Plate Tectonics
Rearranging Earth’s Surface
Tectonic ProcessesGOAL: To
understand the processes behind the distribution of Earth’s continents and oceans
Tectonic Processes
1. Alfred Wegener and how a scientific paradigm operates
2. Structure of Earth allows continents to move
3. Types of Plate Interactions
a. divergence b. convergence c. transform
4. Hot Spots
1. Alfred Wegener and how a scientific paradigm operates
"Scientists still do not appear to understand sufficiently that all earth sciences must contribute evidence toward unveiling the state of our planet in earlier times, and that the truth of the matter can only be reached by combing all this evidence. . . It is only by combing the information furnished by all the earth sciences that we can hope to determine 'truth' here, that is to say, to find the picture that sets out all the known facts in the best arrangement and that therefore has the highest degree of probability. Further, we have to be prepared always for the possibility that each new discovery, no matter what science furnishes it, may modify the conclusions we draw."Alfred Wegener. The Origins of Continents and Oceans (4th edition)
Field work: Atlantic once closed
Shape MatchRocks MatchFossils Match
Lifetime of work
Theory of Continental Drift
Meteorologist proposes
to change geology
1st edition 1915
Earlier, Sir Francis Bacon
wondered about similar shape of South America and Africa coastline (animation in Classroom Resources Folder)
Until 1960s –Utter Rejection by Geologists
Dr. Rollin T. Chamberlin of the University of Chicago said, "Wegener's hypothesis in general is of the footloose type, in that it takes considerable liberty with our globe, and is less bound by restrictions or tied down by awkward, ugly facts than most of its rival theories."
"Utter, damned rot!" W.B. Scott, President of the American Philosophical Society
American Association of Petroleum Geologists organized a symposium specifically in opposition to the continental drift hypothesis.
Today: We Map Tectonic Plates
Classroom Resources Folder
Today: We Accept that most Earthquakes and Volcanoes occur
along boundaries of Tectonic Plates
Today: We Accept Plate Movement
Today: We Accept Pangaeaand its breakup
Classroom Resources
Earth in the FutureAnimation of Drift Over Time
Pangaea Breakup
Online Video Presentations
Why the opposition & shift?• An example of how a discipline can get
“locked into” a pathway of mental thinking for decades (paradigm), rejecting the idea of moving continents.
• An example of how a discipline protects its “turf” – rejecting the thoughts of a meteorologist.
• Enough time for the opponents to die, and a bunch of new geologists in the 1960s to challenge “the establishment”.
• Reminder throughout this class that ideas shift and much depends on the mental framework of the scientist
2. Structure of Earth allows continents to moveWegener could not answer a
fundamental question of HOW continents could move.
But geology advanced to the point to begin to understand Earth’s internal structure & how it allows continental drift
Earth’s Structure
Brittle Crust Floats on Flowing Asthenosphere
Different Crusts: 5/6th submerged(isostatic balance)
Ocean Crust Continental Crust
Dense (sima) Lighter (sial)
Isostacy crust adjusts to
create a balance (equilibrium)
Classroom Resources
IsostacyMtnRoot.swf – animation showing that erosion of a mountain chain creates a condition whereby the root makes the mountain chain rise up to maintain isostatic equilibrium
Human Induced Change
2 continental plates under Tibet
To reach isostatic
balance, Tibet
Plateau
rises
Too thick, so get uplift
Effects Of Erosion on Isostacy
Earth oozes at different rates
Asthenosphere flows
Even the crust adjusts
Reason for Movement
Excess heat from radioactive decay creates liquid outer core (and magnetic field), and forces mantle to have convection
Exactly how
mantle moves?
ASU view
Classroom Resource
DowngoingSlab.swf – animation showing that a subducting plate might descend all the way through the mantle down to the core
Classroom Resources
Animation of a theoretical model showing sinking crust and rising hot plumes – all in the mantle
3. TYPES OF PLATE INTERACTION
Stream Systems on
Dynamic Earth
3. Type of Plate Interactionsa. Divergent Boundaries
Mid-ocean
ridges are
places of
plate divergence
Mid-Atlantic Ridge is the Icon
East
Pacific
Rise
Mid
Atlantic
Ridge
Mid Indian
Ridge
Why a ridge? Why not a Valley?
Heat makes the large structure rise up, but at the axis there is a rift valley from spreading
Sea Floor Spreading: Key to Rejecting the Old Ways
“Plate Tectonics” = continental drift + sea-floor spreading
Rename the theory with new evidence:
1. Potassium-Argon dating
2. Paleomagnetism
Potassium -> Argon Dating
Discovery: middle of ocean ridges youngest
Millions of years before present
Classroom Resources
Animation showing ages of ocean crust getting older away from spreading
Paleomagnetism
Classroom ResourcesVisualizing reversals of
magnetic field and how this creates a mirror image on the two sides of a spreading center
Lava has minerals that “record” the magnetic field
Paleomagnetism in Symmetry
Conclusion
was that
Sea floors
Spreading
apart
Current
“Brunhes”
orientation to
north has lasted
longer than most
(0.8 million).
Current
magnetic pole
moving fast,
Towards Siberia
at 25 mi/yr.
Don’t Worry!
Hollywood movie “The Core” – rediculous
It will take thousands of years to shift
Source for Latest Info:
Website
Continents can also spreadAtlantic Opened up Africa is Rifting
Classroom Resource
EastAfricaRifts.swf – animation showing rifting of East Africa/Red Sea through the process of plate divergence
Obvious question: with making new sea floor, why isn’t Earth expanding?
Old ocean plates undergo “subduction”
3. Type of Plate Interactionsb. Convergent Boundaries
Oceanic-Continental Convergence
Denser Oceanic Crust SubductsTrench Forms
Volcanoes FormLithosphere Subjects Into AsthenosphereEarthquakes Common All Along Boundary
Andes
Nazca plate collide with South American Plate
Cartoon
Latest Research from Nature
Classroom ResourcesVisualizations about the Andes Mtns as from
Convegence of Ocean & Continental Plates
Classroom Resources
Ocean_Cont_Converg.swf – animation showing that an ocean plate subducting under a continental plate explains the sea-floor trench, the mountain belt, earthquakes and volcanoes
From the East-Pacific Rise, the Pacific plate moves towards subduction in the Aleutian Trench
Ocean-Cont.
Convergence
in Pacific
Northwest
Island Arcs from Ocean-Ocean Convergence
Older colder ocean plate
subducts
Classroom ResourceOcean_Ocean_Converg.swf – animation showing
that a cold ocean plate subducting under a warm ocean plate explains the sea-floor trench, earthquakes and a chain of volcanoes called and Island Arc
Convergence of 2 continental platesExample of Himalaya
Classroom Resources
Animations of Himalaya Formation
From Hodges, ASU
Formation of Appalachian Mountains resulted from the collision of the African and European Plates with North American Plate prior to 300 million years ago.
Ural Mountains
Formation of the Ural Mountains related to collision tectonics prior to 300 Ma ago.
Convergence & Tsunami
Effects Of Plate Locking
EQ (or landslide or volcanic eruption ) Movement Generates the Wave that travels about 500 mph
Classroom ResourcesShows how Tsunami
can be made from subduction
Shows time sequence in Tsunami destroying coastal city in Alaksa
Classroom Resource
TsunamiGeneration.swf – animation showing that faulting along an ocean floor from compression can generate a Tsunami
Animated gif
Mega-Quake Set off 2004 Tsunami & will happen
again
Energy ripple
Sri Lanka Sumatra Uplift
Indonesia
Indonesia
Has Happened, Will Happen Again
Hilo, 1960
In Classroom Resources Folder, Animation of Chile EQ generating Tsunami that destroyed Hilo in 1960
3. Type of Plate Interactionsc. Transform Boundaries
Side-by-side motion
along San Andreas zone
Transform motion of the San Andreas Fault can be seen by offset of streams that cross the fault zone.
California Won’t Fall into the Ocean
Continent is 5/6ths submerged, so it is anchored very tight
Classroom ResourceAnimation showing how there used
to be convergence along the California coast, but it turned into a transform boundary
Animation of San Francisco Peninsula Topography shows the “gash” of the San Andreas fault zone
Other Transform Boundaries
4. Hot Spots
Often in the middle of a plate
Focus here on Hawaii & Yellowstone
Hawaiian Hotspot
Classroom Resources
Animation of Hawaii as moves over Hot Spot
Trail of the Hot Spot
Note change in direction 43 myr agoChange in direction 43 myr ago
Alternative Hypothesis
Yellowstone Hot Spot
Online VisualizationsTsunami Visualizationshttp://serc.carleton.edu/NAGTWorkshops/visualization/collections/tsunami.htmlPlate Tectonic Movement Visualizationshttp://serc.carleton.edu/NAGTWorkshops/visualization/PTMovements.htmlIsostacyhttp://serc.carleton.edu/NAGTWorkshops/visualization/collections/IsoGrav.htmlMountain Uplifthttp://serc.carleton.edu/NAGTWorkshops/visualization/collections/mouneros.htmlEarth History
http://serc.carleton.edu/NAGTWorkshops/visualization/collections/geotime.htmlMountain Buildinghttp://serc.carleton.edu/NAGTWorkshops/visualization/collections/orogeny.html
Imagery seen in this presentation is courtesy of Ron Dorn and other ASU colleagues, students and colleagues in other academic departments, individual illustrations in scholarly journals such as Science and Nature, scholarly societies such as the Association of American Geographers, city,state governments, other countries government websites and U.S. government agencies such as NASA, USGS, NRCS, Library of Congress, U.S. Fish and Wildlife Service USAID and NOAA.c