Download IDS 102 Plate Tectonics Questions Part I: Observations

IDS 102
Plate Tectonics Questions
Part I: Observations- Four maps of world are positioned around the room. Answer the questions associated with
each map and record your general observations about the maps.
World topography- this map portrays the elevation of the Earth’s surface by color. See the scale along the side of
the map for the elevations. The elevations are in meters. Recall that a meter is slightly longer than 3 feet, so if you
multiply the values given by 3 you will have a rough estimate of the height of the land or depth of the ocean in feet.
1. Notice that there are two predominate colors on this map. What are the elevations represented by these two
primary colors on this map?
The elevations of the deep ocean (about -3000 meters) and the continents (about 200-400 meters above sea level)
2. Explain why there are these two predominate elevations. (Hint: think about isostasy and the type of materials we
used to demonstrate buoyancy).
The blue color is the ocean crust which is denser and rides lower in the asthenosphere than the continental crust
(green) which is less dense.
3. Does the continental crust end at the shoreline? What is your evidence?
No, from the shoreline there is a gently sloping continental shelf, then the continental slope, and finally the
abyssal plain at the bottom of the ocean. The continents continue to the area between the continental slope. The
abyssal plain is the oceanic crust. The higher elevation of the continental shelf indicates lower density materials.
4. Look carefully at the elevations in the ocean basins. Are there mountains in the ocean basin? If so, describe those
mountains—are they part of a chain of mountains or are they randomly distributed on the ocean floor?
The mountains on the bottom of the ocean are in three categories. There are mountains that are along the
divergent plate boundaries, such as the Mid-Atlantic Ridge or the East Pacific Rise. The second type of mountain
chain is like Hawaii which is created by a hot spot. The hot spot moves only a small amount compared to the
overlying plate. As the plate moves, volcanoes grow over the hot spot and then are carried away by the motion of
the plate. The third type of mountain is an isolated hot spot volcano from a hot spot that does not continue over a
long period of time and forms one or two volcanoes rather than a ridge like Hawaii.
5. Where do you find the deep purple colors on the map? Explain where this color is found and the shape of the
region illustrated by this color.
The deep purple colors are in the deep sea trenches. Notice that these are NOT at divergent boundaries. The deep
sea trench is at a subduction zone.
Earthquake (Seismology) Map:
1. Are the earthquake epicenters randomly distributed on this map? If not, describe the patterns of the earthquake
epicenters.
Most of the earthquakes happen in near linear bands (along major faults or plate boundaries) with a minor
number of isolated epicenters in the middle of plates.
2. The colors of the epicenters indicate the depth of the earthquake focus. Are there more shallow foci or deep foci?
More shallow foci—the rocks near the Earth’s surface are more brittle and produce earthquakes when they
fracture under stress.
3. Describe where you find the shallow earthquake foci. Describe where you find moderate to deep earthquake foci.
Shallow foci are found in many areas. Notice that at the divergent plate boundaries there are only shallow focus
earthquakes. The reason for this is that the rocks under the divergent boundary is too hot and deform plastically
under stress, so no energy is stored in the rock for an earthquake to be released when the rock is deformed.
In the convergent plate boundaries, the pattern from shallow to deep earthquakes indicates a subduction zone in
which a denser plate moves under as less dense plate. The earthquake foci are in the upper part of the subducted
plate.
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4. What do you notice about the patterns of colors in the places that have moderate to deep earthquake foci—in other
words, is a there specific pattern to the colors relative to the ocean and land area when there are moderate to deep
foci?
See the previous answer.
5. Are the shallow foci more or less random on the continents or in the ocean basins?
Some of the shallow foci are nearly “randomly distributed,” but the shallow only foci that are in linear patterns
are likely along divergent or transform plate boundaries.
Volcanoes:
1. What do you notice about the distribution of volcanic eruptions of the Earth’s surface? Is the distribution random,
in specific patterns, or both? If you picked specific patterns or both, describe the patterns and where one finds the
patterns and random distributions.
Like the topography and earthquake patterns, the volcanoes fall into both linear chains of volcanoes, with some
random volcanoes within the plate. The chains are sub-linear (gently arc-shaped). The volcanoes in these arcs
are most often on or near continents (and convergent boundaries). The random volcanoes are from hot spots.
2. When volcanoes are found in a line, are the lines on land or in the ocean? Are the lines of volcanoes running
through the middle of the continents or along the edges?
Most of these bands of volcanoes are on continents, but there some in the ocean, such as Japan or the
Philippines. There are lines of volcanoes in the middle of plates from hot spot volcanic activity, such as the
Hawaii-Emperor Island-Seamount chain.
3. Many people have heard the expression of the Pacific “Ring of Fire”. Where else do we find lines or chains of
volcanoes?
Besides the convergent boundaries and hot spot chains, there are lines of volcanoes along divergent boundaries,
such as the mid-Atlantic ridge or the East African Rift valley in eastern Africa.
4. Would Yellowstone in NW Wyoming be a part of the chain of volcanoes?
Yes, Yellowstone is due to a hot spot and is a part of chain of volcanoes from Oregon, across Idaho, and into the
northwest corner of Wyoming. Our understanding is that the volcanoes west of Yellowstone were over the same
hot spot in past and their current distribution is due to the motion of North America.
Ocean Floor Ages (Geochronology):
1. Describe the age of the rocks that compose the crust west of South America. Where do we find the youngest rocks
and where do we find the oldest rocks? Describe the patterns of the ages.
The plate to the west of South America is the Nazca plate. The divergent boundary between the Pacific plate and
the Nazca plate is the East Pacific Rise. The rocks are progressively older to both the east and west from the East
Pacific Rise.
2. Where do we find the youngest rocks on the ocean floor?
The youngest rocks are at the divergent plate boundaries, such as the Mid-Atlantic Ridge and the East Pacific
Rise.
3. What is the age of the oldest rocks found on the ocean floor? Where are those rocks found on the map?
About 180 million years. These old rocks are found in the western Pacific.
4. What do you notice about the width (distance across) of the red portion of the map? Where do you tend to find
the widest red color pattern? Where do you find a narrow red color width?
The width of the color bands is a function of the speed of the plate. When we did the marshmallow measurements
at Soos Creek we could have measured the fastest part of the stream by determining the distance the
marshmallow floated in a set amount of time. This is very similar to this situation. The color bands are a set
amount of time. The plate that moved the farthest in this period of time will have moved the fastest.
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Part II: Plate boundary map—we will discuss this in class
Part III: Questions about Plate Boundary Features:
Divergent boundaries:
1. Explain why there are mountains at divergent boundaries. (Out pouring of lava from the divergent boundary
alone does not explain why there are mountains here.)
The mountains at divergent boundaries are due to outpouring of basaltic lavas AND the isostatic rise of the crust
because it is hot and less dense than the surrounding rocks.
2. What types of faults would you expect to see (normal, reverse, thrust, or strike-slip) at the divergent zone? There
is a down-dropped valley along the crest of the ridge, what is the origin of this valley?
The faults are normal faults—we did not cover this in IDS, so you will not be expected to know this.
3. One of the observations that we made about the depth of the earthquake foci at divergent boundaries is that they
are all shallow. Why are there no deep earthquakes at divergent boundaries?
The earthquakes foci are shallow because the rocks under the surface are very hot and do not accumulate
enough strain to produce earthquakes. Imagine if you have some peanut brittle. If the peanut brittle is hot, it is
ductile and deforms without breaking; however, a cold piece of peanut brittle fractures with a “pop”. The sound
indicates that strain energy in the rock was converted to sound energy when it breaks.
4. Why is there very little sediment covering the pillow basalts at a divergent boundary?
There has not been enough time for sediment to accumulate because the crust is very young at the divergent
zone.
5. Explain how we could use marine magnetic anomalies (magnetic reversal patterns on the sea floor) to tell us that
divergence is happening at one of these boundaries.
We see a pattern of normal and reversed magnetic intervals in the rocks that is symmetrical parallel to the
divergent boundary. These zones are parallel bands that show that the sea floor is spreading perpendicular to the
divergent zone.
6. How could we determine the past velocity (speed) of plate motion using the marine magnetic anomalies?
The dates of past reversals are known from land-based research. If we measure the distance the plate has moved
between two reversals, we will the necessary data to determine the velocity of the plate over that interval of time.
We would divide the distance the land travelled by the age of the time of the reversal.
7. Explain how basaltic magmas are formed in the upper mantle.
At plate margins that are diverging, the asthenosphere rises into the region where the plates separate. If the rock
in the asthenosphere moves toward the Earth’s surface, the lithostatic pressure will decrease. This change in
pressure causes the rock to melt because:
The decompression permits the rock to expand and it is over the melting temperature for basalt.
Peridotite (mantle rock) moves toward the surface in divergent zones. This movement toward the surface
decreases the pressure on the peridotite causing the partial melting forming basalt from the peridotite. The
basaltic magma then moves toward the Earth’s surface because it is less dense.
8. How are the chimney-like structures in the deep-sea hydrothermal vents (so-called "Black Smokers") formed?
Ocean water circulates through fractures in the oceanic crust at the divergent boundary. The water picks up
sulfur and other elements from the basalt. This water forms hot springs that develop deposits of minerals that
precipitate from the hot water when the water flows into the cold ocean water.
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Ocean-continental Convergent Zone:
1. Which of the two plates (the one with oceanic crust or the one with continental crust) subducts? Why does it
subduct?
The oceanic crust subducts. It is denser.
2. What is a Benioff-Wadati zone and how it is formed?
The Benioff-Wadati zone is the pattern of earthquakes from shallow to deep along the upper part of the
subducting plate.
3. Explain how intermediate to high silica magmas (andesitic, dacitic, or rhyolitic) form at this type of plate
boundary.
Although it may initially seem simple that a subducted plate melts as it sinks deeper in the Earth, we previously
found that this does not work because:
At depth there is too much pressure to permit rocks to melt.
Another way that melting happens in the Earth is due the presence of water. Water causes rock to melt at lower
temperatures than if the rock is dry.
Low
Temperature
High
Low
B
A
liquid
Pressure
solid
High
liquid
C
Notice that point C is now in the liquid zone. So, rock can melt at depth if the rock has water present.
In a convergent plate boundary, how does water get incorporated into in the boundary?
Clays contain water and clays are formed at the divergent zone due to the circulation of hot water through the
crust.
The water in the plate is found in small fractures in the basalts and as water in clays. The water in fractures is
expelled early as the oceanic crust is subducted. The water in the clays must reach a certain temperature to be
expelled, which requires that the plate descend to a certain depth. (This is analogous to water being driven off
from clay in a kiln in a pottery studio.) At this depth the rock above the plate is mantle rock, peridotite. The water
moves into the mantle causing it to melt. What is the partial melting product of peridotite discussed previously in
this handout?
This basalt will rise toward the surface because it is less dense than the surrounding rock. Sometimes this basalt
migrates to the Earth’s surface forming a basaltic lava flow. (Ape Cave at Mt. St. Helens formed as a lava tube in
a basaltic lava flow from St. Helens.)
The basalt generated in the subduction zone may also melt continental crust. Recall that the melting temperature
of the higher silica igneous rocks is lower than basalt. This means that thermal energy from the basalt will melt
the continental crust which is primarily dioritic in composition. This melting generates magmas of a higher silica
content such as andesite or dacite.
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4. Explain why the density of the oceanic crust continues to increase as the plate is subducted.
The pressure from the overlying rocks increases as the plate sinks.
5. What is a modern example of this type of convergent zone? Here in the Pacific Northwest.
Ocean-Ocean Convergent Zone:
1. If both of the plates have oceanic crust, why does one plate subduct? Could we predict which oceanic plate will
subduct? How?
Most likely, one of the plates will be older than the other. The older, denser plate will subduct. We could predict
this based on the ages of the crust.
2. In what ways is this type of boundary similar to a oceanic-continental convergent zone? Be as specific as possible.
There is a deep-sea trench, a Benioff zone of earthquakes, and intermediate (andesitic) volcanism.
3. What is a modern example of this type of convergent zone? The Philippines
Continental-continental convergent zone:
1. This type of boundary is sometimes referred to as a “suture” zone. Explain the use of this term. Does one of the
plates subduct? Why/Why not?
Since the two continental crustal plates are about the same density and both are low density compared to the
oceanic crust, these plates will not subduct very far into the Earth. More likely there will be an overlapping of the
plates and the continental crust will rise in elevation by the principle of isostasy.
2. Would you expect to find a Benioff zone at this type of plate boundary? A deep-sea trench? A volcanic arc?
Explain your answers.
Probably not because there is no currently active subduction zone.
3. What is a modern example of this type of convergent zone? (The Himalayas)
Transform Boundary:
1. Imagine that we have a series of five GPS stations that monitor the motion of the Earth’s surface in different
places. We have drawn vectors to represent the motion of the ground in these five locations. Which of the following
would most likely create a transform boundary?
A
B
C
Explain your answers.
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D
E
C,D, and E—
A is a divergent boundary
B is a convergent boundary
The other three cases have some component of horizontal side-by-side motion. Indeed, all three of these
situations are found on Earth.
2. Why is volcanism not common at transform boundaries?
There is no decompression or addition of water to create the melting described in the divergent and convergent
boundaries, so normally there is no magma at a transform boundary.
Part IV: The Pacific Northwest – we will discuss this in class
E
W
Olympic Mtns
ocean
Label the following features on the diagram above:
1. Juan de Fuca Ridge
2. Benioff zone
3. andesitic volcanism
4. basaltic volcanism
5. oceanic crust
6. “black smoker” vents
7. Juan de Fuca plate
8. continental crust
9. Pacific plate
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Cascade Mtns