GRADE: Seventh

CONTENT STANDARD: Earth and Space Science

CONTENT TOPIC: Earth's History

CONCEPT: The constantly changing nature of the earth is evident.

CONTENT OBJECTIVE: 7Jl.00 To understand the characteristics of geologic time

INSTRUCTIONAL OBJECTIVES: The learner will:

1.01 describe the law of superposition.
1.02 demonstrate which sedimentary rock preserves the best fossils.
1.03 differentiate between a cast and a mold.
1.04 show events in the history of the earth.

I. Law of superposition
II. Fossils
A. Molds
B. Casts

TN COMPONENT OF SCIENCE: Unifying Concepts of Science

TN GOAL:

To enable students to acquire scientific knowledge by applying concepts, theories, principles and laws from life/environmental, physical, and earth/space science.

2.1 SCALE AND MODEL - The development of models provides a conceptual bridge between the concrete and the abstract, while the use of scales allows for a comparison of differences in magnitude between the model and the desired form.

2.1a A model is a representation used to simplify complex phenomena.

BENCHMARK: Models are often used to represent concepts of various magnitudes.

2.1c Models are often used to study processes that happen too slowly, too quickly, or on too small or too large a scale to observe directly.

BENCHMARK: Models are often used to represent processes that cannot be directly observed.

TIME REQUIRED:

Two forty-minute periods

Petrified wood, 3 small pie pans, spoon, patching plaster, sand, gravel, clay, water, paper cups, shell, tape, paper, pencils, meter sticks, reference books on geology, small fossils

Today we will discuss the characteristics of geologic time. (Pass out a few fossils and let the students try to figure out in what time period they flourished.)

Many attempts have been made to measure the age of the earth. Some of these attempts are based on amounts of radioactive elements in minerals within the containing these elements. They attempt to judge ages for other layers by their relationship with the dated layers.

Layered rocks are studied by geologists for clues to their age. They also study fossils, evidence of past life. Fossils are usually found only in sedimentary rocks. Much about geologic history is based on what is learned from sedimentary rocks. Most sedimentary rocks are laid down in horizontal layers, with the oldest beds at the bottom. Younger beds are laid down on the older ones, as time goes on, this layering order is called the law of superposition.

(With teacher help)

Materials: 3 small pie pans, spoon, patching plaster, sand, gravel, clay, water, paper cup, shell

Procedure:
1. Label the pans A, B, and C. Make a firm mixture of sand and water in pie pan A. Make a firm mixture of clay and water in pie pan B. Make a firm mixture of gravel, clay, and water in pie pan

2. Allow the moisture in all three pans to stand for a few minutes. Press the shell into the mixture in each pan. Remove the shell carefully so you leave the most distinct impression possible.

3. Record the appearance of the impression and the clarity of the imprint in the table.

4. Mix a small amount of patching plaster with water in the paper cup. (It should be soft enough to take a form.) Press some of the plaster into your best impression and allow it to harden.

5. When the plaster is hard, break the surrounding rock away and examine the fossil you have made. Record its appearance in your table.

Pan Appearance Type Of Fossil Made

A

B

C

(Ask the class the following questions:)

1. In which of the mixtures did the shell make the best impression? (Clay mixture B takes the best impression.)

2. What sedimentary rock does this mixture resemble most closely? (Pan B represents shale.)

3. Which mixture, if any, resisted your efforts to make an impression enough to distort the shape of the impression? (Sand in pan A crumbles when shell is removed, and gravel in pan C makes it impossible to get a good impression.)

4. What type of fossil did you make in step 2 -- mold or cast? (This impression is a mold.)

5. What type of fossil did the patching plaster make -- mold or cast? (The patching plaster makes a cast.)

6. How did the mold and cast differ in appearance? (A cast shows no internal body parts.)

Scientists called paleontologists study plant and animal fossils. Usually hard parts of animals are preserved. Sometimes entire animals are preserved. (Siberian Woolly Mammoth.)

Another type of impression is carbon. Leaves and soft-bodied animals are buried in soft mud. As the plant or animal matter decays, a coal-like film of carbon is left behind which shows the fine lines of the leaf or animal.

Foot prints are another type of fossil record. Tracts of animals may be left in soft mud.

Some fossils are molds and casts. A mold is a cavity left in the rock when groundwater dissolves and removes a shell or bone around which sand or mud already has hardened. If a mold later is filled with mineral matter or mud, the filling is a cast. A cast has the same shape as the original organism. However, when it is broken open, the lack of inner structure shows it to be a cast.

Mineral matter may seep into shells, wood or bone openings. The mineral may then preserve the fossil. Example: Silica replaces cellulose to form petrified wood which preserves the tree rings, bark and cells. (Pass out pieces of petrified wood so students can see and feel it.)

Earth history is divided into time units based on changes in the fossil record. Divisions of time are based on the appearance and disappearance of certain fossils. These divisions of geologic time are recognized on a worldwide basis.

Materials needed: Tape, paper, pencil, meter-stick, reference books on geology.

Background: It is not always easy to realize the great amounts of time that are involved in the history of the earth. One way to do so is to diagram events on a time line. A time line has a scale in which a given length represents a certain number of years. In making your time line, l mm will be used to represent 1 million years.

Procedure: Cut strips of paper about 10 cm wide. Tape several strips together until you have a strip that is 4.6 m long. Using 1 mm to represent 1 million years, mark the information from the table on your strip. Each date is given in millions of years ago. Use your book and other reference books to mark in other dates you think are important.

Date Label

4,600 Beginning of earth's history 325 Reptiles

4,000 Oldest rock 290 Allegheny Mountains

3,000 Oldest fossils 200 Pangaea

600 Fossils become common 180 Pangaea began breaking up

410 Land plants 135 Dinosaurs

400 Caledonian Mountains 130 Rocky Mountains

380 Insects, amphibians 2 People

1. What length on your time line represents the time from the beginning of earth history to the time fossils became common?

2. What information other than what is in the table did you add to your time line? (Answers will vary.)

3. How does a time line help you show the great length of geologic time?

*Something to think about: Suppose you were to mark dots on a piece of paper, making 1 dot every second. How long would it take you to make a million dots if you could make 1 dot after another without stopping? (114 days, 11 hours, 1 minute, 20 seconds) How long would it take you to make 4,600 million dots? (It would take slightly more than 900 years to make 4,600 million dots!!)

Turn to your neighbor and tell them about 4 things you learned today.

law of superposition - a principle that states that the overlying rock layer is younger than the layers below it.

This is the time this file has been accessed since 04/02/98.

The University of Tennessee at Martin is not responsible for the information or views expressed here.

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