Lab-5

Geology Name: ___________________________ 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: ___________________________ 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? The depth gradually decreases the closer it gets to B. There is also a big gap between A and B where there is nothing. 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 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

Geology Name: ___________________________ Plate Tectonics and Volcanic Arcs Lab Report 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 Oceanic 1. What kind of volcanic activity is happening at the red plate boundaries -- is this spreading center volcanism or arc volcanism ? Arc volcanism, it all connects in one giant arc around the globe. 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? There are so few because they are deep in the ocean and only a few have hotspots that create volcanic figures in the ocean. 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? Divergent: Thinner crust, less density.

Geology Name: ___________________________ Plate Tectonics and Volcanic Arcs Lab Report Convergent: Vary in sizes, can be both thin and thick. Transform: Has a very dense crust. o Remember, the subducting plate is always oceanic crust; it's only the overriding plate that varies. 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? Yes, it affects the distance from the trench to the arc and their densities. 3. Does the type of setting affect trench depth ? If so, what is its effect? Yes, convergent settings typically have the deepest trenches, Divergent vary in depth and Transform boundaries are typically shallow. 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.