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The Dynamic Earth: Plate Tectonics
Lesson 1: Density
Exploration Phase:
Activity 1 – Let’s play the will it float game? [Anyone Dave Letterman’s fans here?]
Materials:
• A glass or graduated cylinder about three-quarters full of water
• A variety of sphere-shaped objects, many of which are of similar size but composed of different
materials such as rubber balls, marbles, and clay balls
• 2 sphere-shaped objects of the same mass but different size (e.g., a polystyrene foam ball
and a bead)
• A balance scale or other scale
• Beakers
• Liquids – cooking oil, water, hair conditioner, vinegar, and maple syrup or molasses.
Think about situations in which you have seen objects float in the past. Write down
a few examples of those situations that you can remember.
Examine the various sphere shaped objects on your table and use words or terms
that you are familiar with to describe the objects and what they are made of.
Write down your predictions about which object will float or sink when placed in
water.
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After you have completed your predictions, perform the following experiment:
- Fill your glass or graduated cylinder about three-quarters of the way full with water. Read
off and write down the water level.
- Drop each sphere shaped object in the cylinder and observe what happens
Write your observations below.
If you were careful with your observations you noticed that several things
occurred. Two questions that might come to mind are (1) What caused the some of
the objects to float and why did the others sink?
What are some possible reasons for each of these observations?
Consider the following hypotheses...
Hypothesis 1
The more mass an object has the more likely that the
object will sink and not float
Experiment
Place each sphere shaped object on a scale, measure the mass of
the object and write it down. Make a list of all the objects in order
from the least massive to the most massive. Compare this list to
the list you made in the first experiment and circle the objects that
floated.
Expected Outcome
Objects with more mass will sink while objects with
less mass will float. Do you agree?
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Hypothesis 2
The more volume an object has the more likely that
the object will sink and not float
Experiment
Start by recording the initial water level in the cylinder and then
determine the volume of each sphere shaped object by recording
the difference in the water level in the graduated cylinder when the
object is completely submerged in the water. For this experiment,
you should use a toothpick or similar object to push down objects
that float until they are completely submerged in the water before
recoding the difference in water level. Make a list of all the objects
in order from the lowest volume to the greatest volume. Circle the
objects that floated on this list.
Expected Outcome
Objects with more volume will sink while objects
with less volume will float. Do you agree?
Conclusions: Take a close look at the two lists that you made for hypothesis 1 and
hypothesis 2. What conclusions can you draw from the two lists?
What do you think will happen in a situation where you have two objects of the
same mass but one has a greater volume? Is mass is the only factor that
determines whether an object will float? Why or why not?
-
Repeat Activity 1 but this time use just two sphere shaped object with about the same
mass but vastly different volume (e.g., a polystyrene foam ball and a bead)
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MISCONCEPTIONS:
Students may think that mass determines whether an object floats or sinks—that lighter objects
float, while heavier objects sink. To confront this misconception, it’s important to include at least
two balls of equal mass but different density, such as a large polystyrene foam ball and a small
bead. It would be even better if a bead that was less massive than the polystyrene foam ball were
included. The activity that we just did will help to confront this misconception.
Students often have trouble understanding the difference between mass and weight. Discuss with
students that mass is the amount of matter in an object, and weight is the amount of mass
multiplied by gravity. Weight is the amount of force with which an object is attracted toward the
Earth. This is why Astronauts experience weightlessness in outer space where the force of
gravity is zero. Emphasize with students that they are measuring the amount of matter in each
object. Model this language with them by asking questions, such as “Which object has the most
matter?”
CONCEPT INTRODUCTION
• Question: What can you tell me about the molecules in the bead compared to the molecules in
the polystyrene foam ball?
• Answer: The molecules in the bead are more tightly packed together than the molecules in the
polystyrene foam ball.
• Question: If the molecules are more tightly packed together in the bead, are there more or less
molecules in each unit of space than compared to the molecules in the polystyrene foam ball?
• Answer: There are more molecules in each unit of space.
Note: The amount of matter (mass) in a certain unit of volume or space is called density.
• Question: When comparing the bead to the polystyrene ball, which object has more mass in a
smaller amount of space?
• Answer: The bead has more mass in a smaller space.
Note: This is why we say that the bead is denser than the polystyrene foam ball.
• Question: So what can you say about what causes objects to float?
• Answer: Objects of lower density float.
Density is mass divided by volume; it is the amount of matter per unit volume. An object that is
less dense has its mass spread out over a larger surface area, which allows it to float; thus, the
shape of an object is also a factor in allowing an object to float. This is what enables large ships
made of steel to float. Mathematically, density can be expressed as follows:
Density = Mass (usually measured in g)/Volume (usually measured in cm3)
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APPLICATION 1
You have been provided with five beakers each half full of one of the following five
liquids: cooking oil, water, maple syrup or molasses, hair conditioner, and vinegar.
You may add food coloring to each liquid to help differentiate between them.
Observe the liquids and record these observations in the space provided below.
Observe the consistency of each liquid by touching the substance with your fingers
and moving the beaker around to see how the liquid moves. Write down your
observations.
What causes some of the liquids to be thicker than others? How might the
molecules of thicker substances differ from thinner substances?
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What do you predict would happen if you poured all five liquids into one container?
Write down your predictions.
Slowly pour half of each liquid into the transparent container on your table and
observe. (You will need the other half of each liquid for the second part of this
activity). Allow the liquid to settle and write down your observations
Thinking Activity:
a. Why do you think one liquid went to the bottom of the container?
b. If one liquid is denser than all the other liquids, what can you tell me about the molecules in
that liquid?
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c. If the molecules are more tightly packed together in the densest liquid, are there more or less
molecules in each unit of space than compared to the molecules in the thinnest liquid?
d. Of the liquids you tested, which do you think have a high density?
Slowly pour the other half of the least dense liquid into another transparent
container on your table and slowly pour two other liquids from this activity into the
same container. Allow the liquid to settle and write down your observations.
What did you observe when you poured the least dense liquid into the container first, followed by
two other liquids from this activity?
What causes this movement of liquids?
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Thinking Activity
Connecting the knowledge of density to understand the composition of the
layers of the Earth.
Question: In the Density Activity, where did the densest substances go?
• Answer:
• Question: In the interior of the Earth, where do you think the densest substances would be?
• Answer:
• Question: Which layer of the Earth do you think is the densest?
• Answer:
• Question: Why is the core the densest layer of the Earth?
• Answer:
• Question: How would you describe the density of the mantle compared to the other layers?
Explain your reasoning.
• Answer:
Note: Each of the layers of the Earth is composed of different materials and has a different
thickness. When the Earth was forming, all of the materials were mixed up.
• Question: What do you think happened to the materials inside the Earth over time?
• Answer: Over time the materials were separated by gravity, with the densest materials in the
center and the least dense materials on top.
Terms: Float, sink, mass, volume, weight, density, molecules, heat, pressure,
matter, gravity, outer space, weightlessness, grams, centimeter cube, solids,
liquids, Earth’s core, Earth’s mantle, Earth’s crust,
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Teacher Notes:
Have students conduct self-directed readings or internet searches to find
answers to the following questions
• Question: What is the thickness and composition of each of Earth’s layers?
• Answer: On average, the crust is about 40 kilometers thick (almost 25 miles). The mantle is
about 2900 kilometers thick (about 1800 miles). The core is almost 3500 kilometers thick (about
2150 miles).
• Question: How does density affect the composition and order of Earth’s layers?
• Answer: The core is composed of iron and nickel, which is the densest layer. The crust is
composed mostly of silicate rock, which is the least dense. The mantle is composed of rock that
has a density between that of the core and crust. The denser materials sunk down to the middle
of the Earth, while the materials of lesser density float on top.
• Question: How would you compare the molecules in each layer?
• Answer: The molecules in the core are more tightly packed together than in the mantle and
core because the materials are very dense. As you travel away from the core to the crust. The
molecules are not as tightly packed and have more space between them.
• Question: How can density affect movement?
• Answer: When a substance that is less dense is under a substance that is denser, the less dense
substance rises or moves above the other substance.
• Question: How are the Earth’s layers like the liquid layers in the Density Activity?
• Answer: (Accept all reasonable answers. Answers may include that just as the liquids
separated into most dense to least dense layers, so are the Earth’s layers separated into most
dense to least dense layers. Just as liquids of lower density rise above those of greater density, so
do the materials inside the Earth.)
• Question: How are the Earth’s layers different from the liquid layers in the Density Activity?
• Answer: (Accept all reasonable answers. Answers may include that the Earth’s layers are solid
and are much larger in mass.)
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Lesson 2: Mantle Convection
Exploration Phase:
Activity 1
Materials for each group:
• 2 or more of the following liquids: oil, water, molasses, conditioner and vinegar
• Cups
• Thermometer
• Hot plate or other heat source
• 2 beakers or glass containers that can be heated (made of material similar to
Pyrex®)
• Food coloring
1. Bridge to inquiry activity.
In the previous lesson on density, we learned that the layers of the Earth are
arranged in order of increasing density starting at the crust and moving towards
the core. In this activity, you are going to design an experiment to help you explore
the question:
– Is it possible to change the density of a substance in order to cause movement?
First step in addressing this question is to come up with a testable hypothesis.
Work with your partner(s) to develop a hypothesis.
Note to Teacher: Based on your experience with your class, there are at least three ways to
proceed from here. Students who would benefit from the opportunity to design their own
experiment should do so. If the class is not ready as a whole, you can also do this as a whole
class project in which everyone contributes to one hypothesis and test. You could also work
with a group who needs more assistance while allowing those who benefit from working on
their own to do so.
2. Work in your groups to refine hypotheses and plan at least one experiment
to test them.
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Note to Teacher: Make sure that the students are thinking in terms of data — what data they
will be collecting, how they will be measuring their data, and how their data is either going
to confirm or refute their hypotheses.
3. Take adequate time to put together you materials and procedures list in
order to figure out what data you will be collecting and how you will measure
it.
4. Share your group hypotheses and experiment plans with the class before
proceeding.
Note to Teacher: Corrections should be focused on the science process, not the correctness
of the hypothesis. An incorrect hypothesis with a solid experimental plan is fine. A correct
hypothesis without a solid experimental plan should be revised..
5. Provide the list of materials that your group needs to conduct your
experiments to the Instructor to confirm that the materials are available
Note to Teacher: Student experiments will vary from group to group. Many students may want to
change the temperature of a substance to see how this affects the substance’s density.
Listed below is a possible idea for an experiment:
Increase in temperature: Students can use two or more of the liquids that were used in Geology
Lesson 3 Density Activity. These liquids were oil, water, molasses, conditioner, and vinegar. The
students could begin the experiment by having all of the liquids at room temperature. The liquids
could then be combined into one cup to determine the order of densities of the liquids. Students
could heat one liquid up using a heat source such as a Bunsen burner, a microwave, or setting a
beaker of the liquid in hot water. The liquids could all be combined once again. One denser
liquid will have an increased temperature, while the other liquids will be at room temperature.
Students can record observations about the densities of the substances and whether the heating
of one of the liquids causes movement. This experiment could be repeated for each of the liquids.
Encourage your students to be creative and use questioning to lead them to the best choices
possible for their experiment. Questions might include: How will you know if density has
increased or decreased? How will you isolate the effect on one liquid?
6. Conduct your experiment and write down your findings to the class. Did
your experiment confirm your hypothesis or not? From insights gained in your
experiment provide answers to the following questions:
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Question: How did a change in the density of a substance affect the location of that substance in
relation to other substances?
• Answer:
• Question: What would happen to the substance if it became less dense than the substances
around it?
• Answer:
• Question: What would happen to the substance if it became denser than the substances around
it?
• Answer:
• Question: What could you do to a substance to change its density?
• Answer:
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Concept Introduction:
Prerequisite Concepts
• Heating and cooling may cause changes in the properties of materials. Many kinds of changes
occur faster under hotter conditions.
• Atoms and molecules are perpetually in motion. Increased temperature means greater average
energy of motion, so most substances expand when heated.
• Humans need the following geologic conditions
– Liquid outer core (coupled with the planet’s rotation) and a thick atmosphere
– Viscous mantle (slow motion)
– Slow motion of crust and upper mantle (lithosphere) of 3 to 5 centimeters per year
• Density is the amount of matter in a certain unit of volume or space and, it is the measure of
how tightly packed molecules are within a substance.
• Substances of greater density will sink below those of lesser density.
• As you go deeper into the Earth, the density of the materials increases.
• A change in density can affect the movement of matter.
Major Concepts
• Convection is the transfer of heat through a substance (solid, liquid, or gas) caused by
molecular motion.
• The density of a substance can change with pressure and temperature.
• As temperature is increased, substances become less dense and thus rise.
• When warmer things are put with cooler ones, the warm ones lose heat and the cool ones gain it
until they are the same temperature.
• When a substance heats up and rises, it comes into contact with cooler substances that cool it
down.
• When the substance cools, it becomes denser and sinks.
• If the gas or liquid sinks toward a heat source, it will warm up and rise again.
• This rising and sinking creates currents or convection cells that transfer heat.
• High temperatures deep inside the Earth heat up the mantle, decreasing density and causing
lower mantle material to rise.
• Mantle material near the crust cools as it comes in contact with cooler material, causing it to
become denser and sink.
• The crust and top part of the mantle that is solid are called the lithosphere (sphere of rock).
• The lower part of the upper mantle composed of some molten rock is called the asthenosphere.
• Below the asthenosphere is the rest of the mantle,
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Application Phase:
In this activity, we model the air movements that occur when air is cold in one
place (for example, at the poles) and hot in another (for example, at the equator)
to illustrate the concept of convection. We use water to represent air in the lower
part of the atmosphere (i.e. the troposphere) because it's easier to observe the
motion of water than it is to observe the motion of air. Keep in mind, however, what
each part of the model represents:
Part of Model
Real Thing That it Represents
Bottom of cake pan
The Earth's surface
Water in the cake pan
The lowermost layer of the atmosphere (the
troposphere)
Air above the cake pan
The stratosphere (the layer of the atmosphere
directly above the troposphere)
Bag of ice
Cold ground near the north (or south) pole
Candles
Warm ground near the equator
Materials: large rectangular clear glass cake pan (15x10x2)
red and blue food coloring
2 eye droppers
3 small candles (the kind that are used to keep food warm)
box of matches
three Styrofoam cups
ice
gallon-size Ziploc bag
red and blue colored pencils
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Activity
1. Fill the cake pan with water.
2. Place the three Styrofoam cups, upside down on the lab table, forming a
triangle that the cake pan can rest on and remain stable. Place the cake pan on
the three cups.
3. Light the candles and place them in a line underneath one end of the cake pan.
This end of the cake pan represents the warm equatorial regions of the earth.
4. Put some ice in the large Ziploc bag and place the bag of ice in the cake pan on
the side opposite the candle. This end of the cake pan represents the cold polar
regions of the earth. The set up should look like the diagram below.
Bag of Ice
Candles
Cup
Cup Cup
Questions
1. On the diagram above, use arrows to show any (invisible) motion of the water
that you think may be occurring, due to the temperature differences across the
cake pan.
2. Explain why you think the water is moving this way.
More Activity
5. After the cake pan has rested undisturbed for a few minutes, place several
drops of blue food coloring in a line along the bag of ice, near where it touches
the water (see diagram below).
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6. Carefully place several drops of red food coloring into the water in a line along
the BOTTOM of the cake pan directly above the candles (see diagram below).
7. Watch the motion of the colored water and answer the following questions.
More Questions
3. On the diagram below, use arrows and colored pencils to show the motion of the
red and blue water in the cake pan.
4. Did the motion you observed match your predictions (Question #1 above)?
If not, explain why the motion that you DID observe occurred.
5. What can you conclude about the density of the water near the candle as
compared to the density of the water near the ice? Where is the water more
dense? Why?
Terms:
Convection, convection cells, rising, sinking, temperature, density,
convection currents, heat, hot, cool, lithosphere, asthenosphere, plates, molecules,
core,
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Thinking Activity & Teacher Notes:
• Question: What is the main difference between the two sections of water — the one that is
rising and the one that is sinking?
• Answer: The main difference between the two sections is the temperature.
• Question: If the water is moving to the top, what is causing it to rise?
• Answer: An increase in temperature is causing it to rise.
• Question: Where did this increase in temperature come from?
• Answer: The heat source provided this increase in temperature.
• Note: So now we understand that when a substance cools, it becomes denser and sinks. We also
understand that when the temperature of a substance is increased, it becomes less dense and
rises.
• Question (point to the top part of the circle of the demonstration): How does the heated water
that rises to the top then become the cooler water that sinks to the bottom of the beaker or glass
container?
• Answer: (Allow students to discuss their ideas.)
• Question: How do you think the movement of the molecules of the heated water compare with
the movement of the molecules of the cooler water?
• Answer: The heated water molecules are moving much faster than the cooler water molecules.
• Question: Do you think the heated water molecules come in contact with the cooler water
molecules?
• Answer: Yes, the heated water molecules come in contact with the cooler water molecules.
• Question: What do you think happens when the heated water molecules come in contact with
the cooler water molecules?
• Answer: (Accept all reasonable answers.)
• Question: How do you think your knowledge of convection and density can help you
understand the interior of the Earth?
• Answer: (Accept all reasonable answers.)
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• Question: Where do you think movement might occur in the Earth? Why?
• Answer: (Accept all reasonable answers)
• Question: What are the main layers of the Earth?
• Answer: The main layers of the Earth are the core, mantle, and crust.
• Note: These layers are based on the composition of the layers. The rocky crust, the rock-metal
mantle, and the metal core are how scientists describe what these layers are made of.
• Note: Scientists can also describe the interior of the Earth based on the movement of the layers.
• Question: What are the layers in the interior of the Earth that move?
• Answer: The layers in the interior of the Earth that move are the lithosphere, asthenosphere,
lower mantle, and outer core.
• Note: Scientists can look specifically at these layers and study their movement. This is called
the mechanical viewpoint of the interior of the Earth.
• Question: If you look at the interior of the Earth from a compositional viewpoint, what are the
layers that make up the Earth?
• Answer: The layers that make up the interior of the Earth based on composition are the crust,
mantle, and core.
•Question: If you look at the interior of the Earth from a mechanical viewpoint, what are the
layers that make up the Earth?
• Answer: The layers that make up the Earth based on mechanics are the brittle, rigid lithosphere,
partially molten asthenosphere, the lower mantle, liquid outer core, and solid inner core.
• Question: What is the lithosphere?
• Answer: The lithosphere is formed by the crust and the uppermost part of the mantle.
• Note: Scientists have evidence that the lithosphere is broken up into sections. These sections of
the lithosphere are called plates.
• Question: What layer is under the lithosphere?
• Answer: The asthenosphere is under the lithosphere.
• Question: How would you describe the asthenosphere?
• Answer: The asthenosphere is a part of the upper mantle below the lithosphere that is partially
molten.
• Note: Because the asthenosphere is partially molten, the plates move on top of the
asthenosphere.
• Note: Material inside the mantle moves in convection cells. We are going to use our knowledge
of convection cells from our demonstration to help us understand the convection cells in the
mantle.
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• Question: In the demonstration, what caused some of the molecules to start rising?
• Answer: An increase in temperature caused some of the molecules to start rising.
• Question: Where did this increase in temperature come from?
• Answer: The increase in temperature came from the hot plate.
• Question: What do you think might provide an increase in temperature inside the Earth?
• Answer: The interior of the Earth is hot. This provides heat energy to the material within the
interior of the Earth.
Note to Teacher: Scientists do not agree on the cause of convection currents in the mantle.
Some believe that the cause is the internal heat, while others believe that convection is
generated by friction from plate movement.
Question: Where do you think there is evidence of convection currents in your home?
• Answer: (Allow students to share their ideas. Students may respond that steam rising in the
shower is an example of convection currents.)
• Question: In a two-story building, where do you think it is warmer? Why?
• Answer: In a two-story building, it is warmer on the second floor than on the first floor due to
heat rising.
• Question: Where would you want to have heater vents in your home? Why?
• Answer: Heater vents would be best located near the floor so that heat rises throughout the
room. When it reached the ceiling, it would cool and sink causing convection cells that would
transfer heat throughout the room.
• Question: What would happen if the heater vents were located near the ceiling instead?
• Answer: Unless there were a strong air flow in the room, the heat would tend to stay at the top
of the room, since there would be nothing cooler to draw it down to the floor.
Question: What are convection cells and how are they helpful?
• Answer: Convection cells are circular currents formed when heated material rises and cooler
material sinks. They transfer heat.
• Question: Why does the material rise or sink?
• Answer: The loss or gain of heat energy results in a change in density that causes the material
to rise or sink.
• Question: How does the inside of the Earth move?
• Answer: Heat from deep inside the Earth heats up the lower mantle. This increase in heat
results in a decrease in density. This decrease in density causes the lower mantle material to rise.
Mantle material near the crust comes in contact with cooler material. During this contact, heat
energy is transferred from the warmer material to the cooler material. Once cooled, the material
is denser, causing it to sink and come into contact with the heat from the interior of the Earth
where it heats up, becomes less dense, and rises once again.
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