Plate Tectonics-Volcanic Arcs Lab Report.14-7-22-1
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Geology
Name: Mysterious Banana Slug
Plate Tectonics and Volcanic Arcs Lab Report
Complete your responses in a color other than black or red
.
Part 1: Seismic Activity at Plate Boundaries
Geology
Name: Mysterious Banana Slug
Plate Tectonics and Volcanic Arcs Lab Report
1.
Based on the different colors of the earthquakes on the map above of South America (see legend at the bottom of map), can you see a different in earthquake depth as you go from west to east along the line (From A to B)? What about earthquake magnitude? What do you notice?
I can see that as you go from point A to point B, the size and depth of the earthquakes change. It is evident that as you approach inland from
the Pacific Ocean, the earthquakes' magnitude decreases, but as you get onto the South American plate, their depth increases.
2.
Use the data from the .csv filed linked on canvas to plot the cross-
section
from A to B, either by hand or using a spreadsheet application
like Microsoft Excel or Google Sheets.
Plot the location of the focus
of each earthquake (x = location east or west of the trench, y = depth) as a dot on the cross section below.
Plot the location of each volcano (x = location east or west of the trench, depth = 0 km) with a color or symbol different from the ones
you used for the earthquakes.
3.
In this location, the ocean plate to the west is subducting under the continental plate to the east. Comparing the diagram of a subduction zone below to the profile with earthquake locations that you just plotted, draw in the following features from the diagram onto your plot:
Label the trench
(zero on the x-axis) with an arrow pointing down and the word "trench".
Draw a line on the cross section to show the subducting plate
as it descends from the surface down into the earth.
Draw arrows on the subducting and overriding plates to show their movement directions
(on the diagram below, these are the fat gray arrows).
Circle the zone of melting
. At what depth (below the surface) and distance (horizontally, from the trench) does the magma that produces the volcanoes probably originate, and why?
The magma that forms volcanoes is produced at a distance of roughly 200–
100 kilometers. As it ascends, the underlying rock melts more and undergoes numerous geological processes that change it, releasing magma that forms volcanoes.
Geology
Name: Mysterious Banana Slug
Plate Tectonics and Volcanic Arcs Lab Report
The crust-mantle boundary (aka “Moho” or Mohorovicic discontinuity) is about 10 km deep under oceanic crust and 50 km deep under continental crust. Draw a straight horizontal line at the depth of the Moho below the oceanic crust
(from the left side of the profile to the trench), and another at the depth of the Moho below the continental crust
(from the right side of the profile to the trench).
Label the part of your cross section where earthquakes are happening
in the mantle
(below the Moho). (this is not labeled in the example subduction zone diagram)
Part 2: Plate Boundary Types 1.
The green, red, and blue highlighted plate boundaries (on figure 4)are the ones at which magma is reaching the surface. What kind of plate boundaries do the different colors of highlighting indicate? (For each color, circle the appropriate word from each set.)
Red: Divergent OR
Convergent OR Transform
| Continental
OR Oceanic
Blue: Divergent OR Convergent
OR Transform
| Continental OR Oceanic
Green:
Divergent OR Convergent
OR Transform | Continental
OR
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Geology
Name: Mysterious Banana Slug
Plate Tectonics and Volcanic Arcs Lab Report
Oceanic
1.
What kind of volcanic activity is happening at the red plate boundaries -- is
this spreading center volcanism
or arc volcanism
?
Arc volcanism is linked to the volcanic activity along red plate borders. These
are created when an oceanic plate subducts beneath another plate, causing magma to be produced from the subducting plate and rising to form the arc, a chain of volcanoes.
2.
Magma is definitely reaching the surface in the blue plate boundaries as well. Why are there so few red triangles (named volcanoes) along the blue plate boundaries?
The reason for the dearth of red triangles near blue plate borders is because,
in general, subduction zone volcanic activity is more explosive and widespread than mid-ocean ridge volcanic activity.
Part 3: Subduction Zone Geometry
1.
How do the thickness
(vertical distance from surface to asthenosphere)
and rock density
(heavy or light) of the overriding plate vary among the three different kinds of settings? o
Remember, the subducting plate is always oceanic crust; it's only
the overriding plate that varies.
Depending on the kind of plate boundary, the overriding plate's thickness and rock density can change. The overriding plate in a subduction zone is usually continental crust, which is thicker and less dense than the subducting
plate's oceanic crust. The density of continental crust is 2.7–2.8 g/cm^, and it can range in thickness from 30 to 50 km. The overriding plate in a transform fault setting is typically oceanic crust, with a density of 3.0 g/cm³ and a thickness of about 6–10 km. Depending on whether a divergent plate boundary occurs in the ocean or on a continent, the overriding plate's density and thickness can change. The overriding plate at a mid-ocean ridge is usually oceanic crust, with a density of 3.0 g/cm³ and a thickness of about 6–10 km. The overriding plate at a continental rift is made up of continental crust, which has an average thickness of around 30–50 km and an average density of about 2.7–2.8 g/cm³. Continental crust is thicker and less dense than oceanic crust.
Geology
Name: Mysterious Banana Slug
Plate Tectonics and Volcanic Arcs Lab Report
2.
Does the type of setting affect the angle of subduction
and the distance from the trench to the volcano
? If so, what is its effect?
The subduction angle and the trench's distance from the volcano can both be
impacted by the type of setting. While the distance from the transform fault to the volcano is unaffected by the subduction angle in a transform fault scenario, a steeper angle of subduction in a subduction zone may result in a shorter distance from the trench to the volcano. The location of the magma source and the pace of spreading can affect how far the volcano is from the rift in a divergent plate boundary.
3.
Does the type of setting affect trench depth
? If so, what is its effect?
Yes, in a divergent plate boundary setting, subduction zone, or transform fault, the trench depth might vary depending on the kind of setting. Because the heavier oceanic plate is sinking into the mantle and creating a deep depression on the seafloor, a subduction zone can have extremely deep trenches. Since there is no subduction in a transform fault situation, there may be a shallow trench or none at all. Instead of a trench, a divergent plate boundary usually consists of a depression called a rift valley that is created when the plates stretch and separate. Depending on the location and rate of spreading, the rift valley's depth can change.
4.
Draw or describe the differences in subduction zone geometry between
the three different types of setting that could produce the differences in trench location and distance to the volcano that you described in questions 2 and 3. (If you draw diagrams, label the diagrams clearly.
) Make sure your answer makes it clear exactly how you think the difference in the type of overriding plate affects the shape of the subduction zone.
Subduction zones vary in shape based on the environment they are in. Both oceanic plates are present in an ocean-ocean subduction zone, with the denser subducting plate sinking more rapidly to form a deep trench near the volcano. The overriding plate is continental in an ocean-
continent subduction zone, while the subducting plate is still oceanic. The trench is less deep and farther away from the volcanic arc because of the ensuing shallower angle of subduction. A high mountain range is created when two continents collide, but subduction doesn't happen and there isn't a volcanic arc or trench.
Geology
Name: Mysterious Banana Slug
Plate Tectonics and Volcanic Arcs Lab Report
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