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DETERMINING the
AGE OF ROCKS AND FOSSILS
MODIFIED
FROM FRANK K. MCKINNEY The age of fossils
intrigues almost everyone. We are not only interested in how old a fossil is,
but also how that age was determined. Some very straightforward principles
are used to determine the age of fossils. As Earth Science students, we
should be able to understand the principles and have that as a background so
that age determinations by paleontologists and geologists don't seem like
black magic. There
are two types of age determinations. Geologists in the late 18th and early
19th century studied rock layers and the fossils in them to determine
relative age. William Smith was one of the most important scientists from
this time who helped to develop knowledge of the succession of different
fossils by studying their distribution through the sequence of sedimentary
rocks in southern England. It wasn't until well into the 20th century that
enough information had accumulated about the rate of radioactive decay that
the age of rocks and fossils in number of years could be determined through
radiometric age dating. PURPOSE
AND OBJECTIVES
This activity will help
you to have a better understanding of the basic principles used to determine
the age of rocks and fossils. This activity consists of several parts. The
objectives of this activity are:
2) To familiarize
students with the concept of half-life in radioactive decay. 3) To have students see
that individual runs of statistical processes are less predictable than the
average of many runs (or that runs with relatively small numbers involved are
less dependable than runs with many numbers). 4) To demonstrate how
the rate of radioactive decay and the buildup of the resulting decay product
is used in radiometric dating of rocks. 5) To use radiometric
dating and the principles of determining relative age to show how ages of
rocks and fossils can be narrowed even if they cannot be dated
radiometrically. MATERIALS
REQUIRED FOR EACH GROUP
1) Block diagram (Figure
1). PART
1: DETERMINING RELATIVE AGE OF ROCKS
(A) With your
team, discuss how to determine the relative age of each of the rock units in
the block diagram (Figure 1). After you have decided how to
establish the relative age of each rock unit, list them under the block, from
most recent at the top of the list to oldest at the bottom. Remember two of
our Earth History “Laws”:
PART
2: RADIOMETRIC AGE-DATING
Some elements have forms
(called isotopes) with unstable atomic nuclei that have a tendency to change,
or decay. For example, U-235 is an unstable isotope of uranium that has 92
protons and 153 neutrons in the nucleus of each atom. Through a series of
changes within the nucleus, it emits several particles, ending up with 82
protons and 125 neutrons. This is a stable condition, and there are no more
changes in the atomic nucleus. A nucleus with that number of protons is
called lead (chemical symbol Pb). The protons (82) and neutrons (125) total
207. This particular form (isotope) of lead is called Pb-207. U-235 is the
parent isotope of Pb-207, which is the daughter isotope. Many
rocks contain small amounts of unstable isotopes and the daughter isotopes
into which they decay. Where the amounts of parent and daughter isotopes can
be accurately measured, the ratio can be used to determine how old the rock
is, as shown in the following activities. Part A- At any moment there is a small chance that each of
the nuclei of U-235 will suddenly decay. That chance of decay is very small,
but it is always present and it never changes. In other words, the nuclei do
not "wear out" or get "tired". If the nucleus has not yet
decayed, there is always that same, slight chance that it will change in the
near future. Very
careful measurements in laboratories, made on VERY LARGE numbers of U-235
atoms, have shown that each of the atoms has a 50:50 chance of decaying
during about 704,000,000 years! In other words, during 704 million
years, half the U-235 atoms that existed at the beginning of that time will
decay to Pb-207. This is known as the half life of U- 235. Many elements have
some isotopes that are unstable, essentially because they have too many
neutrons to be balanced by the number of protons in the nucleus. Each of
these unstable isotopes has its own characteristic half life. Some half lives
are several billion years long, and others are as short as a ten-thousandth
of a second. (i)
Each team should obtain 100 pieces of
"regular" M & M candy. On a piece of notebook paper, each piece
should be placed with the printed M facing down. This represents the parent
isotope. (ii)
Pour the candy into a cup and shake the M & M’s
thoroughly. Pour them back onto the
paper so that it is spread out instead of making a pile. (iii)
This first time of shaking represents one half
life, and all those pieces of candy that have the printed M facing up
represent a change to the daughter isotope. Your team should pick up and set
aside ONLY those pieces of candy that have the M facing up. Then, count the
number of pieces of candy left with the M facing down. These are the parent
isotope that did not change during the first half life. (iv)
Report how many pieces of parent isotope remain in
the first row of the decay table (Figure
2). Next, calculate the
class average for this trial. The same procedure of shaking, counting the
"survivors", and filling in the next row on the decay table should
be done seven or eight more times. Each time represents a half life. (v)
Plot (Figure
3) the number of pieces of candy remaining after each
"shake" and connect each successive point on the graph with a
colored line. On the same graph, plot the AVERAGE VALUES for the class
as a whole and connect that with a different color. AND, on the same graph,
plot points where, after each "shake" the starting number is
divided by exactly two and connect these points by a differently colored
line. (This line begins at 100; the next point is 100/ 2, or 50; the next
point is 50/2, or 25; and so on.) (vi)
Answer the questions on your answer sheet. Part B- U-235 is found in most igneous rocks. Unless the
rock is heated to a very high temperature, both the U-235 and its daughter
Pb-207 remain in the rock. A geologist can compare the proportion of U-235
atoms to Pb-207 produced from it and determine the age of the rock. This
exercise shows how this is done. (i)
Each team should obtain 128 chips. The black side =
U-235, and the white side = Pb-207. Obtain a piece of paper marked TIME, on
which is written either 2, 4, 6, 8, or 10 minutes. (ii)
Place each marked piece so that the BLACK side
is showing. This represents Uranium-235, which emits a series of particles
from the nucleus as it decays to Lead-207 (Pb- 207). When you are ready with
the 128 pieces all showing black, wait for the teacher to announce the start
of a timed two-minute interval. During that time each team turns over half of
the U-235 pieces so that they now show WHITE. This represents one
"half-life" of U-235, which is the time for half the nuclei to
change from the parent U-235 to the daughter Pb-207. (iii)
STOP IF YOUR “TIME” PAPER SAYS 2-minutes! A new two-minute interval begins. During
this time the team should turn over HALF OF THE U-235 (black) THAT WAS LEFT
AFTER THE FIRST INTERVAL OF TIME. Continue through a total of 4 to 5 timed intervals. Again, STOP AFTER THE TOTAL TIME IS
EQUAL TO THE TIME ON YOUR PAPER! (iv)
After all the timed intervals have occurred,
exchange places with another TEAM as instructed by the teacher. Your task is
to determine how many timed intervals (that is, how many half-lives) the set
of pieces they are looking at has experienced. (v)
The half life of U-235 is 704 million years.
Both the team that turned over a set of pieces and the second team that
examined the set should determine how many million years are represented by
the proportion of U-235 and Pb-207 present, compare notes, and discuss any
differences that you determined. We will share results with the class PART
3: PUTTING DATES ON ROCKS AND FOSSILS
(i)
For the block
diagram (Figure 1) at the beginning of this
exercise, the ratio of U-235:Pb-207 atoms in the pegmatite is 1:1, and their
ratio in the granite is 3:1. Using the same reasoning about proportions as in
Part 2b above, determine how old the pegmatite and the granite are. Write the ages of the pegmatite and
granite beside the names of the rocks in the list below the block diagram (Figure
1). (ii)
By plotting the
half life on a type of scale known as a logarithmic scale, the curved line
like that for the M & M activity can be straightened out, as you can see
in the graph in Figure 4. This makes the curve more useful, because it
is easier to plot it more accurately. That is especially helpful for ratios
of parent isotope to daughter isotope that represent less than one half life.
(iii)
For the block
diagram (Figure 1), if a geochemical laboratory
determines that the volcanic ash that is in the siltstone has a ratio of
U-235:Pb-207 of 47:3 (94% of the original U-235 remains), this means that the
ash is 70 million years old (see Figure
4). If the ratio in the basalt is 7:3 (70% of the original U-235
remains), then the basalt is 350 million years old (again, see Figure
4). Write the age of the volcanic ash beside the shale, siltstone
and basalt on the list below the block diagram. (iv)
Answer the
questions on your answer sheet. |