The age of fossils intrigues almost everyone. Students not only want to
know how old a fossil is, but they want to know how that age was
determined. Some very straightforward principles are used to determine the
age of fossils. Students 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.
This activity on determining age of rocks and fossils is intended for 8th
or 9th grade students. It is estimated to require four hours of class
time, including approximately one hour total of occasional instruction and
explanation from the teacher and two hours of group (team) and individual
activities by the students, plus one hour of discussion among students
within the working groups.
This activity will help students to have a better understanding of the
basic principles used to determine the age of rocks and fossils. This
activity consists of several parts. Objectives of this activity are:
1) To have students determine relative age of a geologically complex area.
2) To familiarise students with the concept of half-life in radioactive
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
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.
1) Block diagram (Figure 1).
2) Large cup or other container in which M & M's can be shaken.
3) 100 M & M's
4) Graph paper (Figure 2).
5) Watch or clock that keeps time to seconds. (A single watch or clock for
the entire class will do.)
6) Piece of paper marked TIME and indicating either 2, 4, 6, 8, or 10
7) 128 small cards or buttons that may be cut from cardboard or
construction paper, preferably with a different color on opposite sides,
each marked with "U-235" all on one colored side and "Pb-207" on the
opposite side that has some contrasting color.
Each team of 3 to 5 students should discuss together how to determine the
relative age of each of the rock units in the block diagram (Figure
1). After students have decided how to establish the relative age of
each rock unit, they should list them under the block, from most recent at
the top of the list to oldest at the bottom. The teacher should tell the
students that there are two basic principles used by geologists to
determine the sequence of ages of rocks.
Principle of superposition: Younger sedimentary rocks are deposited on top
of older sedimentary rocks.
Principle of cross-cutting relations: Any geologic feature is younger than
anything else that it cuts across.
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
nucl eus 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
Part 2a Activity — 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.
Atomic nuclei are held together by an attraction between the large nuclear
particles (protons and neutrons) that is known as the "strong nuclear
force", which must exceed the electrostatic repulsion between the protons
within the nucleus. In general, with the exception of the single proton
that constitutes the nucleus of the most abundant isotope of hydrogen, the
number of neutrons must at least equal the number of protons in an atomic
nucleus, because electrostatic repulsion prohibits denser packing of
protons. But if there are too many neutrons, the nucleus is potentially
unstable and decay may be triggered. This happens at any time when
addition of the fleeting "weak nuclear force" to the ever-present
electrostatic repulsion exceeds the binding energy required to hold the
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.
A tasty way for students to understand about half life is to give each
team 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. The candy should be poured into a container
large enough for them to bounce around freely, it should be shaken
thoroughly, then poured back onto the paper so that it is spread out
instead of making a pile. 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. The 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.
The teacher should have each team report how many pieces of parent isotope
remain, and the first row of the decay table (Figure 2)
should be filled in and the average number calculated. 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.
After the results of the final "half life" of the M& M are collected,
the candies are no longer needed.
Each team should plot on a graph (Figure 3) the number
of pieces of candy remaining after each of their "shakes" and connect each
successive point on the graph with a light line. On the same graph each
team should plot the AVERAGE VALUES for the class as a whole and connect
that by a heavier line. AND, on the same graph, each group should 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.)
After the graphs are plotted, the teacher should guide the class into
1) Why didn't each group get the same results?
2) Which follows the mathematically calculated line better? Is it the
single group's results, or is it the line based on the class average? Why?
3) Did students have an easier time guessing (predicting) the results when
there were a lot of pieces of candy in the cup, or when there were very
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. The next part of this
exercise shows how this is done.
Part 2b Activity — Each team receives 128 flat pieces, with U-235
written on one side and Pb-207 written on the other side. Each team is
given a piece of paper marked TIME, on which is written either 2, 4, 6, 8,
or 10 minutes.
The team should place each marked piece so that "U-235" is showing. This
represents Uranium-235, which emits a series of particles from the nucleus
as it decays to Lead-207 (Pb- 207). When each team is ready with the 128
pieces all showing "U-235", a timed two-minute interval should start.
During that time each team turns over half of the U-235 pieces so that
they now show Pb-207. 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
A new two-minute interval begins. During this time the team should turn
over HALF OF THE U-235 THAT WAS LEFT AFTER THE FIRST INTERVAL OF TIME.
Continue through a total of 4 to 5 timed intervals.
However, each team should STOP turning over pieces at the time marked on
their TIME papers. That is, each team should stop according to their TIME
paper at the end of the first timed interval (2 minutes), or at the end of
the second timed interval (4 minutes), and so on. After all the timed
intervals have occurred, teams should exchange places with one another as
instructed by the teacher. The task now for each team is to determine how
many timed intervals (that is, how many half-lives) the set of pieces they
are looking at has experienced.
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 haggle about any differences
that they got. (Right, each team must determine the number of millions of
years represented by the set that they themselves turned over, PLUS the
number of millions of years represented by the set that another team
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, students
can determine how old the pegmatite and the granite are. They should write
the ages of the pegmatite and granite beside the names of the rocks in the
list below the block diagram (Figure 1). By plotting
the half life on a type of scale known as a logarithmic scale, the curved
line like that for the M & MTM 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. 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).
Students should write the age of the volcanic ash beside the shale,
siltstone and basalt on the list below the block diagram.
1) Based on the available radiometric ages, can you determine the possible
age of the rock unit that has acritarchs and bacteria? What is it? Why
can't you say exactly what the age of the rock is?
2) Can you determine the possible age of the rock unit that has
trilobites? What is it? Why can't you say exactly what the age of the rock
3) What is the age of the rock that contains the Triceratops fossils? Why
can you be more precise about the age of this rock than you could about
the ages of the rock that has the trilobites and the rock that contains
acritarchs and bacteria? Note for teachers: Based on cross-cutting
relationships, it was established that the pegmatite is younger than the
slate and that the slate is younger than the granite. Therefore, the slate
that contains the acritarch and bacteria is between 704 million years and
1408 million years old, because the pegmatite is 704 million years old and
the granite is 1408 million years old. The slate itself cannot be
radiometrically dated, so can only be bracketed between the ages of the
granite and the pegmatite.
The trilobite-bearing limestone overlies the quartz sandstone, which
cross-cuts the pegmatite, and the basalt cuts through the limestone.
Therefore the trilobites and the rock that contains them must be younger
than 704 million years (the age of the pegmatite) and older than 350
million years (the age of the basalt). The limestone itself cannot be
radiometrically dated, so can only be bracketed between the ages of the
granite and the pegmatite.
The Triceratops dinosaur fossils are approximately 70 million years old,
because they are found in shale and siltstone that contain volcanic ash
radiometrically dated at 70 million years. Any Triceratops found below the
volcanic ash may be a little older than 70 million years, and any found
above may be a little younger than 70 million years. The age of the
Triceratops can be determined more closely than that of the acritarchs and
bacteria and that of the trilobites because the rock unit that contains
the Triceratops can itself be radiometrically dated, whereas that of the
other fossils could not.