An Introduction to the Seafloor and Plate Tectonics 1 Objectives 1) Investigate the components of the lithosphere and lithospheric plates. 2) Identify the associations among various seafloor features, continental features, and plate boundaries. 3) Compare the different types of lithosphere (oceanic & continental). 4) Compare and contrast the differences among the major ocean basins with respect to the types of plates found in each. Introduction: The Layers of the Earth Approximately 70% of the Earth s surface (covering 361 million square kilometers) is covered by the ocean. The average ocean depth (3800m) is 4.5 times greater than the average elevation on land (840 meters), yet it is only a tiny fraction of the total Earth s volume on a planetary scale (0.13% of the Earth s volume). Although the rest of the Earth is composed of rock, it is not homogeneous. Instead, it consists of distinctive layer at different depths, each characterized by specific physical characteristics. Two classifications have been developed to describe the interior of the Earth. One classification is based on the seismic properties of the Earth s interior, dividing the Earth into the crust, mantle, and inner and outer core (Figure 1A). The second is based on rheology, or rock strength, which divides the Earth into the lithosphere and asthenosphere (Figure 1B and 2). The lithosphere includes the crust and the more rigid portion of the upper mantle, and extends to ~150 km. The asthenosphere is the partially melted, slowly flowing layer of the upper mantle below the lithosphere, and extends to a depth of about 350-650 km. It is important that you understand the differences between these divisions of the Earth. Lithospheric Plate Boundaries Now let s consider the lithosphere. The surface of the Earth is currently composed of 13 lithospheric plates. Some are named relative to the major continent found on the plate: North American, South American, African, Eurasian, Antarctic, Indo-Australian (or Australian), and Arabian Plates. Others are named after an ocean or nearby geographic region: Pacific, Nazca, Cocos, Caribbean, Philippine, and Scotia plates. These lithospheric plates are dense, rigid blocks approximately 100 km thick consisting of the crust and upper mantle. The plates float on the viscous, plastic asthenosphere. 1 Adapted from M. Hardee., MSCI 101 Lab Manual, 2004., Pg 55-73., USC. 1
Figure 1 A. Cross-section through the Earth showing the internal layers determined by seismic properties. B. Cross-section of the crust based on the rock strength. P.R. Pinet, Invitation to Oceanography, 2 nd edition, 1999 Jones and Bartlett Publishers, Sudbury, MA www.jbpub.com. Reprinted with permission. Figure 2 A close-up cross-section of the Earth s lithosphere. The lithosphere includes the oceanic and continental cruse and upper part of the mantle. From Of Sand and Sea: Teaching from the southeastern Shoreline by P. Keener-Chavis and L.R. Sautter. Copyright 2000 by South Carolina Sea Grant Consortium. Reprinted by permission of the author and Consortium. At each lithospheric plate boundary, or margin, the plate moves relative to the other plates surrounding it. There are three types of plate boundaries that describe the movement of a plate relative to others: divergent, convergent, and transform (or strike-slip) margins. The following diagram shows the movement of one plate with respect to another for the three types of margins, and plates are bound on all sides by one or more of these types: Divergent Margins Convergent Margins Transform or Strike-Slip margins Divergent margins are characterized by seafloor features called mid-ocean ridges, which are where new oceanic crust forms and spreads away from this spreading center. Convergent margins are zones of oceanic crust subduction, where crust is destroyed and recycled into the mantle. Deep-sea trenches are features that indicate this process occurring on the ocean floor. 2
At transform margins, two features are found, the transform fault and the fracture zone. The transform fault is the actual boundary between two different plates, whereas the fracture zone is the crack that extends from the transform fault within a single plate (Figure 3). You can see the margins of the lithospheric plates by looking for these specific physical features on a world or ocean basin map. Figure 3 The transform fault is the region between two rift valleys, where two plates are moving in opposite directions (arrows). The fracture zone is the region extending away from an axis, a crack in a single lithospheric plate. P.R. Pinet, Invitation to Oceanography, 2 nd edition, 1999 Jones and Bartlett Publishers, Sudbury, MA www.jbpub.com. Reprinted with permission. Other seafloor features can be seen on an ocean floor map that are not plate boundaries, but are related to plate tectonics in some way. These include volcanic island chains and seamounts. Volcanic island chains (e.g., Japan) are created at a subduction zone when subducting oceanic lithosphere melts and the melt rises beneath another oceanic plate to form an island chain, called a volcanic island arc (Figure 4). Volcanic mountain chains such as the Himalayas occur when the oceanic crust subducts underneath continental crust. Seamounts are volcanic mountains that form over a stationary mantle plume (called a hot spot ) that forms deep in the asthenosphere. An example of a seamount chain is the Emperor Seamounts extending northwest of the Hawaiian island chain. Figure 4.4 Convergent plate boundary between two oceanic lithospheric plates. The subduction of one oceanic plate under another results in the formation of a volcanic island arc in addition to deep-sea trench. From Of Sand and Sea: Teaching from the southeastern Shoreline by P. Keener-Chavis and L.R. Sautter. Copyright 2000 by South Carolina Sea Grant Consortium. Reprinted by permission of the author and Consortium. 3
Puzzling Plates: An Introduction to Plate Tectonics Questions 1a. Consider the Atlantic Ocean. Which plate(s) make up this body? 1. 3. 5. 2. 4. 6. b. What feature is the boundary between these plates? 3. What are 2 key differences between the plates making up the Pacific and Atlantic Oceans? 4. Assemble pieces to form the Indian Ocean. How does this body compare to the Atlantic and Pacific Ocean basins? 5. Complete the puzzle. Consider Antarctica on the puzzle and on a globe. What do you notice about this continent on the two representations? 6a. Other than the South American plate, identify the other plates which contain continental crust. b. What are some plates that contain oceanic crust? 4
c. Certain layers are found in all of the plates. What are these layers? 7. Draw a cross-section of the continental and oceanic crust, label the layers, and compare to the corresponding picture in your book. Discuss some of the differences and similarities. 5
8. Why are so many volcanoes associated with deep-sea trenches? 9. Figure 4.5 above depicts the location of earthquake events worldwide. Examine the figure and answer the associated questions. a. With respect to land masses, where does the greatest number of earthquakes occur? b. With respect to the oceans, where does the greatest number of earthquakes occur? c. Examine the earthquakes associated with the Peru-Chile trench system. What two lithospheric plates are involved in this convergent margin? Are there any earthquakes on the ocean side of this trench, and why do you think this is so? d. Why don t all the earthquakes in the middle of the ocean fall along a straight line? Explain this in terms of plate tectonic boundaries. 6