Download Going Their Separate Ways

Name ____________________________________________
Date _______________________ Period ______________
Objectives:
1. The student should be able to construct a model of seafloor spreading and oceanic crust subduction.
2. Be able to relate magnetic reversal patterns, age of rocks and thickness of sediments to the creation of new crustal material
at mid-ocean ridges.
3. Relate the process of seafloor spreading to divergent plate boundaries, convergent plate boundaries, Subduction, and ocean
trenches.
Background:
It is not easy to see how some processes work, especially if you cannot observe them directly. Thus, it is often helpful to create a
model. In this activity you will make a model that helps to show how ocean-floor spreading works.
Figure 1. Seafloor spreading
The Ocean Floor – A map of the ocean floor shows a
variety of topographic features: flat plains, long
mountain chains, and deep trenches. Mid-ocean ridges
are part of a chain of mountains some 84,000 km long.
The Mid-Atlantic Ridge is the longest mountain chain
on Earth. These ridges are spreading centers or
divergent boundaries where the upwelling of magma
from the mantle creates new ocean floor. The rift zone
is the valley between the two ridges where the plates
are pulling apart and new material is rising. Deep-sea
trenches are long, narrow basins that extend 8-11 km
below sea level. Trenches develop adjacent to
subduction zones, where oceanic lithosphere slides back
into the mantle. (Figure 1.)
Figure 2. Earth’s magnetic field.
Earth’s Magnetic Field – The Earth’s magnetic field is thought to arise from the
movement of liquid iron in the outer core as the planet rotates. The field
behaves as if a permanent magnet were located near the center of Earth, tilted
about 11o from the geographic axis of rotation (Figure 2.). Note that magnetic
north (as measured by a compass) differs from the geographic north that
corresponds to the planet’s axis of rotation.
At present, the lines of force of the Earth’s magnetic field are arranged as shown
in Figure 2; the present orientation of Earth’s magnetic field is referred to as
normal polarity. In the early 1960s, geophysicists discovered that Earth’s
magnetic field periodically reverses; i.e. the north magnetic pole becomes the
south pole and vice verse. Hence, Earth has experienced periods of reversed
polarity alternating with times (like now) or normal polarity. Although the
magnetic field reverses at these times, the physical Earth does not move or
change its direction of rotation.
Basaltic lavas contain iron-bearing minerals such as magnetite, which act like compasses. That is, as these iron-rich minerals cool
below their melting point, they become magnetized in the direction of the surrounding magnetic field. Studies of ancient magnetism
(paleomagnetism) recorded in rocks of different ages provide a record of when Earth’s magnetic field reversed its polarity.
Figure 3. Diagram of magnetic anomalies
on the seafloor. Black represents normal
polarity; white represents reversed polarity.
In 1963, F. Vine and D.H. Matthews reasoned that, as basaltic magma rises to form
new ocean floor at a mid-ocean spreading center, it records the polarity of the
magnetic field existing at the time magma crystallized. As spreading pulls the new
oceanic crust apart, stripes of approximately the same size should be carried away
from the ridge on each side (Figure 3.). Basaltic magma forming at mid-ocean ridges
serves as a kind of “tape recorder,” recording Earth’s magnetic field as it reverses
through time. If this idea is correct, alternating stripes of normal and reversed
polarity should be arranged symmetrically about mid-ocean spreading centers. The
discovery of such magnetic stripes provided powerful evidence that seafloor
spreading occurs.
The age of the seafloor also supports seafloor spreading. As the seafloor spreads, the youngest oceanic crust is found at the ridges
and progressively older crust is found moving away from the ridges towards the continents. The oldest known ocean floor is dated
at about 200 million years, indicating that older ocean floor has been destroyed through subduction at deep-sea floor. The Atlantic
Ocean has not been destroyed by subduction and did not even exist before that time, instead it was created as the continents on
either side of it broke and moved apart.
Procedure:
1. Obtain 2 sheets of unlined paper, a scissor, a colored pencil or marker, and a metric ruler.
2. Using the colored pencil or marker, draw stripes across one sheet of paper, parallel to the short
sides of the paper. The stripes should vary in spacing and thickness. Fold the paper in half
lengthwise. Write the word “start” at the top of both halves of the paper. Then, using the scissors,
cut the paper in half along the fold line to form 2 strips. (Figure 4.)
3.
4.
Take the second sheet of paper and lightly fold it into
eight sections.
Draw lines 5.5 cm long, starting at the fold, on the
middle crease and the 2 creases closest to the ends of
the paper. Then use the scissors to cut along the lines
you drew. Unfold the paper. You should have 3 slits
in the center of the paper. (Figure 5.)
Figure 5.
5.
6.
7.
Figure 4.
Put the 2 striped strips of paper together so that the “Start” labels
touch one another. Insert the strips up through the center slit, then
pull them toward the side slits.
Insert the stripes into the side slits. Pull the ends of the strips and
watch what happens at the center slit.
Practice pulling the strips through the slits until you can make the
stripes come up and go sown at the same time.
Figure 6.
Analysis Questions:
1. What structure does the center slit represent on your seafloor model? ______________________________________________
_________________________________________________________________________________________________________
2.
What structure do the 2 side slits represent? ___________________________________________________________________
_________________________________________________________________________________________________________
3.
What type of plate boundaries are demonstrated in the model? ___________________________________________________
_________________________________________________________________________________________________________
4.
What do colored strips of paper represent? ____________________________________________________________________
_________________________________________________________________________________________________________
5.
What do the stripes on the strips of paper represent? ____________________________________________________________
_________________________________________________________________________________________________________
6.
Describe the process of seafloor spreading in your own words. ____________________________________________________
_________________________________________________________________________________________________________
7.
How is this model useful to studying paleomagnetism? __________________________________________________________
_________________________________________________________________________________________________________
Objective: To work a puzzle using plate tectonics vocabulary terms.
Procedure: Identify the terms described below, then locate them in the puzzle. They may be frontward, backward,
horizontal, vertical, or diagonal.
Descriptions: Write the word being described in the blank after the statement, then find it in the puzzle below.
1. The sideways horizontal movement that occurs along some plate boundaries. _________________________
2. The name of the plate upon which we are traveling. _________________________
3. In a zone of subduction, these creations are produced from the rising of hot magma. ______________________
4. In this zone, a heavy oceanic plate meets a lighter continental plate and dives under. ______________________
5. This describes a plate boundary in which the plates come together. _________________________
6. The name of the supercontinent in which the plates come into one big hunk. _________________________
7. The name of the once single continent in the Southern Hemisphere. _________________________
8. The deformation of Earth materials on a large scale. _________________________
9. A large chunk of the lithosphere. _________________________
10. A rather narrow rise that has steep sides. _________________________
11. A deep valley caused by a break in the lithosphere, following along the crest of the mid-oceanic ridge.
_________________________
12. A long, narrow depression formed in the zones of subduction. _________________________
13. A type of plate boundary in which 2 plates move away from each other. _________________________
14. A large mountain range in Asia formed when 2 plates crunched into each other. _________________________
15. The crust plus some upper mantle. _________________________
16. The mass movement of mantle material caused by heat flow. _________________________
L
I
N
C
O
N
V
E
C
T
I
O
N
O
T
N
E
G
R
E
V
N
O
C
G
G
L
N
H
T
I
I
R
C
U
C
O
O
S
I
E
R
H
J
F
I
A
V
I
N
C
U
T
G
B
I
K
T
D
L
Y
D
T
I
B
H O
R E
O M
M A
O N
P R
G E
U A
W S
A P
R E
N O
D U
S
V
O
L
C
S
N
O
A
I
N
T
C
P
I
D
A
I
S
B
P
X
L
C
C
T
H
D
E
Y
L
E
T
A
L
P
H
E
I
E
O
G
A
M
E
R
I
C
A
N
T
O
R
S
N
S
Q
P
A
N
G
A
E
A
N
E
D
S
U
U
A
L
A
T
E
R
A
L
Harry Hammond Hess: Spreading the seafloor
URL: http://pubs.usgs.gov/publications/text/HHH.html
Last updated: 05.05.99
Harry Hess (1906-1969) in his Navy uniform as Captain of the assault transport Cape Johnson during
World War II. After the war, he remained active in the Naval Reserve, reaching the rank of Rear Admiral.
(Photograph courtesy of Department of Geological and Geophysical Sciences, Princeton University.)
Harry Hammond Hess, a professor of geology at Princeton University, was very influential in setting the
stage for the emerging plate-tectonics theory in the early 1960s. He believed in many of the observations
Wegener used in defending his theory of continental drift, but he had very different views about large-scale
movements of the Earth.
Even while serving in the U.S. Navy during World War II, Hess was keenly interested in the geology of the
ocean basins. In between taking part in the fighting in the Marianas, Leyte, Linguayan, and Iwo Jima, Hess - with the cooperation of his crew -- was able to conduct echo-sounding surveys in the Pacific while cruising
from one battle to the next. Building on the work of English geologist Arthur Holmes in the 1930s, Hess'
research ultimately resulted in a ground-breaking hypothesis that later would be called seafloor spreading.
In 1959, he informally presented this hypothesis in a manuscript that was widely circulated. Hess, like
Wegener, ran into resistance because little ocean-floor data existed for testing his ideas. In 1962, these ideas
were published in a paper titled "History of Ocean Basins," which was one of the most important
contributions in the development of plate tectonics. In this classic paper, Hess outlined the basics of how
seafloor spreading works: molten rock (magma) oozes up from the Earth's interior along the mid-oceanic
ridges, creating new seafloor that spreads away from the active ridge crest and, eventually, sinks into the
deep oceanic trenches.
Hess' concept of a mobile seafloor explained several very puzzling geologic questions. If the oceans have
existed for at least 4 billion years, as most geologists believed, why is there so little sediment deposited on
the ocean floor? Hess reasoned that the sediment has been accumulating for about 300 million years at most.
This interval is approximately the time needed for the ocean floor to move from the ridge crest to the
trenches, where oceanic crust descends into the trench and is destroyed. Meanwhile, magma is continually
rising along the mid-oceanic ridges, where the "recycling" process is completed by the creation of new
oceanic crust. This recycling of the seafloor also explained why the oldest fossils found on the seafloor are
no more than about 180 million years old. In contrast, marine fossils in rock strata on land -- some of which
are found high in the Himalayas, over 8,500 m above sea level -- can be considerably older. Most important,
however, Hess' ideas also resolved a question that plagued Wegener's theory of continental drift: how do the
continents move? Wegener had a vague notion that the continents must simply "plow" through the ocean
floor, which his critics rightly argued was physically impossible. With seafloor spreading, the continents did
not have to push through the ocean floor but were carried along as the ocean floor spread from the ridges.
In 1962, Hess was well aware that solid evidence was still lacking to test his hypothesis and to convince a
more receptive but still skeptical scientific community. But the Vine-Matthews explanation of magnetic
striping of the seafloor a year later and additional oceanic exploration during subsequent years ultimately
provided the arguments to confirm Hess' model of seafloor spreading. The theory was strengthened further
when dating studies showed that the seafloor becomes older with distance away from the ridge crests.
Finally, improved seismic data confirmed that oceanic crust was indeed sinking into the trenches, fully
proving Hess' hypothesis, which was based largely on intuitive geologic reasoning. His basic idea of
seafloor spreading along mid-oceanic ridges has well withstood the test of time.
Hess, who served for years as the head of Princeton's Geology Department, died in 1969. Unlike Wegener,
he was able to see his seafloor-spreading hypothesis largely accepted and confirmed as knowledge of the
ocean floor increased dramatically during his lifetime. Like Wegener, he was keenly interested in other
sciences in addition to geology. In recognition of his enormous stature worldwide, in 1962 Hess -- best
known for his geologic research -- was appointed by President John F. Kennedy to the prestigious position
of Chairman of the Space Science Board of the National Academy of Sciences. Thus, in addition to being a
major force in the development of plate tectonics, Hess also played a prominent role in designing the
nation's space program.
Questions:
1.
What did Hess and his crew do while cruising from one battle to the next during World War II? ____________
_________________________________________________________________________________________________
2. Name the book which was one of the most important contributions in the development of plate tectonics?
____________________________________________
3. How did Hess outline the process of seafloor spreading? _______________________________________________
_________________________________________________________________________________________________
4. What was one of the questions that a mobile seafloor explained? _______________________________________
_________________________________________________________________________________________________
5. How old are the oldest fossils found on the seafloor? _________________________________________________
_________________________________________________________________________________________________
6. How did Hess resolve the problem of continents “plowing” through the ocean floor? _____________________
_________________________________________________________________________________________________
7. What was the evidence that ultimately provided the arguments to confirm Hess’ model of seafloor spreading?
a.
__________________________________________________________________________________________
b. __________________________________________________________________________________________
8. What new information further strengthened the theory of seafloor spreading? ____________________________
_________________________________________________________________________________________________
9. What event happened in 1962? ____________________________________________________________________
_________________________________________________________________________________________________
Observe the diagram below, then answer the questions that follow.
Figure 1. Map of the Seafloor
1.
What is happening at point 1 in the diagram? ____________________________________________________________
___________________________________________________________________________________________________
2.
What type of feature is located at point 2 in the diagram? __________________________________________ What is
happening to the ocean floor at this point? ______________________________________________________________
___________________________________________________________________________________________________
3.
What feature is located at point 3 in the diagram? ________________________________________________________
___________________________________________________________________________________________________
4.
What feature is being formed at point 4? _____________________________________________ Why is this
happening? ________________________________________________________________________________________
5.
What is happening at point 5 in the diagram? ____________________________________________________________
___________________________________________________________________________________________________
Figure 2. Mid-Atlantic Ridge
Part A
1. Locate on the map above (Figure 2.) each feature
listed below. Match each feature to its letter on the
map.
a.
________ Mid-Atlantic Rift
b. ________ Trench
c.
________ Mid-Atlantic Ridge
d. ________ Youngest rocks of the ridge
e.
________ Older rocks of the ridge
2. Follow the directions below to color the map.
a. Color the area red where hot material rises form the mantle.
b. Show the gradual change of young rocks becoming older rocks in the Mid-Atlantic Ridge. Color the
younger rocks orange; color the older rocks yellow.
c. Color the trench green.
d. Draw 2 red arrows to indicate the direction of plate movement along the ridge.
Part B
1. The diagrams below illustrate some of the concepts in the Plate Tectonic Theory. Label each diagram.
2. Match the following examples with the correct type of plate boundary below.
a.
Boundary _____________________________
Example
c.
_____________________________
Boundary _______________________________
Example
_______________________________
b. Boundary _______________________________
Example
_______________________________
d. Energy Source ___________________________