Plate Tectonics

1. Plate Tectonics Dr. R. B. Schultz

2. Plate Tectonics Without question, the theory of “plate tectonics” is the most important advancement in earth sciences in the 20th century. It provides the framework for earth processes that previously were known to exist, but it was unknown why these activities occurred. Because the portions of the earth's interior and differences between continental and oceanic crust are an essential part of plate tectonics, it is worth our while to review these concepts briefly: Earth's crust (lithosphere) is composed of several elements crucial to our existence. In order of their abundance, these eight (8) elements are: 1.    Oxygen 2.    Silicon 3.    Aluminum 4.    Iron 5.    Calcium 6.    Sodium 7.    Potassium 8.    Magnesium

3. Lithosphere and the solid earth: the solid earth lies beneath the atmosphere and the oceans and composes 29% of the earth's surface. It is divided into several distinct units or layers: a.      Lithosphere or crust: two (2) types of crust: oceanic and continental with basic differences *Oceanic crust is thinner and denser and usually darker in color *Continental crust is lighter in weight, less dense, light in color, and tends to float over oceanic crust b.     Mantle: beneath the crust; houses molten rock material called magma c.      Outer core: composed of liquid iron and nickel; very dense material d.      Inner core: composed of solid iron and nickel; extremely dense material The upper mantle and lower crust (lithosphere) are referred to as theasthenosphere. *There is a distinct seismic discontinuity where seismic waves slow down considerably due to the composition of molten rock. This is located in the asthenosphere and is referred to as the Mohorovicic Discontinuity, after the Russian scientist who discovered it. We refer to it as the "Moho".

4. History of Events Leading up to the Formulation of the Theory of Plate Tectonics *Note that plate tectonics is a theory. It is not something that we can directly sample or touch, or for that matter prove. That is why we will refer to it as a theory. *In 1915, a Bavarian scientist named Alfred Wegener (later referred to as the "Father of Plate Tectonics") noticed, while working near the North Pole, that his compass needle did not point to where north "should" have been. In other words, true north and magnetic north were in two separate localities. Wegener theorized that the poles (both North and South) were "wandering" with time. He called this "Polar Wandering". *Subsequent to his theory, he began to also notice how continents fit together like a jigsaw puzzle…most notably the western coast of Africa and the eastern coast of South America. In addition, rocks from these localities were the same rock type, same age, and contained the same age and type of fossils. His revised theory became known as "Continental Drift", because he realized that it was not the poles that shifted, but the continents themselves. *Wegener died of a heart attack on a voyage studying glaciers near the North Pole in early 1930 and his work was virtually forgotten for several decades.

5. Correlation of Africa and South America by Wegener • Evidence used by Wegener: • Shape of continents fit like a jigsaw puzzle. • Similar fossils on both continents • Mountain belts line up • Mineral belts line up

6. Further evidence used by Wegener to support continental drift hypothesis: • Mountains line up in Northern Hemisphere • North America, Europe, South America, and Africa all fit together.

7. Explanation by Wegener that present-day Africa, South America, India, and Australia were all once glaciated.

8. *It was not until the advent of World War II that a technology (Echo Sounding) developed to a degree that a stunning discovery was made by a geologist and seaboat commander, Harry Hess. He noticed that rocks on either side of a prominent geologic feature in the middle of the Atlantic Ocean (called the mid-oceanic rift zone) were a perfect mirror image of each other on either side of the rift zone. He theorized that the rift zone was oozing out magma material from submarine volcanoes and that the material spread laterally across either sides of the rift. As time progressed, Hess took more and more samples to back up his findings, as part of a series of drill voyages aboard the research vessel, Glomar Challenger. *Later in the 1960’s, Frederick Vine and Drummond Matthews, a Cambridge University professor/student team, discovered the principles of magnetic patterns on the ocean floor and went on the road to present their discoveries. They published first in a rather obscure journal and then on the lecture circuit at Scripps Oceanographic Institute. Further findings revealed that not only did the stripes have the same age of rocks, but a magnetic polarity image resulted as well, showing that during earth history there have been several "magnetic reversals" (A time when the compass needle would have pointed south instead of north). *In the late 1960's and early 1970's, two scientists, revisited Wegener's findings and combined them with Hess' discoveries to formulate a new package called "Plate Tectonics". Robert Palmer and Donald Mackenzie are credited with naming and synthesizing the theory of “plate tectonics”.

9. Echo Sounding Device Used By Hess

11. The Dynamics of Plate Tectonics Here, we will learn HOW and WHY plates move and what happens when they collide or pull apart from each other.

12.  The theory of plate tectonics is that rigid lithospheric plates move across the surface of the earth. There are approximately 12 major and 8 minor plates that move in concert with each other. Some pull apart, some push together and some move horizontally against each other. The plate motion is driven by one or more of the following mechanisms: 1. Convection -- heat transferred by movement of a fluid (magma) 2. Conduction -- heat transfer by touching plates 3. Push-Pull Slab -- heavy slabs pull plates downward and magma forced upward pushes plates to the surface (upwelling) *There are several geological processes that occur where plates meet (called plateboundaries or margins): 1.      Volcanoes tend to erupt at plate margins as a result of a process called subduction 2.     Earthquakes occur where plates grind against or over one other 3.     Mountain building occurs as one plate is pushed over another 4.     Seafloor spreading occurs where two oceanic plates pull apart There are three (3) major types of plate boundaries (margins): 1.      Convergent -- plates move towards each other (compression) 2.     Divergent-- plates move away from each other (tension) 3.     Transform-- plates horizontally grind against one another (strike-slip motion)

14. Types of Plate Boundaries: Divergent Convergent Transform

15. Oceanic spreading center with convection of magma occurring in the mantle.

16. Convection Cell Development in Mantle A. Volcanic Arc B. Oceanic Rift Zone C. Transform Fault Zone

17. Volcanic Zones Depicting Plate Boundaries

18. Earthquake Zones in Relation to Plate Boundaries

19. Subducting Plate

20. “Hot Spot” Volcanism

21. *Plate boundaries can occur on landmasses (continents) or in marine settings (oceans) or both at the same time. Convergent plate movement is associated with the following: a.    Compression b.    Reverse faulting c.    Creation of a subduction zone d.    Mountain building processes e.    Collisions of plates: • 1.   Continent vs. continent • 2.   Continent vs. oceanic • 3.   Oceanic vs. oceanic Divergent plate boundaries are associated with the following: • a.      Tension or extension (pulling apart) • b. Normal faulting • c. Rifting (as in the mid-oceanic rift zone) • d. Creation of magma material inside the rift zone Transform boundaries are associated with the following: • a.      Horizontal grinding motion • b.     Strike-slip faulting • c.      Lateral offset of rock units

22. Collision Zones: Continent vs. Oceanic Oceanic vs. Oceanic Continental vs. Continental

23. Continent vs. Oceanic Collision Zone

25. Example of Continent vs. Continent Collision Zone: India vs. Asia

27. Volcanic Zones (both continental and oceanic in origin) associated with Plate Tectonics are located: 1.       Subduction zones a.      Continent vs. ocean collision (ex.: Andes Mts., Pacific NW of U.S Ocean vs. ocean collision (ex.: Japan, Philippines); Basaltic rocks 2. Rift Zones (Spreading centers) are located: a.      Ocean - ocean divergent zones (ex.: mid-oceanic rift); Basaltic rocks b.     Continental rift zone (ex.: East African Rift Zone); Granitic rocks 3. "Hot Spot" Volcanism is located: a.      Oceanic; (ex.: Hawaiian Islands chain); Basaltic rocks b.     Continental; (ex.: Yellowstone Nat. Park); Granitic/Andesitic rocks Seismic (Earthquake) Zones associated with Plate Tectonics: 1.       Subducting oceanic plate; shallow focus as plate subducts 2.     Intermediate focus earthquakes; partial melting and rising of magma; in the "Benioff Zone" 3.     Deep focus as slab of crust is pulled by sheer gravity

28. East African Rift Zone

29. Features/Landforms Associated with Plate Tectonics 1.       1. Continent vs. Oceanic collision zone: subduction zone, deep sea trench associated, volcanic arc, andesitic volcanic rock. 2.     2. Continent vs. Continent collision zone: Granitic rocks, mountain building processes, no volcanism, no magmatic activities. 3. Oceanic vs. Oceanic collision zone: deep sea trench associated, volcanic island arc, basaltic volcanic rock. 4. Divergent zone: Oceanic, basaltic magma, spreading center (Mid-Ocean Ridge). Also, Continental: granitic, (East African Rift Zone)

34. Plate Tectonics Summary Continental vs. Oceanic crust Continental crust = lighter, thicker, less dense (Al, K, Si), Granitic Oceanic crust = darker, thinner, denser (Fe, Mg, Ca) Basaltic Layers of the Earth Lithosphere/crust = outermost Mantle houses magma (molten rock) Outer core is liquid iron and nickel Inner core is solid iron and nickel Evolutionary history of Plate Tectonics Development Alfred Wegener - polar wandering and continental drift (1912-1930) Harry Hess - seafloor spreading (late 1950's and early 1960's) Frederick Vine and Drummond Matthews - magnetic polarities, subduction (1960's) Robert Palmer and Donald McKenzie - synthesizers of previous ideas, named "plate tectonics" Plate Dynamics 12-15 major lithospheric plates; includes both oceanic and continental crust Three types of plate boundaries or margins: Convergent: compression Divergent: tension/extension Transform: strike-slip horizontal motion Occurrences/activities at plate boundaries Earthquakes Volcanoes Mountain building Seafloor spreading

35. Convergent Plate Boundaries Continental vs. Oceanic crust: Cascade Mts. (Pacific NW) and Andes Mts. (Western S. America) Subduction zone with deep sea trench Volcanic arc with Andesitic volcanoes Oceanic vs., Oceanic crust Volcanic island arc with basaltic volcanoes Deep sea trench with NO subduction zone Continental vs. Continental crust Granitic mountains but NO volcanoes Strong earthquakes as in India vs. Asia collision Divergent plate boundaries Mid-oceanic ridge (rift zone) with basaltic pillow lava volcanoes East African rift zone with granitic rock Transform boundaries Ridge-ridge faults - in oceanic setting Strike-slip faults - continental setting (San Andreas Fault in California) Earthquakes in convergent zones Shallow focus near deep sea trench Intermediate focus where oceanic plate melts (Benioff Zone) Deep focus - never deeper than 500 miles (700 km) plate completely melted

36. Volcanoes Andesitic - in subduction zones Basaltic - ocean vs. ocean collision zones and mid-oceanic ridge "Hot Spot" - Hawaiian Islands chain in ocean setting; Yellowstone National Park in continental setting Causes for Plate Movement Convection -cells of magma (lava lamp type movement) - most probable cause Conduction - transference of heat from one plate to another Push-pull slab - especially at leading edge of subduction zones and mid-oceanic ridges Miscellaneous Gondwanaland - S. America, Africa, Antarctica, and Australia bunched at S. Pole 300+ million years ago Pangaea: "Super continent" that split apart 200 million years ago; next "Pangaea" in 250 million years from now

37. Key Terminology Oceanic crust Continental crust Lithosphere Asthenosphere Mantle Magma Inner Core Outer Core Mohorovicic Discontinuity (Moho) Polar Wandering Continental Drift Alfred Wegener Harry Hess Seafloor Spreading Echo Sounding Mid-Oceanic Ridge (Rift) Zone Glomar Challenger Magnetic Reversal Frederick Vine Drummond Matthews Robert Palmer Donald MacKenzie Plate Tectonics Convection Conduction Push-Pull Slab Convergent Divergent Transform Plate Boundary (Margin) Subduction Zone Deep Sea Trench Volcanic Arc Volcanic Island Arc “Hot Spot” Volcanism Collision Zones Benioff Zone East African Rift Zone Gondwanaland Pangaea

38. Pertinent Web Sites Active Tectonics Web Server The Active Tectonics Web Server was established to effectively disseminate ideas resulting from the Active Tectonics initiative. American Geophysical Union (AGU) For over 75 years, American Geophysical Union (AGU) researchers, teachers, and science administrators have dedicated themselves to advancing the understanding of the Earth and its environment in space and making the results available to the public. Ask a Geologist (USGS) Have a geological question? Here is your chance to ask it. Dynamic Earth Tutorial (USGS) An excellent online, interactive plate tectonics tutorial from the United States Geological Survey. Global Earth History The Global Earth history site uses a series of plate-tectonic reconstructions to show the broad patterns of Phanerozoic Earth history. Hawaii Undersea Geo-Observatory (HUGO) The Hawaii Undersea Geo-Observatory (HUGO) is an automated submarine volcano observatory installed on the summit of the undersea Loihi seamount and connected to the shore via fiber optic cable. Hot Spot Activity (Geosphere) An excellent interactive activity on hot spots from the University of Montana's Geosphere. With this activity learners study mantle convection and utilize image processing techniques to determine sizes of geologic formations of Hawaii and Yellowstone National Park from remote sensing data. (The site may be slow to connect, but its usually there.) JOIDES Resolution Page The JOIDES Resolution is the drillship of the Ocean Drilling Program. Map of Plate Boundaries (NEIS) Map of plate boundaries from the National Earthquake Information Center. Marine Geology & Geophysics (NGDC) The Marine Geology & Geophysics Division of the NOAA National Geophysical Data Center (NGDC) and the collocated World Data Center A for Marine Geology and Geophysics (WDC-A for MGG) compiles and maintains extensive databases in both coastal and open ocean areas. Moving Plates Activity (Geosphere) An excellent interactive activity on calculating plate motion from the University of Montana's Geosphere. With this activity learners use numeric representations to identify the positions and perform absolute/relative plate motion calculations using Internet resources. Nordic Volcanological Institute This site contains the geology and tectonics of Iceland. Ocean Drilling Program The Ocean Drilling Program home page with links to related agencies and institutions.

39. Ocean Floor Ages Map A great global map illustrating ocean floor ages from NOAA. Ocean Floor Animations A good visualization of the Pacific hemisphere plate tectonic history. Ocean Floor Datasets Ocean floor data and images from Columbia University. Paleomap This Web site illustrates plate tectonics development of the ocean basins and continents, including projections for the future. Plate Motion Calculator An interactive plate motion calculator from the University of Tokyo. Plate Motion Calculator (UNAVCO) This program is designed for the calculation of plate motion at any location on Earth. You can specify the tectonic plate or, with the AUTO option, let the program determine which plate the point is on. Program returns with the plate model velocity components (Evel, the east velocity, and Nvel, the north velocity) in mm/yr. Plate Tectonics Activity (Geosphere) An excellent interactive activity on plate tectonics from the University of Montana's Geosphere. With this activity learners use current satellite images to determine sea floor spreading rates, identify relationships between earthquake epicenters and sites of active volcanism, and create a scale model of the Earth's layered structure. (The site may be slow to connect, but its usually there.) Plate Tectonics Animations Excellent animations of faults, plate subduction, earthquake wave propagation, and other processes from PBS' Savage Earth program. Plate Tectonics Animations (USGS) Great plate tectonics animations from the United States Geological Survey—a must see! Plate Tectonics Information (NEIS) National Earthquake Information Center (NEIS) plate tectonics information. Plate Tectonics Information (USGS) United States Geologic Survey (USGS) information on plate tectonics and sea-floor spreading. Plate Tectonics Links This Central Michigan University site maintained by Dr. Mark Francek lists several good links to Web sites containing information relative to topics discussed in the chapter.

40. Plate Tectonics Links (Houghton Mifflin) Links to several plate tectonics sites, including class lecture notes, arranged by topic. Plate Tectonics (NASA's Observatorium) A good tutorial on plate tectonics. Plate Tectonics Tutorial (1 of 2) Plate tectonics tutorial from Texas A & M University. Plate Tectonics Tutorial (2 of 2) Plate tectonics tutorial from Texas A & M University. Plate Tectonics Tutorial (VolcanoWorld) An in-depth review of plate tectonics from VolcanoWorld. Plate Tectonics (University of California, Berkeley) This University of California, Berkeley, Web site explains the history and science of tectonics and also contains animations showing the movement of plates. Related Topics Index Information on several topics related to plate tectonics from the University of North Dakota's VolcanoWorld site. San Andreas Fault and the Bay Area A virtual field trip along the San Andreas fault. Scripps Institution of Oceanography The Scripps Institution of Oceanography home page features numerous links to related oceanography Web sites. Seafloor Spreading Calculator This "Seafloor Spreading Rates Calculator" calculates the spreading rate at any point on mid-ocean ridges. The system is developed and maintained by K. Okino and K. Tamaki of Ocean Research Institute, University of Tokyo. Tectonic Plate Motion (Space Geodesy) An excellent site that explains how NASA monitors plate motions. Woods Hole Oceanographic Institution (WHOI) Scientists at the Woods Hole Oceanographic Institution (WHOI), a world leader in the field of oceanography, have been probing the mysteries of the oceans for more than 65 years. World Data Center for Marine Geology & Geophysics The World Data Center for Marine Geology & Geophysics promotes excellence in archiving, managing, and exchanging data obtained from measurements of the seafloor.