Unit 8:  Internal dynamics of the Earth                                    1   2   3

2. Tectonic plates theory

The Plate tectonics theory was formulated in the 1960’s. According to this theory:

     - The outermost solid part of the Earth, the lithosphere, is divided into

        large blocks called tectonic plates that fit together in the same way

        as pieces of a giant jigsaw puzzle.

     - The plates float on top of the asthenosphere, the layer just below

        the lithosphere. The asthenosphere is denser than the lithosphere

        and is partially formed by molten materials that circulate in convection

        currents.

     - The convection currents of the asthenosphere are the engine that moves

        the tectonic plates.

The points where two tectonic plates meet are called boundaries. Earthquakes, volcanic eruptions, formation of mountain ranges and rock deformation

are most likely here.

The plates can move one with respect to another in different ways:

a) Divergent boundaries

      When plates separate they form a divergent boundary.

      Materials from the inside of the Earth rise up between the plates forming

      new lithosphere and provoking their separation.

      The limit between the plates is a ridge.

      For example in the middle of the Atlantic Ocean, the Atlantic Ridge

      separates the African and the South American Plates.

Animation: Divergent boundary (Classzone)

b) Convergent boundaries

     When plates collide they form a convergent boundary.

     These limits can be of two types:

     - When one plate collides by its continental side with other by its oceanic side,

        the oceanic plate sinks underneath the other one (subduction).

        The subducting plate is destroyed, molted into the asthenosphere.

        When plates converge they cause earthquake and form volcanic

          mountain ranges in the continent and a trench in the ocean.

        For example Nazca Plate is subducting underneath South America Plate,

        forming the Andes Mountain Range.

     - When two plates collide by their continental side neither one nor another subduct.

        In this case, a huge mountain range is formed and plates fuse (obduction).

        For example African plate is colliding with Eurasian plate forming the Pirineos

        and the Alpes.

Animation: Convergent boundary (Classzone)

c) Transform boundaries

      When plates slide past each other, moving in opposite directions,

      they form a transform boundary.

      The friction between the plates provokes intense earthquakes,

      but lithosphere is neither created nor destroyed.

      This limit is called transform fault.

      An example is the San Andrés fault between the North American and Pacific Plates.

Animation: Transform boundary (Classzone)

2.1. Define these terms:

     a. Tectonic plate                   

b. Convection current                 

c. Asthenosphere

2.2. Complete the following chart: