Geologic Time

1. As an example of how radioactive decay works, the TA may lead a small demonstration. Each student will receive one penny and stand up. At this point all of the students are parent isotopes. Every student should then flip their penny. Students whose penny lands heads-up should sit down. These students who are now seated are now daughter isotopes. The remaining standing students should again flip their penny, and students whose penny lands heads-up should sit down to become daughter isotopes.

a. How many students started out standing?

All of them

b. How many daughter isotopes were produced after the first flip of the pennies?

About Half

c. How many parent isotopes remained after the second flip of the pennies?

About a quarter

d. How well does each flip (or half-life) actually eliminate half of the remaining parent isotopes?

Decently well

e. Would d.) be improved by making the sample population smaller or larger? Why?

D would be improved by making it larger because larger numbers represent more general populations. Likelihood of eliminating half would be greater.

2.) Sometimes numerical dates are referred to as ‘absolute dates’. Why do you suppose this is? Why might these numerical dates not actually be absolute dates?

Because it gives us a number but there is still a range on the amount of error. Because absolute dates through radioactive dating are really only for igneous/metamorphic rocks and because heat and pressure affect the dates, therefore not making them absolute.

3.) In order to get an accurate numerical date, geologists require on average 0.5-1 grams of material. Why might this be difficult to obtain from some recent (<5,000 years old) volcanic ashes or other thinly bedded deposits?

It’s hard to collect that amount because the ash is so thin that it is easily distributed by wind and water can distribute the ash, making it hard to gather .5-1 grams of the same ash.

4.) In metamorphic rocks, minerals form at different temperatures and pressures. What could this potentially do to radiometric dates from a metamorphic rock?

Because it causes some of the daughter and parent isotopes to leak from minerals, therefore causing inaccurate dates for igneous/metamorphic rock when being radioactively dated.

5. On the side table are three fossil samples. Using figures 7-7 and 7-8, identify the invertebrate group to which each belongs, and indicate the range of geologic period(s) in which each lived.

Specimen #| Invertebrate Group| Range of Geologic Period(s)|
Fossil A| Trilobite| Cambrian – Permian|
Fossil B| Brachiopod| Cambrian – Cenozoic|
Fossil C| Nautiloid| Cambrian – cenozoic|

6. Examine the diagram below to answer the questions a through g. (a) List the relative ages of units A through I in order from oldest to youngest.

Oldest E H C I G A F D B Youngest

(b) Which stratigraphic principle did you use to determine the relationship between unit I and units (C, E,H)?

Law of Cross-Cutting relations

(c) The contact between unit C and unit G is called an angular unconformity. Hypothesize as to the events that occurred between the deposition of these two units.

Events such as collisions in a convergent margin, or any such event that causes uplift, would lead to the angular uniformity of units c and g.

(d) How has the energy of the depositional environment changed between the deposition of unit G and unit A?

Unit G was in a higher energy depositional environment, because conglomerate’s grains are coarser. Unit A was in a lower energy depositional environment because shale’s grains are more fine.

(e) Unit F has been numerically dated at 60 Myr (million years) and Unit I has been numerically dated at 220 Myr. Given this information what can be said about the ages of units G and A relative to F and I?

Unit F is the youngest of the 4 at 60 million years old. Unit A is slightly older. Unit G is older than A, but less than 220 million years. Unit I is the oldest at 220 million years.

(f) Would you expect to find Fossil C, from question 18, in units G and A?

Unit A.

(g) If unit C contained belemnoids and trilobites, how old might these deposits be?

The deposits could be 543 million years old. Belemnoids can be 323 million years old, trilobites can be 543 million years old.

8. Although it may seem strange, the storage of radioactive materials can be related to geologic time. It is believed that after approximately 10,000 years many of the nasty radioactive materials contained in this waste will have experienced one or more (the more the better!) half-life. This means that this waste needs to be stored in a ‘facility’ that will be safe from disturbances for at least 10,000 years.

a. How many generations of humans will come and go during this time? (Assume that the very optimistic life span of any one generation is 100 years.)

10,000 years / 100 = 100 generations

b. Knowing human nature, what are some problems with the waste sitting un-molested for 10,000 years?

As populations expand, people move outwards. This can disturb the waste. Thus, the likelihood of waste sitting untouched for 10,000 years is less.

c. If we are optimistic about human nature, what are some geologic problems to consider? For example, what are some locations (plate tectonic scale) that would be good places to store this waste?

Problems could be earthquakes or related natural disaster. The best place to store the waste would be in the middle of a plate. Less geologic disturbances.

d. What might be some problems that future geologists may have if they tried to date rocks that were storage facilities for radioactive waste?

Different isotopes from surrounding rocks may get on the rocks they try to date, which would disturb the calculations.