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Module 3, Investigation 3: Plate Tectonics Introduction Welcome! You’ve now looked at evidence for shifts in the positions of continents and expansion of the ocean basins, but what actually causes these things to occur? In this investigation, we will examine what is happening below ground to cause these shifts. You will need to refer to your notes about volcanoes, earthquakes, and the Earth’s structure to see the whole picture. Folder 1: Plate Geography & Structure What is a tectonic plate? The entire Earth’s surface is divided into pieces called "lithospheric" or “tectonic plates” and the movement of these plates over geologic time is called “plate tectonics”. There are seven or eight “major” plates, depending on how they are defined, and numerous minor plates. Turn on your Plate Geography folder to see the names and locations of the various plates. Note that the plates are typically named for the continent that occurs on them, but most plates include both land and ocean portions. The plates include two layers, the upper mantle below and crust above. The thickness of the plates depends on their age. In the last investigation, you saw how new oceanic lithosphere is constantly being formed at the mid-ocean ridges. In the ocean, plates are often less than 15 km thick, while on the older land masses, plates can be more than 200 km thick. Another important difference between the two types of plates is that oceanic crust is denser than continental crust. This is because oceanic crust is made of basalt, which has a higher proportion of dense minerals such as magnesium, while continental crust is granitic, which has a higher percentage of less dense minerals such as aluminum. Why is this so important? You will soon see! Folder 2: Rates of Plate Movement How fast do plates move? There are several methods for measuring the movement of tectonic plates. The most widely used is the global positioning system (GPS), which relies on twenty-four GPS satellites that orbit the Earth every 24 hours. Networks of permanent, extremely sensitive, GPS receivers or “monuments” are used to make repeated measurements of their locations. Changes in monument location are then plotted against time to determine the rate at which plates are moving at various locations. Turn on the GPS Monuments folder to see the location of three plate monitoring sites (Neah Bay, WA, Mission Viejo, CA, and Parkfield, CA). Then go to the link below and scroll down to the “Plot Real Time GPS Data” section. For each of the three stations, select the station name from the drop down list and view the graphs of North to South and East to West movements for each of these stations. Click the "analyze" button to see the average velocities over the past 5 years and record them in your field notebook. Click here to open the link. Next, turn on the Earthquakes layer under the Gallery folder in the Layers panel and compare the seismic activity around the three sites in Google Earth. Record your findings in your field notebook. Folder 3: Driving Forces What causes the plates to move? You already know something about the Earth's structure that is the key to making plates move. Can you remember what it is? If not, review your notes from Module 2, Investigation 3, Folder 3: "Voyage to the Center of the Earth". In that investigation, you determined that the asthenosphere is a relatively soft layer between harder sections of the upper mantle below and rigid lithosphere above. This allows the asthenosphere to act as a kind of lubricating layer. The lithosphere, which makes up the tectonic plates, "floats" above it. In Investigation 2, Folder 3: "The Hypothesis", you also learned that liquid iron in the outer core moves in large convection currents. The heat carried by these currents is generated from radioactive decay of several chemical elements in the Earth's core, including potassium, uranium, and thorium. This heat is extracted by the mantle, which also has convection currents that move hot material to the surface and cool material downward. Many scientists believe that these convection currents in the mantle act like conveyor belts, pulling the plates above them along. The process is usually depicted in a simple diagram as shown below. However, mathematical models of how convection might actually occur are far more complex. View the animation here to see for yourself. Folder 4: Plate Interactions Living on the Edge Plate boundaries can either be convergent (colliding), divergent (separating), or transform (sliding past each other). To view each type of boundary, click the play button below and hover your mouse over each button in the interactive diagram. Then, click on the links following the diagram to view animations about each type of plate boundary. Be sure to take notes on each situation in your field notebook and then answer the questions posed. Folder 5: Mystery Tour Mystery Tour Now that you know what happens at the different types of plate boundaries, let’s see if you can identify them “in the field”. You will visit seven locations. At each one, you must determine (1) what types of plates are interacting, and (2) what type of interaction is occurring. Before you begin, turn on the Volcanoes and Earthquakes layers under the Gallery folder in the Layers panel. These layers contain important clues! If you click on an earthquake icon, you can see the depth of the hypocenter. Also, remember that only earthquakes that are recorded by seismographs in an international network will appear in Google Earth. Make sure to record your observations and interpretations in your field notebook. Good Luck! Summary Let's Review! Turn on your Plate Geography folder and the Volcanoes and Earthquakes layers under the Gallery folder in the Layers panel. Zoom in until the earthquakes and volcanoes appear, and take your own tour of the plate boundaries. Would you say that the Earth is geologically active? Why or why not? Do you see that volcanoes and earthquakes tend to occur along plate boundaries? What types of plate boundaries usually have volcanoes? Why do volcanoes form at these boundaries? What types of boundaries usually have earthquakes? Why do earthquakes occur at these boundaries? If you had to live near a plate boundary, what type of boundary would you want it to be?