Download Name Date Class Lesson Outline LESSON 2 Development of a

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Lesson Outline
Development of a Theory
A. Mapping the Ocean Floor
1. Scientists mapped the depth of the ocean floor using a device called
2. In the middle of the oceans are large mountain ranges
a. Existence of these
was confirmed through research
called echo-sounder research.
b. These underwater mountain ranges are much
mountain ranges on land.
B. Seafloor Spreading
occurs when new oceanic crust forms at a mid-ocean
ridge and old crust moves away from the ridge.
a. Molten rock, or
, rises from the mantle through
cracks in the crust. It erupts as
from volcanic vents
along the mid-ocean ridge.
b. The molten rock cools and becomes
, the rock that
forms the oceanic crust.
c. New oceanic crust forms along a mid-ocean ridge, and
crust moves away from the ridge.
2. The topography of the
includes the abyssal plain and
rugged mountains.
a. The rugged mountains that make up the mid-ocean ridge can form in different
ways. One way is through large amounts of
from the center of the ridge, cooling, and building up around the ridge. Another
way is through upward-moving
pushing on the crust
above it, causing it to crack and form jagged, angular mountains on the seafloor.
b. Eventually,
making smooth seafloor called the
forms on top of the oldest oceanic crust,
3. Seafloor spreading helps explain continental drift because it shows that continents
move with the oceanic
as it spreads away from mid-
ocean ridges.
Plate Tectonics
Lesson Outline continued
C. Development of a Theory
1. Evidence to support seafloor spreading first came from studying the
of rocks on the seafloor.
2. Earth’s outer core causes Earth’s
a. The direction of Earth’s magnetic field
b. When a magnetic field causes a magnet to point north, the magnetic field
c. A magnetic field reverses direction during a(n)
d. After a magnetic reversal, a magnet points south because Earth’s magnetic field
form when iron-rich minerals in cooling lava align
with the direction of Earth’s magnetic field.
a. The direction of a magnetic field in minerals can be determined by using a device
called a(n)
b. Magnetometers show
magnetic stripes on either side
of a mid-ocean ridge.
c. These stripes alternate normal polarity and
, showing
that each stripe was formed at the mid-ocean ridge and then moved away.
4. Sediment collected from the seafloor show that sediment farther away from a
mid-ocean ridge is
than the sediment that is closer to
the ridge.
Plate Tecton
Content Practice A
Development of a Theory
Directions: Complete this concept map by choosing terms from the word bank and writing them in the correct
continental drift
mid-ocean ridge
continents move
new oceanic crust
magnetic signatures
older oceanic crust
Seafloor Spreading
is the process
by which
is a way to
explain how
forms along a(n)
is evidenced
by the
of minerals in
and by measurements
of seafloor
moves away from it.
Content Practice B
Development of a Theory
Directions: On each line, write the term that correctly completes each sentence.
are mountain ranges in the oceans, where the crust
is young and there is almost no sediment.
draws dense rock away from mid-ocean ridges.
polarity means magnets orient themselves to point north,
4. As crust
polarity refers to magnets pointing south.
, it records the direction and orientation of Earth’s magnetic field.
5. The movement of the ocean’s crust is confirmed by Earth’s magnetic
6. Magnetic data from the ocean crust support seafloor
move with the ocean crust as the seafloor spreads.
8. What type of boundary is indicated by arrows B? ___________________________
9. What does letter A represent at this boundary?______________________________
10. What landform is represented by letter C? _________________________________
11. What type of boundary does arrows B and E form? __________________________
12. What land form is present at D? _________________________________________
13. What process is occurring as indicated by letter F? __________________________
14. What landforms are likely to occur at these kinds of zones? ___________________
School to Home
Development of a Theory
For this activity, you will model Henry Hammond Hess’s theory of seafloor spreading. You
will need a sheet of paper, scissors, crayons, and tape.
1. Place the sheet of paper on a flat surface so
it is positioned vertically. Cut two strips off
the edge of the paper, each 1 inch wide and
about 8 inches long. Color the strips so each
has four 2-inch × 1-inch sections in the
following colors: red, yellow, blue, green.
2. Place the two strips with the matching
colors facing each other. Tape the bottom of
the green ends together.
3. In the remaining portion of the sheet of paper, cut three 1½-inch slits that are 2 inches
apart from one another.
4. Slide the red ends of both strips up through the middle slit. Then slide one red strip
down through the slit on the left and the other strip down through the slit on the
right. Adjust the strips so the same colors show on both strips.
5. Pull the red ends of both strips. The same color should appear at the same time and
disappear at the same time. The strips of paper represent the old ocean crust moving
away from the mid-ocean ridge. What is happening at the center slit?
6. What does the change in colors on the strips of paper represent?
Plate Tectonics
Key Concept Builder
Development of a Theory
Key Concept What is seafloor spreading?
Directions: Answer each question or respond to each statement on the lines provided.
1. What happens to magma that rises
through cracks on the seafloor?
2. What structures are formed by rapidly
cooling lava in the ocean?
3. Where is the new rock located?
4. What happens to the rock as the
seafloor spreads?
5. Use your answers to help you explain seafloor spreading.
Plate Tectonics
Key Concept Builder
Development of a Theory
Key Concept What is seafloor spreading?
Directions: Answer each question on the lines provided.
What is known about seafloor
1. What forms basalt?
How this supports continental drift
3. What happens to old oceanic crust as new
oceanic crust forms?
2. Which type of rock forms oceanic crust?
4. What happens to the density of rock as
it cools?
5. Where does the crust move as it becomes
6. Which force draws the cooler, denser crust
downward and away from the mid-ocean ridge?
7. What is formed when lava cools and
crystallizes on top of the oceanic crust?
9. What shape does the seafloor take where the
sediment is the thickest?
8. Where is seafloor sediment thickest?
10. What is this area of the seafloor called?
11. What is the ocean crust always doing?
12. What happens as the ocean crust spreads?
13. What does the crust record as it cools?
15. What forms when Earth’s magnetic field
changes direction?
16. What do magnetic stripes confirm?
14. What does basalt contain that makes
this possible?
Plate Tectonics
Key Concept Builder
Development of a Theory
Key Concept What evidence is used to support seafloor spreading?
Directions: Write the word or phrase that correctly completes each sentence on the lines provided.
What has echo-sounder technology shown about the topography of the seafloor?
1. The topography is not
2. Mountain ranges stretch
3. Mountain ranges are located
4. Ocean mountain ranges are longer
What has the magnetometer shown about the magnetic signature of the seafloor?
5. Parallel magnetic stripes are located
6. Each stripe has
7. Magnetic stripes represent
What have temperature measurements beneath the seafloor surface revealed?
8. More thermal energy leaves
9. Less thermal energy leaves
What has dating of sediment that was taken from just above the ocean crust revealed?
10. The sediment that is closest to the mid-ocean ridge is
the sediment that is farther away from the ridge.
Plate Tectonics
Key Concept Builder
Development of a Theory
Key Concept What evidence is used to support seafloor spreading?
Directions: Answer each question on the lines provided. Use complete sentences.
1. What does normal polarity mean?
2. What does reversed polarity mean?
3. How can scientists tell when changes occur in Earth’s magnetic field?
4. How often do magnetic reversals occur?
5. What do Earth’s magnetic stripes indicate about ocean crust?
6. How do Earth’s magnetic signatures support the idea that continents move?
Plate Tectonics
Hot Springs on the Ocean Floor
As part of the developing theory of plate
tectonics, scientists predicted that hot
springs should exist near locations of active
seafloor spreading. Using a submersible
craft, scientists discovered the first
hydrothermal vent on the ocean floor in
1977. Two types of hydrothermal vents
have been discovered. Where the seafloor
crust is spreading apart due to plate
tectonics, seawater sinks deep into cracks in
the ocean floor. In one type of vent, the
ocean water is superheated by magma and
hot rock from Earth’s newly formed crust.
The superheated acidic water spews out,
carrying minerals from Earth’s crust. Thick
deposits of minerals that are rich in copper,
iron, manganese, sulfur, and zinc can form.
When the hot water (380°C or higher)
mixes with the cold seawater (2°C), it cools
quickly. The minerals in the water settle
and gradually form chimney-shaped vent
openings. Because the chimneys spew hot,
mineral-rich water, they can look like
smoke stacks.
Another type of hydrothermal vent
occurs when seawater reacts chemically with
the mantle beneath the ocean crust. This
process is not volcanic, and its heat comes
from chemical reactions. The water that
rises from the seafloor at this type of vent
is cool compared to water at magma-heated
spots and is chemically alkaline instead of
acid. Calcium carbonate and magnesium
precipitate out to form large, white towers.
Hydrothermal vents are located in all the
oceans, but most are along an area of active
volcanoes known as mid-ocean ridges. They
form where ocean plates have separated,
allowing lava to flow and form new crust.
Life in the Hot Springs
Most scientists once thought that
organisms could not survive at such great
depths because of extreme pressure,
absence of sunlight, and low temperatures.
However, because of hot water warming
the area around a vent and the rich
chemical stew being ejected by the vent,
life-forms are plentiful. Bacteria form the
base of the food web around hydrothermal
vents, and they support other organisms,
such as clams and crabs.
The base of food webs everywhere else
on Earth includes photosynthetic
organisms. However, the food web is
different for the bacteria found around
hydrothermal vents. These special bacteria
can convert the toxic sulfur from the vents
into energy. Hydrogen sulfide is the main
chemical used by microorganisms around
the vents to produce food from carbon
compounds. This process of using
chemicals rather than sunlight for energy
is called chemosynthesis.
Applying Critical-Thinking Skills
Directions: Respond to each statement.
1. Explain why the discovery of the chemosynthetic bacteria was important to scientists’
understanding of this ecosystem.
2. Summarize the significance of hydrothermal vents existing only in geologically active
areas on the ocean floor.
Plate Tectonics
Seascape Mapping
The abyssal plain might be somewhat “plain,” but most of the ocean’s floor is busy with
undersea landforms. Draw your vision of the ocean floor along a mid-ocean ridge. Include
the following labels: crust, mid-ocean ridge, seafloor spreading, magma, youngest basalt, older
basalt, oldest basalt, sea mount (undersea mountain), hydrothermal vent, abyssal plain, sediment.
Plate Tectonics
Skill Practice
LESSON 2: 30 minutes
How do rocks on the seafloor vary with age from a
mid-ocean ridge?
Scientists discovered that ocean crust forms at a mid-ocean ridge and spreads away from the
ridge slowly over time. This process is called seafloor spreading. The age of the seafloor is
one component that supports this theory.
Learn It
Scientists use models to represent real-world science. By creating a small three-dimensional
model of volcanic activity along the Mid-Atlantic Ridge, scientists can model the seafloor
spreading process. They can then compare this process to the actual age of the seafloor. In
this skill lab, you will investigate how the age of rocks on the seafloor changes with distance
away from the ridge.
Try It
Do not eat anything used in this lab.
1. Read and complete a lab safety form.
2. Lay the sheet of waxed paper flat on the lab table. Using a plastic spoon, place two
spoonfuls of vanilla yogurt in a straight line near the center of the waxed paper,
leaving it lumpy and full.
3. Lay the two pieces of foam board over the yogurt, leaving a small opening in the
middle. Push the foam boards together and down so the yogurt oozes up and over each
of the foam boards.
4. Pull the foam boards apart and add a new row of two spoonfuls of berry yogurt
down the middle. Lift the boards and place them partly over the new row. Push them
together gently. Observe the outer edges of the new yogurt while you are moving the
foam boards together.
5. Repeat step 4 with one more spoonful of vanilla yogurt. Then repeat again with one
more spoonful of berry yogurt.
Plate Tectonics
Skill Practice continued
Apply It
6. Compare the map and the model. Where is the Mid-Atlantic Ridge on the map?
Where is it represented in your model?
7. Which of your yogurt strips matches today on this map? 60 million years ago?
8. How do scientists determine the ages of different parts of the ocean floor?
9. Conclude What happened to the yogurt when you added more?
Key Concept What happens to the material already on the ocean floor when
magma erupts along a mid-ocean ridge?
Plate Tectonics
Lesson Quiz A
Development of a Theory
Directions: On the line before each definition, write the letter of the term that matches it correctly. Each term
is used only once.
1. Magnets point north when Earth’s magnetic field
has this.
A. magnetic reversal
B. mid-ocean ridge
2. process that forms new oceanic crust and moves it
C. normal polarity
3. takes place when Earth’s magnetic field changes
D. reversed polarity
4. Magnets point south when Earth’s magnetic field
E. seafloor spreading
has this.
5. place where new oceanic crust forms
True or False
Directions: On the line before each statement, write T if the statement is true or F if the statement is false.
6. Seafloor spreading takes place along a mid-ocean ridge.
7. Oceanic crust is older when it is closer to a mid-ocean ridge than it is when
it is farther from the ridge.
8. The high thermal-energy flow at a mid-ocean ridge comes from seawater.
9. Magnetic bands on the ocean floor are evidence that Earth’s magnetic field
10. Knowledge of seafloor spreading could have helped Alfred Wegener gain more
support for his hypothesis of continental drift.
Plate Tectonics
Lesson Quiz B
Development of a Theory
Directions: On each line, write the term that correctly completes each sentence.
1. A(n)
is a mountain range on the ocean floor where new
crust forms.
2. When Earth’s magnetic field has normal polarity, a compass needle
contributes to the high thermal-energy-flow readings near
the center of a mid-ocean ridge.
are changes in the orientation of Earth’s magnetic field.
5. The abyssal plain is flat due to an accumulation of
far from
the ridge.
6. When Earth’s magnetic field has reversed polarity, a compass needle
on the ocean floor are evidence that Earth’s magnetic field
Short Answer
Directions: Respond to each statement on the lines provided.
8. List the steps involved in seafloor spreading in sequential order.
9. Explain how knowledge of seafloor spreading could have been useful to the
acceptance of Alfred Wegener’s continental drift hypothesis.
Plate Tectonics