Download 1.59 Mb.

Share with your friends: