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Plate Tectonics
Name: ________________
INTRODUCTION
Plate tectonics is a well established theory that unifies and provides a framework for
all geologic observations. Most geologic phenomenon observed near the Earth’s
surface are linked in some way to plate tectonic processes. The theory states that the
outer 60-100 km of the Earth is divided into slabs of rigid rock (the lithosphere). These
slabs (the plates) rest upon a semi-viscous layer of easily deformable rock (the
asthenosphere). Thermal convection within the asthenosphere pushes the plates in
horizontal directions at rates ranging from 1 cm to 12 cm/year. This causes the plates
to move in relation to one another. Boundaries between the 8 principle plates and
several smaller plates are zones of rock deformation, earthquakes and volcanism.
This lab utilizes real data that demonstrates and/or validates the theory of Plate
Tectonics. Four exercises, modified from Jones and Jones (2003), follow.
o Part A examines global maps of tectonic plate boundaries and earthquake data
to identify plate boundary locations and assess relative motion between the
plates.
o Part B uses maps of the ocean floor to calculate spreading rates across a midoceanic ridge in the South Pacific.
o Part C interprets maps and utilizes geologic ages for Hawaiian Islands to better
understand movement of the underlying Pacific plate over a “hot spot”.
o Part D examines a geologic map along a portion of the San Andreas Fault to
evaluate the direction and rate of plate movement.
OBJECTIVES
Upon completion of this exercise, you will be able to understand:
1. basic differences between major types of plate boundaries.
2. magnetic stripping and use it to calculate spreading rates
3. the concept of “hot spots” and use this understanding to determine the speed
and direction of movement of plates
4. how to interpret a geological map of the San Andreas Fault and calculate the
rate of movement along the fault
PROCEDURE
Work through the handouts for this lab. Each exercise consists of a brief explanation,
a map, and a series of questions that pertain to the map. Use this information to
interpret the data and see how geologic data supports the theory of Plate Tectonics.
Ask the instructor for assistance if you have questions.
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PART A. PLATE BOUNDARIES
EXPLANATION:
This exercise will familiarize you with Earth’s major tectonic plates, how to identify the
type of plate boundary, and how plates move in relation to each other. Refer to Figure
1, a map of the major plates and boundaries, and Figure 2, a map of earthquake
distribution and depth.
There are three basic types of plate boundaries:
DIVERGENT (constructive) plate boundaries form when two plates are moving away from
one another. This occurs along mid-ocean ridge systems (e.g., the Mid Atlantic Ridge, the
East Pacific Rise). Magma rises from mantle and erupts onto the ocean floor where it
cools and solidifies to create new oceanic crust. The young crust is then pulled apart as
additional lava comes up and newer crust is formed along the center of the ridge. As a
result of this process, oceanic crust moves outward (horizontally) on both sides of an
oceanic ridge, and new crust is continually added to older oceanic crust.
o Characteristics: These boundaries commonly occur in the midst of ocean basins
and they contain numerous transform fault (fracture zone) offsets. Frequently this
results in a Frankenstein-like zipper pattern to the plate boundary. Earthquakes
are common although they are usually shallower and of lesser magnitudes than at
convergent boundaries. The sense of motion is typically perpendicular to and
away from the ridge axis.
CONVERGENT (destructive) plate boundaries occur in the collision zone between two
plates that are moving toward each other. There are three sub-types of convergent
boundaries. Ocean-continent boundaries occur where dense oceanic crust collides with
lighter continental crust. The oceanic crust sinks, or subducts, beneath the less dense
continental crust into the underlying asthenosphere. Ocean-ocean boundaries develop
where oceanic crust collides with oceanic crust from another plate. In this case, the older
(cooler and denser) plate descends and the younger (warmer and less dense) plate
overrides. Finally, continent-continent boundaries form where two continental plates
collide with each other. One plate typically indents into the other resulting in the formation
of a very large mountain chain (e.g., the Himalayas at the boundary between the Indian
Plate and the Eurasian Plate).
o Characteristics: These boundaries can be identified by the presence of deep and
large earthquakes that are initiated along a subducting plate. Hence, epicenters at
the surface of earth occur over a broader geographic zone. Plates move
perpendicular to and toward subuction zone boundaries.
TRANSFORM (conservative) plate boundaries occur in the collision zones where two
plates slide past each other, and no plate is created or destroyed. These plate boundaries
are characterized by long faults between the adjacent plates that are known as transform
faults or strike slip faults (e.g. The San Andres Fault that forms the boundary between the
American Plate and the Pacific Plate in California).
o Characteristics: These boundaries are more difficult to identify. They must be
deduced by analyzing the overall sense of plate motion based on the locations of
divergence and convergence in order to identify regions where plates are forced to
slide laterally past one another (rather than toward or away from each other).
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QUESTIONS:
1. Using the Figures 1 and 2 state weather the following plate boundaries are
Divergent, Convergent, or Transform.
a) South American Plate and the Nazca Plate
_______________
b) Pacific Plate and the Indo-Australian Plate
_______________
c) North American Plate and the Eurasian Plate
_______________
d) Pacific Plate and the North American Plate in Alaska _______________
e) Indo-Australian Plate and the Eurasian Plate
_______________
f) Nazca Plate and the Pacific Plate
_______________
g) Pacific Plate and the Eurasian Plate
_______________
h) Pacific Plate and the North American Plate in California _____________
i) South American Plate and the African Plate
_______________
2. Draw small arrows on Figure 1 showing the sense of motion either sides on the
plate boundaries identified above.
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PART B. SEAFLOOR SPREADING
EXPLANATION:
New ocean crust is formed at mid-oceanic ridges as the rising asthenosphere partially
melts and magma flows into the rift, forcing plates to diverge and the sea floor to
spread. As magma cools it records the magnetic field of the earth. Periodic eruptions
and spreading continues through time causing rocks formed at different times to
record different magnetic polarities as the magnetic field of Earth periodically
reverses. This results in a magnetic striping of the sea floor as illustrated in Figure 3.
Figure 4 depicts magnetic data collected in 1965 by W. C. Pittman III and J. R.
Heirtzler that was originally used to test this hypothesis of seafloor spreading. These
data were published in Science in 1966. The figure shows magnetic data from four
traverses (A through D) over a mid-ocean ridge in the South Pacific, collected by
pulling a magnetometer behind a research ship. The crest of the ridge is shown as a
dashed line. Peaks in the magnetic data correspond to ocean crust that was formed
during times of normal polarity, whereas the troughs (low spots) correspond to ocean
crust that was formed during times of reverse polarity. Four peaks were found by the
researchers to be distinctive enough to correlate from transect to transect. They have
been labeled 1, 2, 3, and 4 on Figure 4. When answering the questions, remember
that these are real data, and there are many ambiguities in nature
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Figure 3. Schematic diagram illustrating the development of magnetic striping on the ocean floor associated
with seafloor spreading. Figure retrieved from http://pubs.usgs.gov)
Figure 4. Data from the south Pacific Ocean (adapted from Pittman and Heirtzler in Science, 1996)
illustrating observed variations in magnetism across themid-oceanic ridge (dashed line) along 4 transects (A
through D). Numbers 1 through 4 indicate positions of magnetic anomalies referred to in questions.
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QUESTIONS:
1. Draw lines on both sides of the ridge in Figure 4 that connect labeled magnetic
peaks (i.e. draw a line from peak 1 on transect A to peak 1 on transect B to
peak 1 on transect C to peak one on transect D and so on for peaks 1 through
4). These lines simulate seafloor striping.
2. Using arrows, indicate on Figure 4 the direction that the plates are moving on
each side of the mid-ocean ridge.
3. Explain how the lines you just drew support the idea of seafloor spreading.
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4. Scientists have determined that peak 4 is 7.0 million years old. The center of
the ridge is 0 years old. Using transect D, calculate the spreading rate (rate at
which the sea floor is spreading). Give your answers in km/ million years, and
the equivalent rate in cm/year. Be sure to show all of your work. (Hint:
calculate the spreading rate on each side of the ridge to get the two halfspreading rates and then add them together to get the full-spreading rate.)
_________________ km / million years
__________________ cm / year
Show your work here:
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PART C. HOT SPOTS
EXPLANATION:
Figure 5 is a map of the islands and seamounts (submerged volcanoes) that form the
Hawaiian Islands and the Hawaiian-Emperor Chain. All of the islands and seamounts were
formed as volcanoes, and are younger than the ocean crust that they sit upon. In 1963, J.
Tuzo Wilson proposed that all of these volcanoes were formed by a “hot spot” – a relatively
stationary hot plume that rises through the earth’s mantle and melts part of the overriding
plate.
QUESTIONS:
1. Assuming that the big island of Hawaii is directly over the hot spot, and that the hot spot
has been stationary since these islands and seamounts formed, use arrows to show the
relative motion of the Pacific Plate over the hot spot. You should use two arrows to
account for the bend in the Hawaiian-Emperor Chain.
2. What explanation can you give for the bend in the Hawaiian-Emperor Chain? ________
______________________________________________________________________
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3. The table below presents age data for each island or seamount (based on radiometric
dating), and it’s distance from Kilauea (the active volcano in Hawaii that is inferred to be
directly over the hot spot). Using the graph paper on the next page, plot the age versus
distance from Kilauea for each of the volcanoes listed in the table.
Draw 2 best-fit straight-line segments through the data, one from Mauna Kea to
Laysan and one from Laysan to Suiko.
Volcano Name
Distance from
Age in Millions of Years
Kilauea (km)
Mona Kea (on Hawaii)
54
0.375 ± 0.05
West Maui
221
1.32 ± 0.04
Kauai
519
5.1 ± 0.20
Nihoa
780
7.2 ± 0.3
Necker
1058
10.3 ± 0.4
La Perouse Pinnacle
1209
12.0 ± 0.4
Laysan
1818
19.9 ± 0.3
Midway
2432
27.7 ± 0.6
Abbott
3280
38.7 ± 0.9
Daikakuji
3493
42.4 ± 2.3
Koko (southern)
3758
48.1 ± 0.8
Jingu
4175
55.4 ± 0.9
Nintoku
4452
56.2 ± 0.6
Suiko (southern)
4794
59.6 ± 0.6
Suiko (central)
4860
64.7 ± 1.1
Original source for data USGS Professional Paper 1350
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4. Why do the two lines have different slopes?
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5. Calculate the rate of plate movement for each of the line segments you drew by dividing
the distance traveled by the time interval over which the travel took place. Give your
answer in cm/yr.
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Figure 5. Map of the Hawaiian-Emperor Chain in the Pacific Ocean
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TRANSFORM FAULTS
EXPLANATION:
Figure 1 shows a geologic map along part of the San Andreas Fault, a transform fault that
is the boundary of the North American and Pacific plates. At this location, the two plates are
sliding past each other. Wallace Creek flows to the northwest from the Tremblor Mountains
across the fault. Figure 1 shows the distribution of younger (3680 years to today) and older
(10,000 to 3680 years) sediments deposited by the creek. These ages were derived from
carbon-14 dating.
QUESTIONS:
1. Why does the present day stream valley bend sharply at the San Andreas Fault? _____
______________________________________________________________________
______________________________________________________________________
2. What evidence is there on the map for vertical (up and down) movement along the
fault? _________________________________________________________________
______________________________________________________________________
______________________________________________________________________
3. What do you predict will happen to the course of the creek in the future? Draw on the
map if it helps. __________________________________________________________
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______________________________________________________________________
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4. Draw arrows on Figure 4 to show the direction of movement of the plates on either side
of the fault.
5. What is the rate of movement along the fault in the past 10,000 years? This can be
determined by dividing the distance of the stream offset across the fault by the age of
the oldest sediments in each of the channels. Calculate the offset rate based on data for
each channel. Use 10,000 yrs for the age of the sediment in the old channel, and 3,680
yrs for the age of the sediment in the younger channel. Give your answers in cm/yr.
10
3
2
1
1
2
2
3
1
2
1
3
http://pasadena.wr.usgs.gov/
Figure 1. Geologic map (top) of the Wallace Creek (bottom) crossing the San Andreas
Fault, California.
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