Plate Tectonic - Plate Tectonics (5)
Pre Lab 

   
OBJECTIVES:
  • Exploring how the crust moves.
  • Investigating the patterns produced by volcanoes and earthquakes.
VOCABULARY:
  • asthenosphere
  • inner core
  • outer core
  • crust
  • lithosphere
  • mantle
  • plate tectonics
MATERIALS:

Students research patterns of earthquakes and volcanoes on the Internet.

Click on image to enlarge
plate01_.gif (51459 bytes)

A map of the plates and active volcanoes, from the USGS

BACKGROUND:

Geologists have distinguished three main internal subdivisions of the Earth, based on the behavior of seismic waves and laboratory experiments. The outermost layer is the crust. The Earth has two types of crust. Continental crust underlies much of the Earth’s land surface. The ocean floors are underlain by oceanic crust. These materials are composed of different kinds of silicate minerals. The continental crust is composed mainly of the minerals that are in the igneous rock granite, and the oceanic crust is mainly composed of minerals that are in basalt.

Underlying the crust is the second layer, the mantle. It comprises the largest portion of the Earth. Like the crust, it is also composed of silicate minerals. The innermost portion of the Earth is the core. It is composed of metallic elements, primarily iron and nickel. The core is subdivided into two parts. The outer core is liquid metal, while the inner core is solid. The radius of the Earth is 6371 km (inner core = 1200 km; outer core = 2200 km; mantle = 2900 km; and crust = 71 km).

Remember that the composition of the inner part of the Earth has not been directly observed. The layers presented above are based on the interpretation of seismic waves and experimental data by geologists and seismologists. Remind students that we cannot even drill all the way through the crust!

The plates are composed of the crust and the uppermost part of the mantle. These two layers are often called the lithosphere because they are both composed of solid rock. The underlying, partially molten part of the mantle, on which the plates slide, is called the asthenosphere. In contrast to the crust–mantle-core division, which was based on composition, the lithosphere and asthenosphere are separated based on strength. The lithosphere is strong, rigid rock, while the asthenosphere is a weaker, very viscous fluid. The plates are layers of rigid, solid rock. However, as they move, plates interact at their edges or boundaries. These interactions generate earthquakes and volcanoes. There are three basic directions or types of boundary interactions. At diverging plate boundaries, earthquakes occurs as the plates pull away from each other. Volcanoes form between the plates, as magma rises upward from the underlying mantle. Second, two plates may come together, at a converging plate boundary. Two situations are possible at converging plate boundaries. First, only earthquakes occur when two plates collide (obduct), building a mountain range. Second, both volcanoes and earthquakes form where one plate sinks under the other, instead of colliding. This process is called subduction. Finally plates can also slide past each other horizontally. Transform plate boundaries commonly have only earthquakes.

PROCEDURE:

  1. Introduce the students to the internal structure of the Earth. The physiographic globe illustrates these layers well, or use the presentation image below. Make sure to give them the thickness of each layer.
      

  2. Introduce students to the basics of plate tectonics. Describe each type of plate boundary, the type of motion occurring there, and whether or not earthquakes and volcanoes are generated.

  1. As a homework assignment, have the students search the Internet for information on the global distribution of earthquakes and volcanoes. You may wish to tell them to look for patterns and/or exceptions to the occurrence of these phenomena at plate boundaries. Here are some suggested websites:
      
    http://pubs.usgs.gov/publications/text/dynamic.html  
    This Dynamic Earth - an introduction to plate tectonics from the U. S. Geological Survey
      
    http://www.ucmp.berkeley.edu/geology/tectonics.html 
    The plate tectonics page at the UC Berkeley Paleontology museum.

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