FOR SEA Institute of Marine Science
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FOR SEA Grade 7
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How Old?

Part 1 – Sweet Stratigraphy
You are on the trail of a rare prehistoric animal. It lived only a short time. Few fossils of the exotic animal are in existence. You will be given a sample of rock from the time period it is thought to have lived. The rock was formed as sediment settled in layers called strata. Fossils are also found in the strata. Your task is to find out when this animal lived.

How to date your sample

  1. Look for the sediment layers. Can you see them clearly? If not, gently cut one rock face so that you can. Find a fossil specimen, but leave it in place.You don’t want to destroy any important clues!

  2. Now, record the depth of each sediment layer in your sample. Use a centimeter ruler to measure each layer. Use this information to make a scale drawing of the sample.

    Hint: You can use two centimeter rulers to help you make your drawing. First, place one centimeter ruler along the bottom of your sample. Put the end of the ruler next to the end of the sample.

    Hold the second upright so that the 0 point is directly over the end of the first ruler.

    Measure the height of each sediment layer. Record this information on your data grid. Continue by moving the upright ruler to the 1 cm mark on the horizontal ruler and repeating your steps.

    Continue until all sediment layers are marked.

  3. Locate the fossil in the strata sample. Accurately place it in your drawing. The ruler may be a help.

  4. On your drawing, note the presence of other fossils. Label the location with the name of the fossil. These other fossils may provide important clues. They may help you age the rare fossil.

  5. Some things are known about these layers of rock. Your teacher has that information. With your group, decide on questions to ask your teacher. Write down each question on a piece of scratch paper. You may give your teacher one question at a time.

    As you work, you may learn the ages of some of the features of your strata. Add this information to your drawing.

Analysis and Interpretation
  1. a.  Age of your rare fossil:          

    b.  How do you know this?

  2. Let’s say that no sediments have been removed from the surface. Let’s also say that sediments have always built up at the same rate. How long has it taken for a centimeter of sedimentary rock to form on your sample?
    (Hint: Take the age of the fossil and divide that number by its depth in centimeters.)

  3. Some fossils are known as index fossils. Index fossils tell us certain things. They may give us a very clear understanding of when that animal or plant lived. Or, they may let us know the conditions under which it lived.

    a.  How does the presence of an index fossil help us increase our knowledge of other plants and animals found in the same sediment layer?

    b.  In this activity, which items served as index fossils for determining the age of the m&m; candies?

Part 2 – How Old?
What evidence do scientists use to reconstruct the story of the first whales? Most of the information comes from the rocks of the earth itself. Today, at many places on earth, we can watch the wearing away of rocks. This wearing away is called erosion. Scientists believe erosion has been happening for as long as the history of the earth. Over the years, in other places, the tiny worn away pieces have come together. New rocks, built up of layers, or strata have formed. These rock layers give us a clue to the past.

A simple rule helps us understand the record of these rock strata. Let’s look at rocks where the layers are horizontal, or nearly so. In these rocks, each overlying bed is younger than the one beneath it. This is called the principle of “superposition”. (This makes sense. Think about the clothes in your laundry basket (or on your floor). The ones on the bottom have been there the longest time.)

  1. Look at the drawing below. Label the oldest and youngest rock layers (strata).

  2. Why can we assume this principle is true of most rock layers?

    However, the earth’s crust is in motion. This means that it is not always as easy as it might sound to tell the age of the different layers. The diagram below shows a cross-section of a piece of the earth’s crust. Motion in the crust has moved sections of layers of rocks and soil. This rearranged section of earth is difficult to “read.” Geologists are scientists that study the earth. They have devised methods to “read.” these confusing rock records.

  3. What might have occurred in the rock layers above to make it difficult to “read.” their age?

    The position of the layers of rock tells us which rocks are oldest. It does not tell us how old the rocks are in terms of years. How can we tell their age in years? Geologists can make reasonable guesses of the age of rocks. First, they note how long it takes to deposit, or lay down, a layer of rock. This is called the rate of deposition. They then measure the thickness of the layers. From this information, they can determine the age of the layer. Let’s see how.

  4. The diagram below shows a series of rock layers. The labeled section is 1 kilometer deep. It was deposited at a rate of 1 centimeter per year. How old is the bottom layer? Please show your work.
    (Hint: Let’s write the question mathematically:

    1 centimeter/year = 1 kilometer/? years. This is the same as:

    1 centimeter
    1 kilometer
    ? years

    This is easy! Remember that:
    1 kilometer = 1000 meters and 1 meter = 100 centimeters. All you need to find out is the number of centimeters in a kilometer.)

    These methods gave early geologists an idea of the age of certain rock layers. Today, scientists have powerful new tools to date rocks. We know that radioactive atoms are unstable. These unstable atoms change from one form to another over time. This is called radioactive decay. We also know the rate at which the change takes place. For any rock, we can measure how many atoms have changed and how many have not. From this information we can determine the age of the rock in which these materials are found.

  5. Uranium 238 changes to Lead 206 by radioactive decay. It takes 4,500,000,000 (4.5 billion) years for half (50%) of the uranium in a given rock to change to lead.

    a.  If a rock contains 50% lead and 50% uranium, how old is the rock?

    b.  If a rock contains 25% lead and 75% uranium, how old is the rock?

    So, now we have an idea of what the layers of rocks tell us and how old each layer is. How does this information relate to our history of the whale?

    Early whales lived near coasts. When they died, some settled into the soft mud and sands. Most of the whales disappeared. But in some cases, their skeletal materials were replaced by harder minerals. These skeletons were transformed into rock. We call these special rocks “fossils”. Fossils are naturally preserved remains of animals and plants.

    Certain fossils are called index fossils. Index fossils are useful to geologists. They are found in rock layers of only one geologic age. They serve to identify specific rock layers no matter where they are found. By understanding the clues found in rock strata and the fossil record, we can reconstruct the origin of whales.

  6. How can index fossils be helpful in telling when the first whales lived?

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