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Module: Earth’s Skin: Plate Tectonics
ANSWER KEY
Document Contents:
Exercise 3 .
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Earth’s Skin: Plate Tectonics Answer Key
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Name: ___________________________
USING COMPUTERS TO FIND PATTERNS IN EARTHQUAKES AND VOLCANOES
OBJECTIVES
• To explore the correlation between plate boundaries and tectonic activity
• To understand why and where earthquakes and volcanoes occur
• To understand the relationship between downgoing oceanic slabs, mid-ocean ridges, hotspots, and
tectonic activity.
DIRECTIONS
1. Start up the Seismic Eruption program on your computer. Your teacher will tell you where to access
the program.
2. Locate the controls that manipulate the display. Ask your teacher for help if you don’t understand how
to navigate through the program.
3. Once you feel comfortable with the program, begin working on the worksheet. BE SURE TO READ
THE DIRECTIONS FOR EACH EXERCISE.
EXERCISES
1. Click the ‘World’ view; turn the eruptions and the earthquakes on Speed up the display rate to 2
years/sec. Once the display has advanced to the present day, answer the following questions:
a) What does the color of each eruption represent? The color of the symbol indicates the type of ejecta –
red indicates lava, yellow indicates volatiles, pink indicates both, light grey indicates inactive, and dark
grey indicates unknown. The size of the symbol indicates the size of the eruption.
b) What does the color of each earthquake symbol represent? What does the size of each earthquake
symbol represent? The color of the earthquake symbol indicates the depth of the earthquake. The size of
each symbol indicates the size of the earthquake.
c) How are the earthquakes and volcanoes distributed around the world? Volcanoes and earthquakes are
heavily concentrated along plate boundaries. Nearly all of the volcanic activity occurs near plate
margins. There is very little volcanic activity inside of the plates. Volcanoes do occur over mantle
hotspots such as Hawaii and Yellowstone.
d) What kind of plate margin interaction seems to be most conducive to generating volcanic activity? At
a convergent margin, the motion of the downgoing slab creates an arc of volcanic activity on the
overriding continental slab. Prime examples include the Pacific Rim island arcs and the western edge of
South America.
2. Click the ‘North America GroupÆAlaska Group.’ Watch the ‘Cook Inlet Cross-Section’ and
‘Cook Inlet 3D.’ Read the accompanying information to gain a better understanding of what
you’re looking at.
a) What happens when the oceanic crust of one plate collides with the continental crust of another plate?
The oceanic plate is subducted beneath the continental plate. This “downgoing slab” is recycled in the
mantle.
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b) As you go deeper into the Earth, do the earthquakes get bigger or smaller?
Larger earthquakes tend to occur at greater depths.
c) Sketch a profile of a compressional plate margin where one plate is being subducted by the other.
Show where the earthquakes and volcanoes are likely to occur.
3. Click the ‘North America GroupÆAlaska Group.’ Watch the ‘Alaska 1964.’ This was the
biggest recorded earthquake in North America. Read the accompanying information to gain a
better understanding of what you’re looking at.
a) How long is the time span of this animation? Did most of these earthquakes occur on land?
The animation is only two days long. Almost all of the earthquakes occurred in the ocean.
b) What caused all the smaller earthquakes after the first big one? What are these smaller quakes called?
These smaller quakes are called aftershocks. They occur as a set of faults readjust themselves to the new
stress regime that the original earthquake caused.
c) What other natural disasters occurred as a result of this earthquake? Did the earthquake cause most of
the fatalities? If not, what did? ? Landslides both above and below water occurred. These landslides in
turn caused a tsunami. This tsunami was responsible for nearly all of the fatalities associated with the
incident.
4. Click the ‘North America GroupÆPacific Northwest GroupÆPacific Northwest.’ Read the
information to learn about Yellowstone.
a) What causes the earthquake activity in Yellowstone? Does the author describe this feature as
stationary or moving? Mantle hotspot activity causes earthquakes around Yellowstone. The author
describes the hotspot as stationary. This is the conventional description in nearly all textbooks.
b) Is Yellowstone near a plate boundary? Do you have to be near a plate boundary to experience an
earthquake?
Yellowstone is not near a plate boundary. You don’t have to be near a plate boundary to experience an
earthquake, but chances are very small that you would experience one if you’re not.
c) Locate your home town. Have many earthquakes occurred near it in the past 50 years? Why or why
not?
(Of course this answer will depend on your home town) There was one earthquake near Austin, Texas in
1993 located at 28.8ºN, 98.1ºW. Earthquakes are rare in Austin, TX because the city is not located near
a plate boundary or a mantle hotspot. However, this city is located along the Balcones fault, so there
exists a potential for future earthquakes.
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Write a brief summarizing paragraph that addresses the three objectives given at the beginning of this
worksheet.
The vast majority of volcanoes and earthquakes occur along plate boundaries. Convergent plate
boundaries often form arcs of volcanoes, whereas earthquakes are prevalent across all types of plate
boundaries. Earthquakes occur when a sudden release of stress occurs at a fault in the subsurface.
Volcanoes occur when molten material rises up through the crust and is extruded out onto the surface.
Volcanoes are most common where a downgoing oceanic slab is subducted below a continental plate.
The downgoing material melts and rises to the surface. Molten material also occurs as the surface at midocean ridges. These spreading centers are the site of many earthquakes and are responsible for creating
new oceanic crust. Finally, hotspots occur where mantle heat reaches the earth’s surface. These hotspots
can create volcanoes (such as the Hawaiian island chain) or earthquakes (such as those around
Yellowstone).
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