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ES 131 Lab: Plate Tectonics 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.  Part A examines global maps of tectonic plate boundaries along with maps of earthquake and volcanic activity to identify plate boundary locations and assess relative motion between the plates.  Part B uses maps of the ocean floor to calculate spreading rates across mid ocean ridges in the South Pacific and Atlantic Oceans  Part C interprets maps and utilizes geologic ages for Hawaiian Islands to better understand movement of the underlying Pacific plate over a “hot spot”.  Part D examines a geologic map along a portion of the San Andreas Fault to evaluate the direction and rates of plate movement. OBJECTIVES Upon completion of this exercise, you will be able to understand: 1. the basic differences between major types of plate boundaries 2. what magnetic stripping is and use it to calculate spreading rates 3. the co ncept of “hot spots” and use this under standing 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, accompanying map(s), and a series of questions that pertain to the map(s). Use this information to interpret the data and see how geologic data supports the theory of Plate Tectonics. PART A. PLATE BOUNDARIES EXPLANATION: This exercise will familiarize you with Earth’s major tecto nic plates, how to identify the type of plate boundary, and how plates move in relation to each other. You will need to refer to Figure 1 , a map of the major plates and boundaries, Figure 2 , a map of historic volcanic activity, and Figure 3 , 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.  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 zig-zag pattern to the plate boundary. Earthquakes are common although they are usually shallower and of lesser magnitudes than at convergent boundaries. Volcanic activity is sporadic and may not be centered exactly on the plate boundary. 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).  Characteristics: These boundaries can be identified by the presence of deep and large earthquakes that are initiated along a subducting plate. Earthquakes are shallow adjacent to the trench and get progressively deeper in the direction of subduction. Hence, epicenters at the surface of earth occur over a broader geographic zone. Plates move perpendicular to and toward subuction zone boundaries. Volcanic activity is prevalent on the margin of the overriding plate and is expressed as linear chains of volcanic islands or continental volcanic arcs 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 is a well-known transform fault. The largest number of transform faults are formed in association with divergent seafloor-spreading regions where they serve as accommodation zones for rates of differential movement along the ridges).  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). There is no volcanic activity associated specifically with this type of plate margin and although small shallow earthquakes may be associated with transform faults on the seafloor, large destructive earthquakes are associated with onshore transform faults in southern California, and portions of Asia north of the Mediterranean Sea.

FIGURE 1. PLATE BOUNDARY MAP Plate boundaries are indicated by irregular lines surrounding continental landmasses (light gray shading) or oceanic regions as depicted on map. Names of plates are as indicated. This map is part of “Discovering Plate Boundaries,” a classroom exercise dev eloped by Dale S. Sawyer at Rice University. Additional information about this exercise can be found at http://terra.rice.edu/plateboundary

Figure 2. VOLCANOLOGY MAP Red dots indicate currently or historically active volcanic features. List obtained from the Smithsonian Institution. This map is part of “Discovering Plate Boundaries,” a classroom exercise developed by Dale S. Sawyer at Rice University. Additional information a bout this exercise can be found at http://terra.rice.edu/plateboundary

SEISMOLOGY MAP Earthquake locations 1980-1996 (magnitudes 4 and greater) Color indicates depth: Red 0-33 km, Green 70-300 km, Blue 300-700 km . This map is part of “Discovering Plate Boundaries,” a classroom exercise developed by Dale S. Sawyer at Rice University. Additional information about this exercise can be found at http://terra.rice.edu/plateboundary

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