Continental Drift and Plate Tectonics––#

CONTINENTAL DRIFT AND PLATE TECTONICS

  1. Introduction

    1. Here we examine the proposition that the earth is not fixed, but in constant motion
    2. Our acceptance or rejection of this proposition will have a profound effect on our understanding of the history of life––for life is dependent on the earth for substrate and sustenance

  2. Early views of the earth

    1. A fixist view of the earth suggests that earth structure has remained virtually unchanged since either the beginning of life or Noah’s flood
    2. The contracting earth model

      1. A popular form of this model was proposed by James Hall (19th century)
      2. Assumption––earth began as ball of hot matter that subsequently cooled
      3. Surface cooled first and contracted producing great cracks
      4. Cracks (geosynclines) were filled in by eroded sediments
      5. Interior then began to cool and shrink
      6. Shrinking interior caused surface to wrinkle creating

        1. Anticlines (arches)
        2. Synclines (troughs)

      7. Problems with contracting earth model

        1. Earth would have had to have cooled by thousands of degrees to create the magnitude of folding observed––This seemed unlikely
        2. Some mountain chains are obviously younger than others, yet are of the same general size. Differences in the times of cooling for various parts of the earth seemed unlikely

      8. By the early 20th century, the contracting earth model had lost favor among earth scientists

  3. The continental drift model

    1. Alfred Wegener (1880-1930), a German meteorologist, was aware of the problems with the contracting earth model
    2. In 1912, Wegener proposed the continental drift model

      1. The North and South American continents were once joined with Europe, Africa, Antarctica, Australia, India and Asia––a supercontinent called Pangaea
      2. During the Late Carboniferous, Pangaea began to break apart
      3. The continents reached their present positions by “drifting” into place

    3. Wegener’s evidence in support of his model

      1. East coastline of New World matches west coastline of Old World––Fit is even more impressive if continental shelves are included in match
      2. Similar rock strata and fossils on either side of the Atlantic Ocean

        1. Land snails
        2. Glossopteris (the tongue fern)

      3. Distribution of Carboniferous and Permian glacial deposits in S. South America, India, S. Australia and Antarctica (Gondwanaland)
      4. Vast coal deposits in North America which show evidence of a tropical climate
      5. Folded mountains which, according to Wegener, resulted from the collisions of continents with one another

    4. Wegener’s proposed mechanism for continental drift

      1. Wegener was well aware of how difficult it was to propose a mechanism for continental drift
      2. He hypothesized that two types of forces were involved

        1. Pole-fleeting force

          1. Rotation of earth on its axis creates a centrifugal force
          2. This force deflects somewhat the pull of gravity toward the equator
          3. Wegener suggested that this force was responsible for moving Gondwanaland to the north

        2. Tidal attraction of the sun and moon to move North and South America toward the west

      3. Scientists quickly recognized that these forces were inadequate
      4. By 1930, when Wegener died, very few people defended his model

  4. The rebirth of continental drift after World War II

    1. After World War II, there was a sustained effort by the U.S. to chart the ocean floor
    2. This exploration, combined with several other discoveries, led to a rebirth of the continental drift model
    3. By the late 1960s, virtually all geologists accepted continental drift. This was a major paradigm shift––The textbooks had to be rewritten
    4. What evidence led to this thought revolution?

      1. Seafloor spreading

        1. Since World War II research vessels with sonic depth recorders have crisscrossed the oceans, resulting in the construction of detailed maps of the ocean surface
        2. Mid-ocean ridges were found to be dominant features of the ocean floors
        3. Examples

          1. Mid-Atlantic Ridge
          2. East Pacific Rise

        4. These ridges are more than 3000 m high and 2000 km wide -- they surpass the Swiss Alps in size
        5. Terrestrial mountain ranges show evidence of being folding under pressure from both flanks
        6. By contrast, mid-ocean ridges are volcanic in origin and look as though they are under tension from the middle

          1. In 1960 it was announced that each ridge is bisected at the top by a huge trench several km wide and 1-2 km deep
          2. Temperature measurements showed that heat flow is highest at the tops of the ridges

        7. Moreover, alternating bands of rock, magnetically oriented 180o differently from flanking bands, are found on either side of the ridges

          1. Band patterns are mirror images of one another on either side of the ridge
          2. Also, there is a positive correlation between the age of a and its distance from the ridge

        8. The inescapable conclusion seems to be that the sea-floor is spreading out from ridge centers at a currently measured rate of 1-2 cm per year

      2. Paleomagnetism and polar wandering

        1. The earth is structured as if a giant bar magnet is oriented north-south within the earth
        2. The orientation today is not exactly north-south, but is off by 11 degrees
        3. Compass needles line up with magnetic field
        4. We don’t completely understand why the earth acts as a magnet, though it is probably related to its liquid metal outer core
        5. Perhaps this metal core acts as a generator, or dynamo, which generates its own electric field––a field that changes over time.
        6. What is the evidence for this view?

          1. Records have been kept for about 300 years on compass orientations
          2. Orientations have changed during this time
          3. For example, compass needle deviate 6o west of true north in Tokyo
          4. 150 years ago, compass needle deviated 3o east of true north––a difference of 9o
          5. This is called secular variation in geomagnetism

        7. Paleomagnetism

          1. Paleomagnetism is the study of the earth’s past magnetic fields
          2. How are past magnetic fields studied?

            1. Volcanic rocks are molten when they first form
            2. Each molecule in a cooling volcanic rock aligns with the earth’s magnetic field
            3. Once solidified, the molecule’s orientation is a permanent record of the earth’s magnetic field orientation at the time the rock cooled

          3. During the 1920s it was found that half the volcanic rocks in Japan and Korea were oriented with the magnetic field of today, and half were oriented in the opposite direction
          4. By the late 1950s evidence for the same thing turned up for volcanic rocks in the whole world
          5. The conclusion is that the earth’s magnetic field must have undergone reversals in times past

        8. Polar wandering

          1. If the paleomagnetic orientations of volcanic rocks of different levels of the geologic column are determined, the positions of the north and south poles when the rocks were formed can be calculated
          2. These calculations reveal that the poles have been wandering in the past
          3. The magnetic poles indicated by rocks of the same age in different continents correspond only when corrected for their presumed locations relative to one another while drifting
          4. This was seen as strong evidence for continental drift

  5. Plate tectonics and current views

    1. Structure of the earth’s crust and upper mantle

      1. Lithosphere

        1. About 70 km thick
        2. Rigid

      2. Asthenosphere

        1. About 190 km thick
        2. Partially molten

      3. Plates

        1. Twelve large blocks of the lithosphere
        2. Each plate contains

          1. Elevated continental regions
          2. Lower ocean basins

        3. The continents are high because they have deep roots

          1. The principle of isostacy states that the lower the bottom part of a buoyant object floats, the higher its upper surface will reach above the surface of the fluid
          2. Icebergs illustrate this principle

        4. Ocean basins are low because they have shallow roots.

    2. Plate movements

      1. The rigid plates move relative to one another.
      2. Three types of plate-plate boundaries occur

        1. Diverging boundary

          1. Typically occurs at mid-oceanic ridge where new plate material is being formed
          2. Plates are moving away from one another at a diverging boundary, much like diverging conveyer belts

        2. Converging boundary

          1. Creates trenches or subduction zones where one plate pushes beneath another
          2. Mountains often buckle up in response to collision (e.g., Himalayas)
        3. Transform fault

          1. Plates move relative to one another at their boundaries along the same plane
          2. The famous San Andreas fault in California is an example of this type of boundary––This is the contact point between the Pacific Plate and the North American Plate

    3. Models of driving mechanisms

      1. Mantle convection model

        1. Heat convection occurs in mantle
        2. Convection currents responsible for plate movements
        3. Appears overly simplistic––cannot alone account for plate movements

      2. Sinking-slab model

        1. Plate is formed by molten material at mid-ocean ridge
        2. The high elevation of ridge, the low elevation of the subducting portion of the plate, the mass of the plate and various other forces interact to create plate movement
        3. Driving forces (move plate away from ridge)

          1. Slab pull

            1. Subducting slab beneath trench is colder and thus denser than surrounding mantle
            2. This density difference creates negative buoyancy which pulls the slab down.
            3. This slab pull force is transferred to entire plate causing it to move away from the ridge
            4. Thought to be one of the most important driving forces

          2. Ridge push

            1. Ridge has an elevated topography
            2. The elevated topography gives the plate at the point of the ridge high potential energy
            3. This energy is transferred to the entire plate causing it to move away from the ridge

        4. Resistive forces (resist movement away from ridge)

          1. Slab resistance

            1. Resistance of mantle to subducting slab
            2. This force is proportional to the velocity of underthrusting
            3. This force is probably concentrated along the leading edge of the slab

          2. Colliding resistance

            1. Resistances between colliding plates
            2. Responsible for creating shallow earthquakes

          3. Transform fault resistance

            1. Resistances along transform faults between plates
            2. Also responsible for creating shallow earthquakes (e.g., the earthquakes along the San Andreas fault)

        5. Driving or resistive forces

          1. Mantle drag force

            1. Proportional to the area and velocity of the plate relative to the asthenosphere
            2. Can be driving or resisting depending on relative movement of asthenosphere

          2. Continental drag force

            1. This is the same as the mantle drag force, except that it refers to forces on the continental part of the plate
            2. Can, of course, also be driving or resisting depending on relative movement of asthenosphere

    4. Plate velocities

      1. Effect of continent size

        1. There is a significant correlation between pla velocity and continental area
        2. The larger the plate, the greater the mantle resistance and the lower the velocity of plate movement

      2. Effect of subducting slab size

        1. For the underthrusting plate, velocity is positively correlated with trench length
        2. This finding is what led geophysicists to think that slab pull force was very significan

    5. Plate movements and the Pacific "ring of fire"
    6. E. Plate movements and hot spots

      1. Hawaiian Islands and the Emperor Seamounts
      2. Yellowstone National Park

  6. Conclusion

    1. The theory of plate tectonics and continental drift has revolutionized the way scientists view the earth
    2. This theory explains many features of the earth's surface which could not previously be explained

      1. Mountain ranges
      2. Mid-ocean ridges
      3. Patterns of volcanism
      4. Earthquake activity
      5. The shapes of the continents
      6. Distribution patterns of organisms
      7. Island archipelagos

    3. Thus, the theory of plate tectonics and continental drift is a unifying theory in geology