Earth image Plate Tectonics

Dr. Pamela Gore
Georgia Perimeter College

Objectives

  1. Explain the basics of the theory of plate tectonics.
  2. Explain what is meant by the terms "lithosphere" and "asthenosphere", and the difference between them in terms of plate tectonics.
  3. Discuss the major evidence in support of the theory of plate tectonics.
  4. Explain what is meant by "seafloor spreading".
  5. List the three types of plate boundaries and describe the type of motion involved at each.
  6. Explain what Pangea was.
  7. Describe what convection currents are, how they form, and how they move.
  8. Explain how convection currents are related to plate tectonics.
This section addresses, in whole or in part, the following Georgia GPS standard(s):
  • S6E3b. Describe the composition, location and subsurface topography of the world's oceans.
  • S6E5d. Recognize that lithospheric plates constantly move and cause major geological events on the Earth's surface.
  • S6E5e. Explain the effects of physical processes (plate tectonics, erosion, deposition, volcanic eruptions, gravity) on geological features including oceans (composition, currents, and tides).

This section addresses, in whole or in part, the following Benchmarks for Scientific Literacy:
  • The interior of the earth is hot. Heat flow and movement of material within the earth cause earthquakes and volcanic eruptions and create mountains and ocean basins. Gas and dust from large volcanoes can change the atmosphere.
  • Some changes in the earth's surface are abrupt (such as earthquakes and volcanic eruptions) while other changes happen very slowly (such as uplift and wearing down of mountains). The earth's surface is shaped in part by the motion of water and wind over very long times, which act to level mountain ranges.
  • The solid crust of the earth-including both the continents and the ocean basins-consists of separate plates that ride on a denser, hot, gradually deformable layer of the earth. The crust sections move very slowly, pressing against one another in some places, pulling apart in other places. Ocean-floor plates may slide under continental plates, sinking deep into the earth. The surface layers of these plates may fold, forming mountain ranges.
  • Earthquakes often occur along the boundaries between colliding plates, and molten rock from below creates pressure that is released by volcanic eruptions, helping to build up mountains. Under the ocean basins, molten rock may well up between separating plates to create new ocean floor. Volcanic activity along the ocean floor may form undersea mountains, which can thrust above the ocean's surface to become islands.

This section addresses, in whole or in part, the following National Science Education Standards:
  • Lithospheric plates on the scales of continents and oceans constantly move at rates of centimeters per year in response to movements in the mantle. Major geological events, such as earthquakes, volcanic eruptions, and mountain building, result from these plate motions.
  • The earth processes we see today, including erosion, movement of lithospheric plates, and changes in atmospheric composition, are similar to those that occurred in the past. earth history is also influenced by occasional catastrophes, such as the impact of an asteroid or comet.

The Theory of Plate Tectonics

Discoveries leading to the plate tectonic theory came in part from research associated with the International Geophysical Year (or IGY) (July 1957-December 1958), when scientists from 67 nations around the world worked together to study various geophysical phenomena in fields such as cosmic rays, geomagnetism, glaciology, gravity, longitude and latitude determination, meteorology, oceanography, rocketry, and seismology. (One of the goals of the research was to launch the first artificial satellite into orbit around the Earth.) One of the most significant achievements of the IGY (in terms of geology) was the mapping of large parts of the ocean floor and delineating the mid-ocean ridge system with its central valley.


Image of the surface of the Earth, derived from a database of land and sea-floor elevations. World view Mercator projection. Click on image for a larger version. Click here for a selection of other views of sea floor topography, including on a globe in various orientations. Image from National Oceanic and Atmospheric Administration (NOAA).

The theory of plate tectonics has revolutionized the understanding of geology. Many seemingly unrelated geologic facts have been found to be interrelated as a result of plate tectonic theory. These include:

  1. The distribution of volcanoes along the edges of continents and around the rim of the Pacific Ocean (Ring of Fire)
  2. The association of deep ocean trenches with volcanic mountain chains,
  3. The presence of a huge undersea mountain range with a central valley, which encircles the globe (see image above),
  4. The geographic distribution of mountain ranges (and their various ages and types of deformation),
  5. The geographic distribution of earthquakes, which occur in lines, and deep earthquakes which occur along inclined planes,
  6. The geographic distribution of certain types of fossils
  7. The distribution of certain types of sedimentary rocks which can be used as paleoclimate (ancient climate) indicators,
  8. The age of the oceanic crust
  9. Sediment thickness distribution patterns in the ocean basins.

Plate tectonics is a unifying theory which helps us understand the underlying causes of the major topographic features of the Earth, as well as the reasons why some areas of the world are frequently devastated by earthquakes and volcanic eruptions.

According to the theory of plate tectonics, the earth's surface (the rigid rocky layer of the lithosphere) is broken or divided into about a dozen "plates" that are moving relative to one another. These plates ride atop a part of the Earth's mantle that is hot, dense and partially molten (but not liquid). This part of the mantle is called the asthenosphere, and it flows with a type of movement called convection.

map of the Earth's tectonic plates
Map of the Earth's tectonic plates from the US Geological Survey.
Image courtesy of U. S. Geological Survey.

The lithosphere consists of the Earth's crust and part of the uppermost mantle. The Earth's surface or lithosphere is divided into about 7 large plates and 20 smaller ones.

Asthenosphere is a partially molten part of the mantle, below the lithosphere.

Two types of crust are present in the lithospheric plates:

  1. Thin (5 - 7 km), dense (3.0 g/cm3), basaltic oceanic crust (dark, fine-grained igneous rock)
  2. Thick (35 - 40 km, ranging to 60-70 km in some mountain ranges), low density (2.7 g/cm3), granitic continental crust (light-colored, coarse-grained igneous rock)

The tectonic plates move very slowly relative to one another. The rates and directions of plate movements vary. The rate of movement has been determined to be approximately 5 - 10 cm per year (2 - 6 inches per year), depending on location.

All plates are moving. As plate movement occurs, the plates sometimes stick together and then slip. This sudden slippage causes vibrations known as earthquakes.

Tectonic plate boundaries tend to be sites of relatively intense geologic activity. Earthquakes and volcanic eruptions occur predominantly along plate boundaries. The interiors of plates tend to be less geologically active than the boundaries.

Types of plate boundaries

There are three major types of tectonic plate boundaries. These include:

  1. Divergent plate boundaries where plates move apart from one another.
  2. Convergent plate boundaries where plates move toward one another.
  3. Transform plate boundaries where plates slide past one another.


Artist's cross section illustrating the main types of plate boundaries. Cross section by Josť F. Vigil from This Dynamic Planet -- a wall map produced jointly by the U.S. Geological Survey, the Smithsonian Institution, and the U.S. Naval Research Laboratory. Image courtesy of U. S. Geological Survey.

  1. Divergent - where the plates are moving apart.


    Animation of divergent plate motion at a mid-ocean ridge. Courtesy of U. S. Geological Survey..

    Examples: mid-ocean ridges such as the Mid-Atlantic Ridge (the site of sea-floor spreading), and continental rifts such as the east African Rift system.

    Animation of divergent plate motion.
    Animation of divergent plate motion.

  2. Convergent - where the plates are moving toward one another.


    Example of a convergent plate boundary. In this case, ocean-to-continent convergence is shown. The deep sea trench is the site of a subduction zone, where oceanic crust is being carried down into the mantle, where it begins to melt. The magma rises to form volcanoes along the edge of the continent. Image courtesy of U.S. Geological Survey.


    Example of a convergent plate boundary. In this case, ocean-to-ocean convergence is shown. The deep sea trench is the site of a subduction zone, where oceanic crust is being carried down into the mantle, where it begins to melt. The magma rises to form a volcanic island arc. Image courtesy of U.S. Geological Survey.


    Example of a convergent plate boundary. In this case, continent-to-continent convergence is shown. Continental crust collides with and slides over other continental crust, forming high mountain ranges like the Himalayas. Image courtesy of U.S. Geological Survey.

    Examples: subduction zones which occur at deep sea trenches such as the Marianas Trench, and sites of continental collision forming mountain belts, such as the Himalaya Mountains, the Ural Mountains, the Appalachian Mountains, and the Alps.

    Animation of convergent plate motion.
    Animation of convergent plate motion.

  3. Transform - where the plates are sliding past one another, such as one sliding to the north and the adjacent plate sliding to the south.

    Examples: transform faults (easily seen where they cut at right angles to the mid-ocean ridges); includes the San Andreas fault.

     Animation of transform plate motion.
    Animation of transform plate motion.


Evidence in support of the Theory of Plate Tectonics:

  1. Shape of the coastlines - Africa and South America would fit together like jigsaw puzzle pieces.

  2. Fossil evidence
    1. Glossopteris flora - a type of Late Paleozoic seed ferns (plant fossils) that were found in Gondwanaland (India, Africa, Australia, S. America, Antarctica)
    2. Mesosaurus, a freshwater aquatic reptile whose fossils were found in South America and Africa


    The locations of certain fossil plants and animals on present-day, widely separated continents form definite patterns (shown by the bands of colors), if the continents are put back together. Click image for a larger version. Diagram courtesy U.S. Geological Survey.

  3. Rift Valleys of Africa - (evidence for a continent breaking up)

    East African Rift Valley
    Map of the East African Rift Valley showing plate boundaries (solid black lines), rift zones (dashed black lines), and volcanoes (red triangles). Several deep lakes are present in the rift valleys. Saudi Arabia (the Arabian Plate) has rifted away from the African Plate, forming a rift valley which has been flooded by the Red Sea. The African and Arabian Plates meet in a "triple junction", where the Red Sea meets the Gulf of Aden. This area (shaded orange) is called the Afar Triangle. A new spreading center appears to be developing along the East African Rift Valley. As rifting occurs, normal faults or tensional cracks form. Magma rises from below into the cracks or faults, and in places erupts onto the earth's surface forming volcanoes. Image courtesy of U. S. Geological Survey.

  4. Geologic similarities between S. America and Africa
    1. Same stratigraphic sequence (i.e. same sequence of types of layered sedimentary rocks)
    2. Mountain belts and folded rocks would line up if you could push the continents back together

  5. Paleoclimatic evidence (Paleo = "ancient", climatic = "climate")
    If you could push the continents back together, the ancient climatic zones, as indicated by the rock types, would 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.

    Glaciers start to form on continents from the buildup of snow. As they grow through snow accumulation, they begin to move outwards. They do not form in the sea and move onto the land.

    Directions of glacial striations on the Gondwanaland continents only make sense if you could push the continents back together to form a single landmass in the past.
    The directions of glacial movement would not make sense in the absence of plate tectonics.

  6. Youth of ocean basins and sea floor.
    Before the seafloor rocks were sampled and dated, the hypothesis is that they would be very old and would be covered by thick accumulations of sediment. The deep sea drilling efforts have revealed that the basaltic rocks of the seafloor are actually quite young. Seafloor basalt dates to less than 200 million years (most is younger than 150 million years).
    The seafloor is much younger than we expected. This is because new seafloor is continually forming at the mid-ocean ridges and spreading outward away from the ridge in conveyor belt fashion. The Earth is not expanding. Older seafloor rocks have been subducted, or carried down into the Earth's mantle in deep sea trenches and melted.


    The age of the oceanic crust is shown by colors. The youngest ocean crust is shown in red along the mid-ocean ridges (0 to 9.6 million years old), and increasingly older crust is shown in orange, yellow, green, and blue. The areas of the oldest ocean crust are shown in blue (156.6 to 180.0 million years), and are located farthest from the ridges, such as along the eastern coast of North America, and the northwestern coast of Africa, and in part of the western Pacific Ocean. Click on image to seer a largerversion. Image courtesy National Oceanographic and Atmospheric Administration (NOAA).

  7. Sediment thickness is greatest along the edges of continents.

    When deep sea drilling began, scientists expected to see a complete record of sediment deposition over hundreds of millions of years or more. Contrary to expectations, they found that there was only a thin layer of sediment on the ocean floors, and that the ocean crust beneath was quite young.


    Total sediment thickness of the world's oceans. Click on image for a larger version. Image courtesy National Oceanographic and Atmospheric Administration (NOAA).

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


    Map of part of the Pacific basin showing the volcanic trail of the Hawaiian hotspot-- 6,000-km-long Hawaiian Ridge-Emperor Seamounts chain. (Base map reprinted by permission from World Ocean Floor by Bruce C. Heezen and Marie Tharp, Copyright 1977.) From "This Dynamic Earth - The Story of Plate Tectonics", U. S. Geological Survey.


    Map of world hot spots. Image courtesy of U.S. Geological Survey.

    Vigorous scientific debate has ensued regarding volcanism at "hotspots" in recent years. New studies suggest that hotspots are neither deep phenomena nor "fixed" in position over geologic time, as assumed in the popular plume model. See http://www.mantleplumes.org.

  9. Evidence for subsidence in oceans - Guyots - flat-topped sea mounts (erosion when at or above sea level)
  10. 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

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

    A Benioff Zone (sometimes called a Wadati-Benioff Zone) is an inclined zone or plane of earthquake foci near deep sea trenches. In the 1940's, Hugo Benioff of the California Institute of Technology, studied earthquake patterns in the Pacific Ocean and concluded that the deep ocean trenches were associated with faults. Earthquakes along these faults fall into three groups - shallow (less than 70 km deep), intermediate (70 to 250 or 300 km) and deep (300-600 km), as seen on the map above. The earthquake foci are shallowest nearest the trench, and become deeper in a landward direction (or toward a volcanic island arc). The earthquake foci plot along a dipping plane at an angle of 33 to 60 degrees. In plate tectonic theory, the faults indicated by the Benioff Zones are interpreted as subduction zones. In subduction zones, cold lithosphere descends into the asthenosphere along the faults associated with the deep sea trenches. Earthquakes are caused as one plate slides beneath the other.


    Benioff zone A dipping planar (flat) zone of earthquakes that is produced by the interaction of a downgoing oceanic crustal plate with a continental plate. These earthquakes can be produced by slip along the subduction thrust fault or by slip on faults within the downgoing plate as a result of bending and extension as the plate is pulled into the mantle. Also known as the Wadati-Benioff zone.

  12. Magnetic stripes on the sea floor - One of the chief lines of evidence for seafloor spreading comes from the examination of patterns of ancient magnetic reversals that are recorded in the basalts of the oceanic crust. (See below.)


Contributions to plate tectonic theory from paleomagnetism

Igneous rocks, such as seafloor basalts, become magnetized in alignment with the Earth's current magnetic field as they cool through a specific temperature known as the Curie point. Magnetization in older rocks has a different orientation because older rocks cooled and crystallized when the Earth's magnetic poles were reversed, and/or in a different location. (The magnetic poles are in slow but continual motion. This is known as polar wandering.) It is possible to determine the direction and distance to the north magnetic pole from the inclination and declination of the magnetic field preserved in the rock. Ancient rocks preserve the past positions of the north magnetic pole.


The Earth acts like a great spherical magnet, in that it is surrounded by a magnetic field. The Earth's magnetic field resembles, in general, the field generated by a dipole magnet (i.e., a straight magnet with a north and south pole) located at the center of the Earth. The axis of the dipole is offset from the axis of the Earth's rotation by approximately 11 degrees. This means that the north and south geographic poles and the north and south magnetic poles are not located in the same place. Image courtesy of NOAA.

  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


    Animation courtesy of U.S. Geological Survey.

  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.


    Animation courtesy of U.S. Geological Survey.

  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)


What forces drive plate tectonics?

In 1962, Harry Hess proposed the hypothesis that midocean ridges represent narrow zones where ocean crust forms (called 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 along with ocean crust, and do not plow through it.

Plate tectonics is driven by the convection in the asthenosphere (part of the Earth's mantle).


Conceptual drawing of assumed convection cells in the mantle. Below a depth of about 700 km, the descending slab begins to soften and flow, losing its form. Image courtesy of U.S. Geological Survey.


Sketch showing convection cells commonly seen in boiling water or soup. This analogy, however, does not take into account the huge differences in the size and the flow rates of these cells.


Pangea

Pangea was a supercontinent which existed during the Permian Period about 225 million years ago.

Diagram of five maps of the Earth showing Pangea and the positions of the continents as they split apart over time
Diagram of five maps of the Earth showing Pangea and the positions of the continents as they split apart over time, from the U.S. Geological Survey. According to the continental drift theory, the supercontinent Pangaea began to break up about 225-200 million years ago, eventually fragmenting into the continents as we know them today. Continental drift was the forerunner of the theory of plate tectonics.


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

Page created March 9, 2005
Image links updated January 18, 2007
Image links updated June 12, 2008