- The age of sedimentary deposits can be determimed through knowledge of rates of sedimentaton, the presence of index fossils, and by measurement of radioactive decay.
- An index fossil is a fossil from an organism, about which we have a clear understanding when it lived, or the conditions under which it lived. The presence of an index fossil enables us to make inferences about the sage and/or environmental conditions of the sediments in which it is found.
- By knowing the time and conditions under which fossils were deposited, we are able to better reconstruct the origins of species like the California gray whale.
How can we learn about the origins of an animal such as the California gray whale? The sciences of geology and paleontology are especially useful in determining the evolutionary history of a species since most of the information comes from the rocks of the earth itself. Geology includes the study of earth’s crust and rocks, and the age and origins of rocks. Paleontology is the scientific study of forms of life existing in former geologic periods, as represented by their fossils. Both disciplines contribute to our understanding of the origins of whales and both have branches which form integral parts of the marine sciences.
Certain geologic principles are especially helpful in determining the age of origin of a species. The law of original horizontality states that materials are deposited in horizontal layers. The principle of superposition states that when the layers of rock are horizontal, or nearly so, each overlying bed of rock is younger than the one beneath it. And finally, the principle of uniformitarianism states that the processes (erosion, deposition, etc.) which we observe today have been occurring throughout the history of the earth. These three principals let us "read" rock formations and determine their relative ages; that is, which is oldest, youngest etc., not their age in years.
Paleontologists also contribute important information through the identification of "index fossils". Index fossils are found in rock layers of only one geologic age. They serve to identify specific rock layers no matter where they are found. Not only do index fossils tell scientists the age of the rock formation in which they are found, they also provide information about the environmental conditions present during the time the rock layer was formed.
By understanding the clues found in rock strata and the fossil record, we can reconstruct the origin of whales. It is wise to point out, however, that fossils can be transported and redeposited. As such, index fossils are not a fool-proof method of aging rock formations. For example, whale fossils have been moved from the ocean floor to Pacific coastlines by the spreading of the sea floor. Such observations are the anomalies that make geology and paleontology such exciting fields of study.
For each group of 2-3 students:
For each student:
- one "fossil-bearing" cupcake
- scalpel or sharp knife
- 2 centimeter rulers
- recycled paper cut in quarters
- "Sweet Stratigraphy" and "How Old?" activity pages
"How Old?" is a two part lesson that provides information about the detective processes of geology, archaeology and paleontology which have given us our current picture of whale descent. In Part 1, "Sweet Stratigraphy", students determine the age of a "fossil" in a sample of "rock strata". In Part 2, students explore basic geologic principles and the role of index fossils in determining geologic ages.
Part 1 – Sweet Stratigraphy
Note that "How Old?" is not intended to be a comprehensive coverage of stratigraphy. It is intended to provide supporting evidence for theories on origins of cetaceans and to provide a springboard for further explorations in geology. Earth science texts provide a good source for additional information about the topics covered in this section.
Part 1, "Sweet Stratigraphy", is designed to give your students experience determining the age of a fossil in a sample of rock strata containing fossils of known ages, known as index fossils. In this activity, a cupcake or piece of layer cake with layers of colored batter will serve as rock strata, an m&m candy will be the specimen of uncertain age, and peanuts, raisins and chocolate chips will be index fossils.
An added twist in this activity is that students are not given all information about their sample and its index fossils outright. They must generate specific questions to ask, much as scientists need to generate questions to find out what is already known about a field prior to embarking on research, as well when they design their actual study.
Directions are given for cupcakes; however, you may find that squares of cake baked in a sheet pan provide more uniform samples. If you choose the latter, be sure to place sufficient m&m specimens in the samples that students can locate one easily.
If you are using "Voyage of the Mimi" in conjunction with this unit, "Expedition 4: Whale Bones" correlates well with the above lesson.
- Use white cake mix to prepare the fossil-bearing cupcakes. In determining the number of cupcakes needed, recall that this activity is best accomplished by student teams of 2-3. Preheat oven and line muffin tins with paper liners.
- Prepare the white cake mix according to the package directions. The batter should be the consistency of heavy cream so that objects you add to the batter don’t settle out. If it is too thin, add additional flour to the batter. Stir the batter as you add the flour to keep the batter from getting too stiff. If the batter is too stiff, you may find it difficult to form horizontal layers.
- Divide batter into three parts. Tint one portion red, one portion green, and leave the third white.
- Assemble the following materials close to where you will be working: peanuts, chocolate chips, raisins, and m&m candies. If you can find them, white raisins are preferable to dark raisins in that they are easier to distinguish from chocolate chips.)
- Add 1/4 cup chocolate chips and 1/4 cup peanuts to the green batter and place about a 1/2 inch of this mixture into each tin.
- Over the green batter, lightly place 4-5 m&m candies, including some so close to the edge that they are likely to be visible once the paper liner is removed.
- Add 1/4 cup chocolate chips and 1/4 cup of raisins to the white batter and gently place about 1/2 inch of this mixture over the m&ms and green batter.
- Stir 1/4 cup of raisins into the red batter and place a final 1/2 inch of this batter over the other layers.
- Bake as directed on package mix.
Information you may give out if asked:
- Provide each student team with a sample "rock" (cupcake). You may wish to call the cupcake "fossil-bearing strata" or some similar term. Explain to them that they are teams of paleontologists in search of a rare prehistoric fossil. (Show them an m&m.) Their job is to pinpoint as nearly as possible the age of the fossil. They will probably have to make use of any and all clues available to them, such as the kind of sedimentary material it is found in, or knowledge other prehistoric creatures that may have lived at about the same time about which more information is known. As paleontologists, they may need to make careful excavations into exposed outcroppings of strata layers to gather more information.
- Suggest that students begin by finding a specimen of the fossil in their rock sample, but caution them against removing it for fear of destroying important clues.
- Offer students the use centimeter rulers and introduce the procedure for mapping an outcrop of their "fossil-bearing rock." You may wish to have them us the grid found on their student activity pages to make a full-scale diagram of a vertical "rock face". Their diagram should show the position of rock layers, the m&m fossil, and additional fossils (e.g., peanuts, chocolate chips, and raisins) they find in the layers. Students will need to make horizontal and vertical measurements using a centimeter ruler in order to apply all features to the grid paper on their data sheet.
- Tell students that certain information about these sedimentary deposits is known, but that they will have to make inquiries of "scientific literature" to gain access to this information. To do this, student groups may send one representative to ask you a specific question about a characteristic of the sedimentary materials in their samples. Stress that you will answer no vague questions. In order to encourage thoughtful and systematic questioning, you may want to limit the number of questions groups can to ask to 5. Answer only one question at a time per group, and to eliminate eavesdropping, communicate with student representatives through the written word only. Tell them that some questions do not yet have answers, and therefore you may not be able to answer every question you are asked.
- Chocolate chips first appeared about 1,000,000 years ago and went extinct 4000,000 years ago.
- Peanuts appeared on earth about 1,500,000 years ago and went extinct about 600,000 years ago.
- Raisins appeared 400,000 years ago and are still living today.
In the discussion that follows the activity, you might point out that fossils can be transported and redeposited, and allow students to suggest possible ways that could happen. Students should understand that index fossils are not a fool-proof method of aging rock formations.
Part 2 – "How Old?"
Note that although the student reading "How Old?" is presented here as following "Sweet Stratigraphy" as a way of reinforcing the concepts presented, it can be used before that activity to introduce those concepts.
- cross-section – a vertical section or slice of anything, such as rock layers
- dating – to determine the age of something
- erosion – the weathering or wearing away of land features
- fossil – any remains, impression, or trace of a living thing of a former geologic age, as a skeleton, footprint, etc.
- index fossil – a fossil of an organism of known age or habitat needs, useful for dating or characterizing the strata in which it is found
- lead 206 – a form of lead found naturally in the environment, used in dating rocks through measurement of radioactive decay
- radioactive decay – the transformation of a radioactive element into a different element through release of radioactive particles
- sediment – materials which settles out of water
- strata – layers
- uranium 238 – a form of uranium found naturally in the environment, used in dating rocks through measurement of radioactive decay
- Take your class on a fossil hunt at a known site. To find out about possible destinations, a phone call to the geology department of your local university is usually a good starting place. Be sure to get permission of any property owners, and make sure you have a clear understanding of whether you and your students may take materials from the site. A trip of this nature provides abundant opportunities for recording observations, student reporting, and involving your class in creating a display on paleontology in your school.
- Natural history museums are a resource not to be overlooked. In addition to providing interpretive displays on prehistoric life and geology, many also make loans of fossil specimens for classroom use.
- As an art extension, have students "make" fossils by pressing shells, leaves or bones into a soft substance, such as damp pottery clay to leave a negative imprint. Then plaster of Paris can be poured into the negative mold. Other substances to try include salt-flour dough and even pottery clay, which can later be fired in a kiln.
Part 1 – Sweet Stratigraphy
Answer Key – cont.
Analysis and Interpretation
- Students diagrams will vary, depending on the characteristics of their "strata." Students are asked to include the depth of each sediment layer, the placement of the index fossil, and the location and labeling of other fossils.
- a. If the students’ fossils have remained in the position they were placed during the baking process, they should arrive at the age of 600,000 years.
b. Answers will vary depending upon the strategies employed to determine the age. Do not be too concerned if dates vary so long as students have followed a rational process to arrive at their answers.
- The answer will depend somewhat on the properties of the cake. If the fossil is 3 cm below the surface and its age is 600,000 years, one centimeter of sediment would have taken 200,000 years to accumulate.
- a. The presence of an index fossil allows us to apply known information on age and environmental conditions of the index species to other organisms found in the same sediments.
b. In this activity the chocolate chips and raisins served as index fossils for dating the m&ms.
Part 2 – How Old?
- A correctly labeled drawing is found below.
- We can assume the principle is true of most rock layers because sedimentary materials are continually deposited from above. If no other disruptive forces are at work, the youngest sediments will always lie above older layers.
- Since the question calls for an opinion, students should be encouraged to speculate. As such, answers may vary. Some of the layers appear to be folded by movements of the earth’s crust. There has probably been erosion and then additional layers deposited over much earlier ones. Volcanic lava intrusions are extending up into the layers as well.
- This simple math problem may cause your students some initial frustration. If this is the case, provide them with help useful in setting up this (and other similar) problems.
1 centimeter/year = 1 km/? years
Since 1 km = 1000 meters and 1 meter = 100 cm, then 1 km = 100,000 cm. At the rate of 1 cm/year, the it has taken 100,000 years to deposit 1 km of sedimentary rock.
- a. Your students can determine the approximate age of the rock by noting that the uranium has decayed to 50% of its original amount, the definition of half life indirectly given in the text. So, 50% of Uranium left = 1 half life = 4.5 billion years.
b. If a rock contains 25% lead and 75% uranium, .5 half life has elapsed or 2.25 billion years.
This is the best answer your students can give based on the information they have. This answer is only an estimate, however, because radioactive decay is not a straight line function. In reality, during each year that passes, 1/7,700,000,000 of the amount of uranium is changed to lead.
- Index fossils are found in rock layers of only one geologic age and hence are useful in providing information about when the first whales lived. They serve to identify specific rock layers no matter where they are found.