Chapter 14 Lesson 2, "The Ages of Rocks and Fossils"

Materials:

• Worksheet, "The Ages of Rocks and Fossils"

Objective:

• Students will explain how a rock's relative age is determined.
• Students will explain how a rock's absolute age is determined.

Anticipatory Set:*

Students will view a jar filled with layers of gravel that are different colors. Then the students will be asked which layer the teacher put in the jar first. Show a picture of sedimentary rock layers. Explain just as they knew what layer the teacher put in the jar first, scientists know what rock layer is the oldest and was laid down first. In almost all cases there is a lot of changes to rock over millions of years so it is not as simple as it seems. Show the students a picture of a sedimentary rock that has folds, faults, and bends. Then show this five slide representation of how that rock may have been created. Then scientists can tell that if a certain kind of fossil is found in one of those rock layers then the same kind of fossil found in a rock layer 500 miles away must be about the same.

Teacher Input:* This kind of comparison of rock layers and fossils is called relative dating. Define relative dating. There is also a scientific principle that was developed out of these ideas. It is known as the principle of superposition. Define the principle of superposition. This seems quite simple but remember that the earth beneath your feet is constantly changing. Look at this diagram of a cross section of earth and put these events in the proper order of how they happened: Here is the correct sequence from first to last: (E, G, L, C, H, M, D, A, Erosion, N, K, B, F, Erosion. What we just did is known as the principle of crosscutting relationships. Define it.

This procedure works alright to compare rock layers that are all in the same place. However, how would you compare a sandstone in here in Wisconsin with an igneous rock found in Nulato. To do this scientists use an index fossil. Define index fossil. Explain that the trilobite had a hard shell so it fossilized well. It also lived on earth for a relatively short period of time. Scientists can then use these fossils to compare rocks that are not in the same area. When other fossils are found in the same layer as the trilobite we know that the two organisms lived at the same time.

Another way to tell how old rocks and fossils are is to use a method called Absolute dating. Define Absolute Dating. Show this picture and describe that there are some radio active elements that naturally break down into other elements. The original creation of the radioactive element differs but the breakdown takes place over a known amount of time. (Show this picture) This table shows how the percent of mother vs. dater elements can date the age of the fossil.

Here is an example of how one of these absolute dating techniques works.

Carbon-14 dating works like this: (Show this picture and describe the following)

1. Cosmic rays from the sun strike nitrogen-14 atoms in the atmosphere and cause them to turn into radioactive carbon-14, which combines with oxygen to form radioactive carbon dioxide.
2. Living things are in equilibrium with the atmosphere, and the radioactive carbon dioxide is absorbed and used by plants. The radioactive carbon dioxide gets into the food chain and the carbon cycle.
3. All living things contain a constant ratio of carbon-14 to carbon-12. (1 in a trillion).
4. At death, carbon-14 exchange ceases and any carbon-14 in the tissues of the organism begins to decay to nitrogen-14, and is not replenished by new C-14.
5. The change in the carbon-14 to carbon-12 ratio is the basis for dating.
6. The half-life is so short (5730 years) that this method can only be used on materials less than 70,000 years old. Archaeological dating uses this method.) Also useful for dating the Pleistocene Epoch (Ice Ages).
7. Assumes that the rate of carbon-14 production (and hence the amount of cosmic rays striking the Earth) has been constant (through the past 70,000 years).
Scientists used this method to tell the age of the iceman that was found melting out of a glacier in Italian Alps in 1991. They determined that he lived just over 5,000 years ago.

The half-life of carbon 14 though is so short that other elements have to be used for things older than 70,000 years.

Guided Practice:* Students will take turns answering the questions on Page 344.

Independent Practice:* Students will complete the worksheet, "The Ages of Rocks and Fossils"

Check for Understanding:* Students will complete the quiz on Chapter 14 Lesson 2, "The Ages of Rocks and Fossils"

Duration:

30 minutes + 10-15 for Independent Practice (Could be longer if taken for homework).

Alaska Content Standards Addressed in this lesson:

• SCI A-1 Students will understand models describing the nature of molecules, atoms, and sub-atomic particles and the relation of the models to the structure and behavior of matter (Structure of Matter);
• SCI A-2 Students will understand the physical, chemical, and nuclear changes and interactions that result in observable changes in the properties of matter (Changes and Interactions of Matter);
• SCI A-3 Students will understand models describing the composition, age, and size of our universe, galaxy, and solar system and understand that the universe is constantly moving and changing (Universe);
• SCI A-7 Students will understand how the earth changes because of plate tectonics, earthquakes, volcanoes, erosion and deposition, and living things (Processes that Shape the Earth);
• SCI A-8 Students will understand the scientific principles and models that
  describe the nature of physical, chemical, and nuclear reactions;
  state that whenever energy is reduced in one place, it is increased somewhere else by the same amount; and
  state that whenever there is a transformation of energy, some energy is spent in ways that make it unavailable for use (Energy Transformations);
• SCI A-9 Students will understand the transfers and transformations of matter and energy that link living things and their physical

• SCI A-15 Students will use science to understand and describe the local environment (Local Knowledge); and: