Plate Tectonics

Pamela J. W. Gore
Georgia Perimeter College

Objectives

  1. List the basic types of plate boundaries (divergent, convergent, transform) and describe the relative motions of each. Give examples of where each type of plate motion occurs on the Earth.
  2. List the driving mechanisms that might cause plate motion.
  3. Recognize features produced by mantle plumes or hot spots.
  4. Be able to cite the lines of evidence in support of the Plate Tectonic Theory, Continental Drift and Sea floor Spreading.
  5. Understand how geomagnetic reversals provide evidence for sea floor spreading.
  6. Understand the tectonic processes resulting in the formation of major Earth features such as mountain belts, ocean basins, deep sea trenches, and volcanic areas.
  7. Know the history of the development of the theory of Plate Tectonics, and recognize the contributions of various geoscientists (Alfred Wegener, Harry Hess, Fred Vine and D. H. Matthews).
  8. Identify the various features associated with the basic types of plate boundaries (rift valley, subduction zone, deep sea trench, Benioff zone, etc.).


Evidence in support of the Theory of Plate Tectonics:

  1. Shape of the coastlines

  2. Faunal similarities

  3. Fossil evidence
    1. Glossopteris flora (plant fossils)
      Late Paleozoic seed ferns
      Gondwanaland (India, Africa, Australia, S. America, Antarctica)
    2. Mesosaurus (aquatic reptile)

  4. Rift Valleys of Africa - (continent breaking up)

  5. Geologic similarities between S. America and Africa
    1. Same stratigraphic sequence (i.e. same sequence of layered sedimentary rocks)
    2. Mountain belts and folded rocks line up

  6. Paleoclimatic evidence (ancient climatic zones match up)
    Layers of glacial deposits are found at same place in sequence of rocks
    Note directions of glacial ice movement as indicated by striations or grooves in the rock

  7. Apparent youth of ocean basins and sea floor
    Thin layer of sediment on basalt
    Basalt dates to less than 200 million years (most less than 150 million years)

  8. Evidence for subsidence in oceans -
    1. guyots - flat-topped sea mounts (erosion when at or above sea level)
    2. Chains of volcanic islands that are older away from site of current volcanic activity -
      Hawaiian Islands and Emperor Sea Mounts
      (also subsiding as they go away from site of current volcanic activity)

  9. Mid-ocean ridges located near ocean centers
    1. High heat flow
    2. Seismic wave velocity decreases due to high temperatures
    3. Valley along center of ridge (graben)
    4. Volcanoes along ridge
    5. Earthquakes along ridge

  10. Benioff Zones - inclined zone of earthquake foci (plural of focus) near deep sea trenches

  11. Magnetic stripes on the sea floor


Contributions to plate tectonic theory from paleomagnetism

  1. Recently magnetized rocks show alignment of magnetic field consistent with Earth's current magnetic field

  2. Magnetization in older rocks has different orientations (as determined by magnetometer towed by ship)
    Can determine direction to north magnetic pole and distance to north magnetic pole from inclination and declination of magnetic field in the rock

  3. Polar wandering curves
    Different polar wandering paths seen in rocks of different continents.
    Put continents "back together" and the polar wandering curves are superimposed (match up)

  4. A test of the hypothesis of sea floor spreading (Vine and Matthews, 1963)
    Magnetic reversal "stripes" are SYMMETRICAL about the ridge.

  5. Magnetic reversal time scale -
    Pattern of reversals in sea floor basalts matches known reversal time scale as determined from rocks exposed on land.
    Width of magnetic stripes on sea floor is related to time.
    (Wide stripes = long time; narrow stripes = short time)


In 1962, Harry Hess proposed the hypothesis that midocean ridges represent narrow zones where ocean crust forms.

Spreading centers

Mantle material moves upward, carrying heat.
Heat causes expansion and rising of sea floor.
Volcanism occurs (also earthquakes)

Convection in the mantle drives the system
Large scale thermal convection in the mantle.
Convection cells. Roughly circular.
Mantle heat probably due to radioactive decay

If the rising part of a convection cell is beneath a continent, it will cause it to RIFT apart.
Also causes seafloor to rift apart at mid-ocean ridge.

Continents move alont with ocean crust, and do not plow through it.


Many seemingly unrelated geologic facts are unified by the plate tectonic theory.


Basic terminology

Lithosphere is divided into plates (about 8 large plates and several smaller ones).
Brittle crust and uppermost mantle.

Asthenosphere is partially molten part of mantle. Below lithosphere.

Types of plate boundaries:

  1. Divergent - mid-ocean ridges and continental rifts
  2. Convergent - subduction zones and sites of continental collision forming mountain belts
  3. Transform - transform faults (easily seen where they cut at right angles to the mid-ocean ridges)

    Types of plate tectonic motion:


Rates and directions of plate movements vary.

All plates are moving

At subduction zones, cold lithosphere descends into the asthenosphere in deep sea trenches
Associated with volcanoes and deep-focus earthquakes (over 190 miles deep).
"Ring of Fire" - around Pacific Rim.

Island Arcs = chains of volcanoes adjacent to subduction zones.

Hot spots - thermal plumes (heat rising in mantle).
Plates move over hot spots creating a chain of volcanoes.
Hawaiian Islands, Emperor Sea Mounts

Triple Junction - where three plate boundaries meet.
Associated with divergent plate boundaries.
Rift zones
Two plate boundaries tend to be most active, and one less active (failed rift)

Regions of continental rifting have:

  1. Normal faults
  2. Mafic dikes and sills (basalt - rich in Mg and Fe)
  3. Thick continental sedimentary sequences in fault-block basins (normal faults)


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This page created by Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA

Page created April 10, 1996
Modified January 17, 1997
Modified December 16, 1998
Last modified July 17, 1999
Links updated August 15, 2009