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Unit 32


Unit Overview

This unit discusses the movement and interactions of lithospheric plates. The main sections are as follows:

  • Continental drift
  • Continents, plate tectonics, and seafloors
  • Distribution of plates
  • Movement of plates

The continents once formed a "super-continent" called Pangaea. The continents separated by means of continental drift, whereby the continents┬┐drifting as a result of plate tectonics┬┐function as rafts on the ocean. Midoceanic ridges are corridors of seafloor spreading, where new crust is created and subsequently moved away from these linear zones. The midoceanic ridges mark many of the boundaries between the lithospheric plates. In addition to these zones of plate formation are zones of destruction, where plate collision occurs.

The largest plates are as follows: Pacific, Eurasian, African, South American, Australian, Indian, and Antarctic. Earthquakes and volcanoes are typically located at plate boundaries. The plates move relative to one another and that movement is directly responsible for many of the Earth's major landscapes and landforms. The plates diverge, converge/collide, and displace laterally. Divergence results from crustal spreading, which is not unique to midoceanic ridges. If plates form and spread outward in certain areas of the crust, then they must converge and collide in other zones.

The three types of convergent plate boundaries are oceanic continental, oceanic-oceanic, and continental-continental. The oceanic-continental convergence zones are characterized by subduction zones, which are areas where an oceanic plate subducts beneath a continental plate. These subduction zones are places of intense tectonic activity because of the melting of crust in the asthenosphere. Subduction zones can also occur at oceanic-oceanic plate boundaries.

Continental-continental convergence involves negligible subduction; it instead produces massive deformation and considerable buildup of crustal rock mass. The final type of plate contact involves lateral motions. This lateral plate contact is characterized by transform faults, where plates slide past each other. Like other plate boundaries, such plate movement is associated with earthquakes and crustal deformation.

Unit Objectives

  • To introduce the concepts of continental drift and plate tectonics
  • To identify the major plates of the lithosphere
  • To discuss the important boundary zones between the lithospheric plates in which rifting, subduction, and transform faulting occur

Glossary of Key Terms

Continental drift The notion hypothesized by Alfred Wegener concerning the fragmentation of Pangaea and the slow movement of the modem continents away from this core supercontinent.
Crustal spreading The geographic term for seafloor spreading; not all crustal spreading occurs on the ocean floor.
Gondwana The southern portion of the primeval supercontinent, Pangaea.
Island arc A volcanic island chain produced in a zone where two oceanic plates are converging; one plate will subduct the other, forming deep trenches as well as spawning volcanoes that may protrude above sea level in an island-arc formation.
Laurasia The northern portion of the primeval supercontinent, Pangaea.
Lithospheric plates One of the fragmented, rigid segments of the lithosphere (also called a tectonic plate, which denotes its active mobile character); these segments or plates move in response to the plastic flow in the hot asthenosphere that lies just below the lithosphere.
Pacific Ring of Fire The Circum-Pacific belt of high volcanic and seismic activity, stretching around the entire Pacific Basin counterclockwise through western South America, western North America, and Asia's island archipelagoes (from Japan to Indonesia) as far as New Zealand.
Pangaea The primeval supercontinent, hypothesized by Alfred Wegner, that broke apart and formed the continents and oceans as we know them today; consisted of two parts - a northern Laurasia and a southern Gondwana.
Rift An opening of the crust, normally into a trough or trench, that occurs in a zone of plate divergence.
Rift valley Develops in a continental zone of plate divergence where tensional forces pull the crustally thinning surface apart; the rift valley is the trough that forms when the land sinks between parallel faults in strips.
Seafloor spreading The process wherein new crust is formed by upwelling magma at the midoceanic ridges, and then continuously moves away from its source toward the margins of the ocean basin.
Subduction The process that takes place when an oceanic plate converges head-on with a plate carrying a continental landmass at its leading edge; the lighter continental plate overrides the denser oceanic plate and pushes it downward.
Transform fault A special case of transcurrent faulting in which the transverse fault marks the boundary between two lithospheric plates that are sliding past each other.

Unit Outline

  • Continental drift
    • Proposed by Alfred Wegener, 1915, The Origin of the Continents and Oceans
    • All continents were formerly part of the Pangaea supercontinent
      • Laurasia in the north
      • Gondwana in the south
    • Continental drift is the fragmenting of Pangaea and slow movement of continents away from it
      • fossil evidence of plants and animals
      • jigsaw-like fit of continents
  • Continents and seafloors
    • Arthur Holmes (1939) proposed existence of convection cells deep inside Earth that dragged the continents
    • Midoceanic ridges discovered to be formed by seafloor spreading: the continuous creation and upward deposition of new crust, and its subsequent movement away from its source
      • Earth divided into lithospheric (tectonic) plates
      • if ocean floor is created and spreads in one area, must be crushed and destroyed in another location
        • volcanoes
        • earthquakes
        • mountain building
    • Distribution of plates
    • Pacific Plate
    • North American Plate
    • Eurasian Plate
    • African Plate
    • South American Plate
    • Australian Plate
    • Indian Plate
    • Antarctic Plate
      • Many smaller plates associated with the eight major plates
    • Location of plate boundaries
      • represented by linear earthquake zones
    • continental and submarine volcanism
      • Pacific Ring of Fire
      • Movement of plates
  • Plates maintain their direction of movement for millions of years
  • Movement of plate directly creates earth's landscapes
  • Plate divergence
    • midoceanic ridges spread apart by rising magma, rifts in seafloor created
    • rift valley occurs when tensional forces are underneath a landmass
    • seafloor spreading now called crustal spreading because it is not confined to just the oceans
  • Plate convergence
    • oceanic-continental plate convergence: subduction occurs when an oceanic plate and a continental plat collide; the lighter continental plate overrides the oceanic plate, and pushes it downward
    • oceanic-oceanic plate convergence: convergent plate densities are the same, when collision occurs, huge contortions formed, deep trenches and huge volcanoes result
    • continental-continental plate convergence: convergent plate densities are the same, huge distortions made, lower plate not forced downward, earthquakes
  • Lateral plate contact
    • boundary where two plates are sliding past one another
    • create transform faults
    • earthquakes and crustal deformation
    • San Andreas Fault

Review Questions

  1. What is the Pacific Ring of Fire?
  2. Describe generally the formation of a rift valley, using Fig. 32.6 as a guide.
  3. Contrast oceanic-oceanic plate convergence with oceanic-continental plate convergence.