Assignments

1. Understanding Earth:
   Chapter 2 Plate Tectonics: The Unifying Theory
   Chapter 3: Earth Materials pp. 66-69 (Rock Cycle)
   Chapter 10: Evolution of Continents
2. Earth Revealed: Courtesy of Anneberg Media, URL <http://www.learner.org/resources/series78.html>.  Requires Windows media Player. Sign in and go view #5 Birth of a Theory, and #6 Plate Dynamics.
3. University of California Museum of Paleontology (USMP) > Plate Tectonics / The History Behind Plate Tectonics / Alfred Wegner / The Mechanism

Terms: convection, fault, subduction, seafloor spreading, divergent, convergent, transform, fault, sedimentary, metamorphic, igneous, oceanic crust, continental crust, slab pull, ridge push, lithospheric plate, rock cycle, wilson cycle, mid-oceanic ridge, trench, rift valley, orogeny

Plate tectonics is the most important paradigm in geology. It is the unifying theory that relates the formation of mountains, volcanoes, earthquakes, the differences between the ocean basins and continents to a understandable and predictable global process.

The Theory of Plate Tectonics beautifully illustrates evolution of a theory through scientific method.  Alfred Wegner's hypothesis of Continental Drift was the seed from which the theory evolved.  Unfortunately for Wegner his original premise that tidal and Coriolis forces drove the movement of continents was known to be impossible, and the technology of the time was insufficent to modify the hypothesis to the next step.  The development and application of sonar during World War II, used to map the sea floor, was the first step in the resurection of his hypothesis and development in the modern theory of Plate Tectonics.

Plate Tectonics: Exercise 1 From the lecture, text, and Internet sites given below answer the following questions: How does Wegner's hypothesis of Continental Drift differ from the theory of Plate Tectonics? What was Wegner's evidence and why was his hypothesis dissmissed? What evidence do we now have that was unavailable to Wegner supporting the theory of Plate Tectonics?

Additional Internet Sites: Alfred Wegner, On the shoulder of Giants (Nasa Earth Observatory) <http://earthobservatory.nasa.gov/Library/Giants/Wegener/>. Alfred Wegner's hypothesis of Continental Drift was the forerunner to the Theory of Plate Tectonics. You need to know about this guy! On the Move...Continental Drift and Plate Tectonics: Evidence Supporting Continental Drift. <http://kids.earth.nasa.gov/archive/pangaea/evidence.html> USGS Dynamic Earth <http://pubs.usgs.gov/publications/text/dynamic.html#anchor10790904> Historical Perspectives and Developing the TheoryHarry Hess: Spreading the Sea Floor <http://pubs.usgs.gov/publications/text/HHH.html> Earth Revealed Movie: The Birth of a Theory. <http://www.learner.org/resources/series78.html> Requires Windows media Player. Sign in and go to #5.

The Theory of Plate Tectonics

Plate tectonics is the theory that the earth's rigid lithosphere is broken into approximately 15 moving plates that float on the denser, plastic asthenosphere.   Motion is accommodated by the creation and destruction of lithosphere along zones of subduction and sea-floor spreading respectively. 

Deformation occurs along the boundaries where the plates interact.  Where they meet the plates grind and push building up stress.  Eventually the rocks rupture producing earthquakes.  Therefore from earthquakes (seismicity) geologists can interpret not only the location of boundaries, but the motion that occurs along them.   Plate motion also drives processes that generate melting. So volcanic features are typically concentrated along plate boundaries. 

World seismicity map containing earthquakes occurring 9/6-9/11 2000. Courtesy of the USGS National Earthquake Information Center (NEIC). <http://earthquake.usgs.gov/regional/world/seismicity/index.php> Color scale indicates depth of earthquakes.  Earthquakes along subduction zones form broad well-defined belts and range from shallow, near the trench, to deep, beneath the over-riding plate.  Earthquakes along oceanic divergent boundaries are shallow and occupy very narrow linear trends.  Broad diffuse belts of earthquakes occur along continental-continental convergent zones and splintering zones of continental rifting.  Can you identify areas of subduction and sea-floor spreading?

Significance of Plate Tectonics

Plate tectonics explains the origin and distribution of:

1. most earthquakes

   Can you label the plates on this map and determine from the distribution of earthquakes which boundaries are divergent and which are convergent?

   See the most recent events from the National Earthquake Information Center

2. most volcanoes, with the exception of hotspots

   Map of the distribution of volcanoes (Michigan Technological University Volcanoes Page): Compare this map with the seismic map above. How are they similar? Why?

   The ring of fire was observed long before plate tectonics became a theory.  Now we know that the volcanic activity around the Pacific Ocean is caused by the subduction of the Nazca, Pacific, Cocos, Juan de Fuca, and Philippine plates.

3. major physiographic features, such as mountain belts, deep sea trenches, oceanic ridges and rises, and continental rift valleys. In fact the occurrence of these features enable us to locate and distinguish plate boundaries on a physiographic map.

   World Ocean Floors (Bruce C. Heezen and Marie Tharp): Interactive version presented by platetectonics.com.  Explore this map to learn how many of the earth's physiographic features were formed.

Indirectly plate tectonics also plays a major role in determining:

• size and flow direction of large drainage systems
• wind currents and weather patterns
• width of continental shelves, which influences coastal
  • wave height
  • tidal range

Some modern Evidence for Plate Tectonics:

• seismology: distribution of earthquakes, movement, and seismic tomography
• paleomagnetism: polar wandering and polarity reversals
• GPS: measures real-time movement of plate motion

Lithospheric Plate Boundaries

A single lithospheric plate may contain oceanic or continental crust, or both oceanic and continental crust together. The latter case is most common. For example, the large Antarctic, African, Eurasian, North and South American plates are all composed of both oceanic and continental crust. Plate boundaries may underlie ocean basins and cut through continents. In some instances, such as the boundary between the Nazca and South American plates a boundary my lie along the border of a continent and ocean basin.

Plate boundaries are classified by:

• the motion along the boundary (divergent, convergent, transform), and
• the type of crust in contact along the boundary (oceanic or continental)

Types of boundaries

1. Convergent

a. oceanic-oceanic convergent boundary

Features: subduction zone, trench, accretionary prism, volcanic island arc

b. oceanic-continental convergent boundary

Features: subduction zone, trench, accretionary prism, volcanic arc

c. continental-continental convergent boundary

Features: Alpine mountains

Review descriptive cross section of plate boundaries from the USGS Dynamic Earth. What is occurring along each boundary?  What are the feature produced? If you were asked to draw a cross-section of a boundary could you do it?

2. Divergent

a. oceanic-ocean divergent boundary - region of sea-floor spreading

Features: Oceanic ridges and rises with axial rift valleys, small transform faults

b. continental- continental divergent boundary

Features: continental rift, incipient ocean basin

3. Transform

• Features: Long transform fault
• crust is neither created or destroyed

Internet Sites to Explore:

• Christopher Scotese's Paleomap Project: Earth History Explore the earth's past, present and future.
• Introduction to submarine volcanoes (NODAK VolcanoWorld) Excellent illustrated summary of volcanoes and their plate tectonic setting. Includes a good map of plate boundaries.
• Plate Tectonics: General Index of Plate Tectonic websites.
• Downloadable PDF map from the USGS Dynamic Planet showing the Earth's physiographic features, the current movements of its major tectonic plates, and the locations of its volcanoes, earthquakes, and impact craters.

Introduction to rocks and the rock cycle in the context of Plate Tectonics

Three Categories of rocks: (rock classifications within each category are based on texture and composition)

1. Igneous
2. Sedimentary
3. Metamorphic

The Rock cycle (Wikipedia)

Concepts: Any rock can be produced from any other preexisting rock as a result of

• Sedimentary processes: Weathering, erosion, deposition and lithification
• Igneous processes: Melting and solidification, or
• Metamorphic processes: Heating (without melting) and increasing pressure

Image from Overview of geologic fundamentals, USGS 3D parks, URL: http://3dparks.wr.usgs.gov/nyc/common/geologicbasics.htm

Plate tectonics plays a major roll in recycling rocks. Through sea-floor spreading and subduction the ocean floor is recycled over a period of approximately 180 million years.  Continental rocks are recycled through collision driven deformation and uplift, erosion, and deposition occurring on scales ranging form a few tens of millions of years to a billion years or more.

The Wilson Cycle is a theory put forth a Canadian Geologist Tuzo Wilson who believed in the cyclical nature of ocean basin formation and destruction.  After visiting the sites listed below try to answer the following questions.

How are the Wilson and rock cycles similar? How did they differ?
What are the processes involved in the recycling of oceanic and continental crust?

Internet sites on the Wilson Cycle: Circular Wilson Cycle and One Page Wilson Cycle from The Wilson Cycle by Lynn Fichter, James Madison University.

Additional Online Resources

The Dynamic Earth - The story of Plate Tectonics, Kios and Tilling, United States Geological Survey <http://pubs.usgs.gov/gip/dynamic/dynamic.html>

The Engine that Drives Earth--Peering into the mantle to reveal the inner working of our planet, 2004, Robert Detrick, Senior Scientist, Geology and Geophysics Department, Oceanus, Woods Hole Oceanographic Institution (pdf)