Time, and Time Again
Unit Developed by: Jonathan Hoffman
Grade level: 7th-11th
Estimated time:
Lesson 1: 60-90 minutes
Lesson 2: 45-60 minutes
Lesson 3: 45-60 minutes
Topics covered: Geologic time, the Geologic Time Scale, relative dating, absolute dating
Standards and Benchmarks:
Grade 11: Content Standard 1: Concepts and Processes
Benchmark 9: Origin and Evolution of the Earth
Grades 5-8: Standard 1: Concepts and Processes
Benchmark 9: The Earth’s History
Objectives:
The goals of this lesson plan are to increase the students’ understanding of:
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the depth of geologic time
how the Geologic Time Scale was constructed and what evidence was used
how the Geologic Time Scale has been corroborated by additional lines of
evidence.
These goals can be divided into three lessons and these lessons can be presented in
accordance to time constraints and grade level. These units are:
1) Geologic Time: “Walk through time”
2) Relative Dating and the Geologic Time Scale
3) Absolute Time and Radioactive Decay
Note: Lessons 1) is recommended for grades 7-9, while Lessons 2) and 3) are
recommended for grades 10-11, time permitting.
Lesson 1: Geologic Time: “Walk through time”
Vocabulary:
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eon = The largest unit of time.
era = A unit of time shorter than an eon but longer than a period.
period = A unit of time shorter than an era but longer than epoch.
epoch = A unit of time shorter than a period but longer than an age.
Archean = “Ancient” eon from 4,500 Ma – 2,500 Ma.
Proterozoic = “Early life” eon from 2,500 Ma – 540 Ma.
Paleozoic = “Ancient life” eon from 540 Ma – 248 Ma.
Mesozoic = “Middle life” eon from 248 Ma – 65 Ma.
Cenozoic = “Recent life” eon from 65 Ma to Present.
Holocene = “All recent” epoch from 10 Ka to Present
Ma = Mega annum, i.e. million years ago before present.
Ka = Thousand years ago before present.
Materials:
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46 ft. of butcher paper
markers and/or crayons
Preparation:
Before class, measure out 46 feet (1 ft/100 million years) of butcher paper and use it to
trace the perimeter of the classroom. If your classroom is too small, you can use a shorter
stretch of paper; just make sure that you calculate the proper ratio of feet to years. You
can also make your scale longer and lay it down on a football field or in a gymnasium.
Use a marker to draw lines at the following time boundaries:
 2500 Ma (21 ft. from the origin)
6 students
 540 Ma (40 ft. 7.2 in.)
6 students
 248 Ma (43 ft. 6.25 in.)
5 students
 65 Ma (45 ft. 4.2 in.)
3 students
 10 Ka (45 ft. 11.99 in.)
Label these divisions, along with the dates of each boundary, as the:
 Archean Eon (4,500 Ma – 2,500 Ma)
 Proterozoic Eon (2,500 Ma – 540 Ma)
 Paleozoic Era (540 Ma – 248 Ma)
 Mesozoic Era (248 Ma –65 Ma)
 Cenozoic Era (65 Ma – Present).
Do not label the last division, the Holocene Epoch (10 Ka-present), since there is not
enough room. Instead, color that thin strip of paper with a bright marker (orange or
yellow, maybe).
Summary of Lesson Plan:
Begin by asking the students to list major events in human/American history and their
own lives (e.g. their births) to construct a time scale of what they perceive of human
history on the chalkboard. A few suggestions for historical events include:
 construction of the first of the Great Pyramids in ~2600 B.C.
 the era of Ancient Greece from ~1100 B.C. to 146 B.C.
 the Magna Carta is issued in 1215 A.D.
 Columbus arrived in America in 1492 A.D.
 the Declaration of Independence is signed in 1776 A.D.
 Wyoming becomes a state in 1890 A.D.
 World War II ends in 1945 A.D.
 Neil Armstrong walks on the moon in 1969 A.D.
The students will likely focus on events within their lifetimes, so add a few of the above
suggestions to give them some historical perspective.
Next, divide the students into five groups and assign each group to graphically depict the
major events of each of the following:
 the Archean Eon
 the Proterozoic Eon
 the Paleozoic Era
 the Mesozoic Era
 the Cenozoic Era
Since the different divisions are composed of vastly different stretches of time, you may
find it easiest to divide students into groups that are sized proportional to the length of the
time division. The major events for these time divisions are as follows:
Archean (“Ancient”) Eon:
Formation of the oceans and sea floors, meteor
bombardment, formation of small volcanic islands, and
the evolution of blue-green algae, floating in the oceans
as algal mats (the only known life at that time).
Proterozoic (“Early Life”) Eon: Formation of two supercontinents, still a lot of volcanic
activity, and the evolution of multi-celled organisms at
the very end of the Proterozoic.
Paleozoic (“Ancient Life”) Era: (in order) the evolution of fish, evolution of land plants,
evolution of terrestrial insects (e.g. giant dragonflies and
centipedes) evolution of terrestrial tetrapods (fourlimbed vertebrates similar to amphibians).
Mesozoic (“Middle Life”) Era: (in order) evolution of small dinosaurs (e.g. Coelophysis),
the break-up the supercontinent Pangea, evolution of
sauropod dinosaurs (“long-neck” dinosaurs such as
Diplodocus and Brachiosaurus), evolution of small
rodent-like mammals, evolution of ceratopsian (e.g.
Triceratops) dinosaurs and Tyrannosaurus rex, evolution
of birds, and the giant meteor impact at the end of the
Mesozoic.
Note: There are many different types of dinosaurs that
appear in the Mesozoic and the students should not be
limited to just those mentioned above. The dinosaurs
mentioned above are merely some of the more
recognizable dinosaurs.
Cenozoic (“Recent Life”) Era: evolution of modern mammals, build-up of the
Himalayas, continents attain their modern positions.
Allow 10-20 minutes for the drawing. Also, encourage the students to incorporate any
available images from their textbook (e.g. the Proterozoic supercontinents, Paleozoic
terrestrial plants). Encourage the students working on the longer time divisions to cover
as much of their division with pictures as possible (e.g. long stretches of oceans and sea
floors in the Archean). Since the Cenozoic strip of paper is only about 8 inches long, the
students (more than 3 will make it difficult) working on it will probably be able to only
fit a portion of the Cenozoic events. Encourage them to spend a significant portion of the
time allowed to choose what they think are the most important events to draw. Also,
make sure that the students working on the Cenozoic Era do not mark over the thin strip
of the Holocene Epoch.
Once the drawing is complete, gather the class at the beginning of the “Walk through
time.” Take them down the pathway, giving a brief account of the major events and
noting the beginning and end of each time division. Once you reach the end of the
walkway, inform the students that the thin strip at the end of the walkway represents the
Holocene (“Entirely Recent) Epoch, starting 10,000 years ago. Then, stress to them that
their human time scale on the board (at most a fifth of the Holocene), fits into a small
fraction of this small strip of the Earth’s history.
Lesson 2: Relative Dating and the Geologic Time Scale
Vocabulary:
 index fossil = A fossil species that existed for a relatively short period of time and
is often geographically widespread. They are useful for dating rock units.
 sedimentary rock = A rock formed from the deposition of sediment.
 igneous rock = A rock formed by the crystallization of magma or lava.
 metamorphic rock = A rock, either sedimentary or igneous, that has been altered
by heat, pressure, or chemical reactions.
 fault = A fracture in rock in which movement has occurred.
 stratigraphic column = A visual depiction of the stratigraphic sequence, or order
of rock layers, for a given locality.
 outcrop = An exposure of bedrock.
 faunal succession = The sequence of fossil organisms from layer stratum to the
next.
Materials:
 variously colored (blue, green, red) play-doh or clay
Preparation: The students should already be familiar with the rock cycle and the three
main types of rocks (sedimentary, igneous, and metamorphic).
Background Information:
Introduce the basic principles of relative age relationships:
1. Principle of superposition: In a sequence of undeformed sedimentary rock layers, the
rocks get older from top to bottom (i.e. the bottom layer is the oldest layer and the top
layer is the youngest).
2. Principle of original horizontality: When sedimentary rock layers are being deposited,
gravity forces them to be deposited as flat, horizontal layers. Once the sediment has
solidified and become rock, they can be tilted or folded.
3. Principle of faunal succession: Groups, or assemblages, of plant and animal fossils
appear in the geologic record in a specific order. These assemblages can be used to
identify certain periods of geologic time.
4. Principle of crosscutting relations: An igneous rock unit or fault that cuts across
another rock unit must be younger than the unit it cuts across. In other words, the other
rock unit must have already been there for the igneous rock or fault to cut across it.
5. Principle of inclusion: A rock unit that contains inclusions of preexisting rocks must be
younger than the rock unit from which the inclusions came.
6. Metamorphic rocks: A metamorphic rock is always older than the non-metamorphosed
rocks around it. The metamorphic rock must have formed before the surrounding rocks,
otherwise they would be metamorphosed as well.
Summary of Lesson Plan:
Part A. Relative Dating: Which Rock Layer Formed First?
This section is designed to introduce and illustrate the principles of relative age
relationships. Break the students up into groups and give each group the activity
worksheet and a set of clay (blue, green, and red). Allow the students to use the clay to
progress through the first section of the worksheet, “Relative Dating: Which Rock Layer
Formed First”. Once they have completed this activity, they will be able to determine
relative ages in the subsequent cross-sections in the worksheet. You may want to lead the
students through the first problem. List the units, from oldest to youngest, and cite the
principle you used to determine each step of the sequence. The relative age problems can
be removed as seen fit for time constraints.
Part B. Fossil Ages: There and Back Again
This section is designed to show students how fossil assemblages can be used to
determine the ages of rocks. Hand out the Fossil Ages: There and Back Again worksheet
and let the students work through it. The end of the worksheet refers to the use of index
fossils to refine ages. It is worth noting that most index fossils are invertebrate fossils.
References:
McLelland, Christine. “A Relative Age Dating Activity.”
http://www.geosociety.org/educate/LessonPlans/Relative_Age.pdf
Lesson 3: Absolute Time and Radioactive Decay
This final unit explains how radioactive decay occurs. Radioactive decay is the main
principle behind absolute dating techniques that have corroborated the sequence of the
geologic time scale as well as provided absolute dates.
Vocabulary:
 radioactive decay = Spontaneous disintegration of an atomic nucleus into a lighter
one, releasing alpha, beta, or gamma particles.
 isotope = Forms of an element with the same number of protons but different
numbers of neutrons.
 parent isotope = An isotope that radioactively decays into a different (daughter)
isotope.
 daughter isotope = The product of the radioactive decay of an isotope.
 half-life = The time required for 50% a radioactive substance to decay into the
daughter isotope.
 logarithmic growth/decay = Growth or decay described by the function y=logx.
 exponential growth/decay = Growth or decay described by a constant being raised
to the power of the variable x.
Materials: wooden “isotope” blocks, demonstration chart, calculators
Summary of Lesson Plan:
Divide the students into groups and give each a set of 16 wooden blocks and a
demonstration chart. Introduce the principles of radioactive isotopes and half-lives. You
can use the wooden blocks (use only 8 to give the students an example that is slightly
different from the exercise they are about to complete). After this demonstration, the
students should be able to work through the worksheet together to understand absolute
dating. If you have extra time available or advanced students, you can hand out the
“Nuclear Waste Disposal” exercise.
References:
Myers, James and Erin Campbell-Stone. 2008. University of Wyoming Department of
Geology and Geophysics. “Relative and Absolute Time.”