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Geosciences LibreTexts

4: Plate Tectonics

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  • Learning Objectives

    After completing this chapter, you should be able to:

    • Explain several lines of evidence supporting the movement of tectonic plates. 
    • Accurately describe the movement of tectonic plates through time.
    • Describe the progression of a Hawaiian Island and how it relates to the Theory of Plate Tectonics.
    • Describe the properties of tectonic plates and how that relates to the proposed mechanisms driving plate tectonics. 
    • Be able to describe and identify the features that occur at different plate boundaries.

    • 4.1: Introduction
      In chapter one, we reviewed the scientific method and the exact meaning of a theory, which is a well-supported explanation for a natural phenomenon that still cannot be completely proven. A Grand Unifying Theory is a set of ideas that is central and essential to the field of studies such as the theory of gravity in physics or the theory of evolution in biology. The Grand Unifying Theory of geology is the theory of Plate Tectonics.
    • 4.2: Evidence of the Movement of Continents
      The idea that the continents appear to have been joined based on their shapes is not new. In fact. this idea first appeared in the writings of Sir Francis Bacon in 1620. The resulting hypothesis from this observation is rather straightforward: the shapes of the continents fit together because they were once connected and have since broken apart and moved. This hypothesis is discussing a historical event in the past and cannot be directly tested without a time machine.
    • 4.3: Lab Exercise (Part A)
      This lab will use two different ways to input your answers. Most of the questions will be multiple choice and submitted online as you have in previous labs. Other questions will give you a blank box to input your answer as text. Your professor will manually grade this text, such that the format is not as important as your answer. This format allows you the opportunity to show your work using simple symbols and allows the instructor to better see your thought process.
    • 4.4: Hot Spots
      Another line of evidence that can be used to track plate motion is the location of hot spots. Hot spots are volcanically active areas on the Earth’s surface that are caused by anomalously hot mantle rocks underneath. This heat is the result of a mantle plume that rises from deep in the mantle toward the surface resulting in melted rocks and volcanoes. These mantle plumes occur deep in the Earth such that they are unaffected by the movement of the continents or the crust under the ocean.
    • 4.5: Lab Exercise (Part B)
      Type “Hawaii” into the search bar of Google Earth and examine the chain of Hawaiian Islands. On a separate sheet of paper please draw yourself a map of the islands and label the following on your map (making sure to include the names), which will be used to answer the following questions.
    • 4.6: Plate Materials
      By now you can see many different lines of evidence that the tectonic plates are moving (there are many additional lines of evidence as well). To build a theory we need an explanation or a mechanism that explains the patterns that we see. The theory of plate tectonics states that the outer rigid layer of the earth (the lithosphere) is broken into pieces called tectonic plates and that these plates move independently above the flowing plastic-like portion of the mantle (Asthenosphere).
    • 4.7: Lab Exercise (Part C)
      An important property of geological plates is their density (mass/volume). Remember the asthenosphere has fluid-like properties, such that tectonic plates ‘float’ relative to their density. This property is called isostasy and is similar to buoyancy in water. For example, if a cargo ship has a full load of goods it will appear lower than if it were empty because the density of the ship is on average higher.
    • 4.8: Plate Boundaries
      Tectonic plates can interact in three different ways they can come together, they can pull apart, or they can slide by each other (Figure 4.6). The other factor that can be important is the composition of the plates (oceanic or continental crust) that are interacting as was explored in the previous section. These three types of motions along with the type of plates on each side of the boundary can produce vastly different structures and geologic events.
    • 4.9: Lab Exercise (Part D and E)
      Magma is formed from the melting of rock at both convergent and divergent boundaries. However, the processes that occur to melt the rock are quite different. Three different processes are involved in the melting of rocks as we will explore in the following exercise. In Figure 4.7 you can see a graph depicting a variety of temperature and pressure conditions.
    • 4.10: Plate Tectonic Mechanisms
      The question still remains, why do tectonic plates move? The answer comes down to gravity and mantle convection. You have already studied in chapter two how the mantle flows through time creating convection currents. These convection currents flow underneath the plates and through friction pull them along at the surface as well as when they are subducted which is a force called slab suction. Related to this force is slab pull.
    • 4.11: Lab Exercise (Part F)
      This page contains the lab exercise regarding the mechanisms of plate tectonic.
    • 4.12: Student Responses
      The following is a summary of the questions in this lab for ease in submitting answers online.