BAESI: Earth and Life Through Time

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BAESI Earth and Life Through Time

• Plate Tectonics Overview
• Jonathan Hendricks
• SJSU Department of Geology
• jhendricks_at_sjsugeology.org

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Plate Tectonics Overview

• Lecture Overview
• Have the positions of continents moved through
  time?
• What is the history of the idea of continental
  drift?
• How does plate tectonics work?
• How do plates interact at their boundaries?

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Plate Tectonics

• Plate Tectonics The scientific theory that the
  surface of the Earth (lithosphere) is divided
  into plates that move relative to one another and
  that interact at their boundaries.

• Slide shows an image of one tectonic plate being
  subducted beneath another.

Image from USGS
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Map of the World

• Slide shows a satellite map of the world.

Image from NASA
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History of Plate Tectonics

• Slide shows two images made by geographer Antonio
  Snider-Pellegrini, 1858. One shows the
  continents separated. The other shows North and
  South America connected with Europe and Africa.

Maps by geographer Antonio Snider-Pellegrini, 1858
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Glossopteris Seed Fern

• Slide shows a photo of a Glossopteris fossil, as
  well as locations on the world map where
  Glossopteris fossils have been found.

Image from USGS
Image from NASA
Stars show places where Glossopteris fossils have
been found.
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Glossopteris Flora and Land Bridges?

• Slide shows a photo of a Glossopteris fossil, as
  well as locations on the world map where
  Glossopteris fossils have been found. A circle
  shows the position of the ancient land mass known
  as Gondwana.

Gondwana
Image from USGS
Image from NASA
Was sea level lower during late Paleozoic?
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Alfred Wegener (1880-1930)

• German meteorologist who proposed idea of
  continental drift idea that continents moved
  (and continue to move) horizontally over the
  surface of the Earth.
• In 1915 presented evidence for a single
  supercontinent, which he called Gondwana.
• Early evidence presented by Wegener and other
  workers (especially Alexander du Toit) in support
  of continental drift
• Continental fit.
• Rock sequences.
• Glacial flow directions.
• Rift valleys.
• Distributions of fossils.

• Slide shows a photograph of Alfred Wegener.

Image from USGS
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Glacial Flow Directions

• Slide shows two images. One shows a picture of
  scratch marks on rocks caused by glacial flow.
  The second shows ancient glacial flow directions
  on the modern world map. The third shows that the
  glacial flow directions in the southern
  continents only make sense if the continents were
  once connected.

Image from USGS
Image from NASA
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Rift Valleys of Africa
From NASA

• Slide shows two images. One is a map of
  northeastern Africa showing the positions of
  major rift valleys. The second shows a satellite
  photo of a rift valley.

Image from USGS
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Fossil Evidence

• Slide shows the distributions of Glossopteris,
  Lystrosaurus, and Mesosaurus fossils when the
  southern continents are reunited as Gondwana.

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History of Plate Tectonics

• Despite the extensive evidence that the positions
  of the continents have changed through time, most
  geologists rejected the idea of continental
  drift.
• This was because there was no known mechanism
  that could produce such change.

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Interior of the Earth

• Inner core mostly solid iron
• Outer core mostly liquid iron
• Mantle rocky material
• Crust
• Oceanic crust
• Continental crust
• Pressure increases with depth.

• Slide shows two figures that detail the different
  parts of the interior of the Earth

Images from USGS
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Earths Magnetism

• Motion of iron-rich outer core creates a magnetic
  field.
• Earth acts like giant bar magnet with N and S
  poles.
• Geographic and magnetic poles offset.

• Slide shows a cartoon image of Earths magnetic
  field.

Image from USGS
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Magnetism is Recorded in Rocks

• Some rocks contain iron minerals.
• These minerals align themselves to Earths
  magnetic field as the rock forms.
• Iron particles in sedimentary rock align as they
  fall out of suspension from water.
• Iron particles in magma (igneous rocks) align
  before the magma cools.
• Frozen orientations preserve record of the
  ancient orientations of Earths magnetic field.

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Study of the Seafloor

• The seafloor became much better explored during
  the 1940-1960s.
• WWII, sonar.
• Complex topography.
• Mid-oceanic ridges with central furrow.
• Volcanoes often associated with ridges.

• Slide shows an artists painting of the
  Mid-Atlantic Ridge.

Image from USGS
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Harry H. Hess Seafloor Spreading

• Hess Hypothesis of Seafloor Spreading
• 1962
• Continental and oceanic crust move together.
• New oceanic crust forms from rising magma at
  mid-continental ridges
• Oceanic crust moves away from ridge as it cools.
• Mechanism thermal convection.

• Slide shows a photograph of Harry Hess.

Image from USGS
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Thermal Convection

• Thermal convection is thought to be the process
  driving the movement of plates.
• Earth is hotter (due to radioactive decay -
  fission) in some portions of the deep mantle than
  in others.
• This causes the formation of convection cells
  that drag along overlying lithospheric plates -
  acts like conveyor belts.
• Think about a container full of boiling water.

• Slide shows two images. One is a cross-section
  through the Earth showing how convection cells in
  the mantle may operate. The other shows a
  container of boiling water.

Images from USGS
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Testing Hess Hypothesis

• How could one test Hess hypothesis of seafloor
  spreading?
• What pattern should one find on either side of
  mid-ocean ridge systems if Hess hypothesis is
  true?

• Slide shows an image of the Mid-Atlantic Ridge.

Image from Google Earth
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Magnetic Reversals

• The polarity of Earths magnetic field has
  flipped many times throughout the geologic
  past.
• The reason(s) why are not at all clear.
• Durations of normal and reversed polarity
  highly variable in length.

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Test of Hess Hypothesis

• During the early 1960s, it was discovered that
  changes in Earths magnetic polarity have been
  recorded into rocks on the seafloor (oceanic
  crust) as they cooled.
• Symmetrical banding on each side of mid-oceanic
  ridge systems.
• Younger rock near ridge, older away.

• Slide shows two images that illustrate magnetic
  reversals on either side of a mid-ocean ridge.
  These reversals form symmetrical patterns on each
  side of the mid-ocean ridge.

Images from USGS
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Ages of the Worlds Ocean Basins

• Slide has one image showing the ages of different
  oceanic crust rocks in the worlds ocean basins.

Image from USGS
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Hess Hypothesis Was NOT FalsifiedEnough
support has since been provided for plate
tectonics that the idea is now accepted as a
unifying theory for geology.Simple idea with
great explanatory power.
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Major Plates of the World

• Slide shows the major plates of the world.

Image from USGS
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Plates Interact at Their Boundaries

• Slide shows a figure that details where recent
  earthquakes have occurred. Most occurred near
  plate boundaries.

Image from USGS Earthquakes over last 30 days
(http//neic.usgs.gov/neis/qed/)
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Different Plate Boundaries

• Three major types of plate boundaries
• Divergent - plates diverge from each other.
• Convergent - plates converge toward each other.
• Oceanic-Continental - oceanic crust (denser)
  subducts (goes under) beneath continental crust.
• Continental-Continental - neither body of
  continental crust subducts (equal density).
• Transform - plates slide past each other.

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Different Plate Boundaries

• Slide shows a figure that provides an overview of
  the three major types of plate boundaries
  divergent plate boundaries, convergent plate
  boundaries, and transform plate boundaries.

Image from USGS
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Divergent Plate Boundary
Mid-Atlantic Ridge

• Slide shows two images. The first shows a
  picture that illustrates a divergent plate
  boundary. The second shows the mid-Atlantic ridge
  system.

Image from USGS
Image from Google Earth
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Oceanic-Continental Convergent Plate Boundary
Andes, South America

• Slide shows two images. On is an illustration of
  a oceanic-continental convergent boundary. The
  second is an image of the west coast of South
  America, which is an oceanic-continental
  convergent boundary.

Image from USGS
Image from Google Earth
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Continental-Continental Convergent Plate Boundary
Himalaya Mountains, Asia

• Slide shows three images. One is an illustration
  of a continental-continental convergent boundary.
  The second is an image of the Himalayan
  mountains, which is an example of a
  continental-continental convergent boundary. The
  third is a cartoon that shows how India crashed
  into Asia.

Image from USGS
Image from USGS
Image from Google Earth
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Transform Plate Boundary
Northwestern United States

• Slide shows two images, both of which illustrate
  the positions of transform plate boundaries near
  the northwest coast of the United States.

San Andreas Fault
Image from USGS
Image from Google Earth
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Earth Today
Plate tectonic maps and Continental drift
animations by C. R. Scotese, PALEOMAP Project
(www.scotese.com)
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Earth in the Cretaceous
Plate tectonic maps and Continental drift
animations by C. R. Scotese, PALEOMAP Project
(www.scotese.com)
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Earth in the Jurassic
GONDWANA South America, Africa, Antarctica,
India, and Australia
Plate tectonic maps and Continental drift
animations by C. R. Scotese, PALEOMAP Project
(www.scotese.com)
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Earth in the Permian
SUPERCONTINENT OF PANGEA!!
Plate tectonic maps and Continental drift
animations by C. R. Scotese, PALEOMAP Project
(www.scotese.com)
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Earth in the Devonian
Plate tectonic maps and Continental drift
animations by C. R. Scotese, PALEOMAP Project
(www.scotese.com)
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Earth in the Ordovician
Plate tectonic maps and Continental drift
animations by C. R. Scotese, PALEOMAP Project
(www.scotese.com)
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Earth in the Cambrian
Plate tectonic maps and Continental drift
animations by C. R. Scotese, PALEOMAP Project
(www.scotese.com)
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Earth in the Late Proterozoic
Plate tectonic maps and Continental drift
animations by C. R. Scotese, PALEOMAP Project
(www.scotese.com)
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Earth Today
Plate tectonic maps and Continental drift
animations by C. R. Scotese, PALEOMAP Project
(www.scotese.com)
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Earth in 150 Million Years
Plate tectonic maps and Continental drift
animations by C. R. Scotese, PALEOMAP Project
(www.scotese.com)
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Earth in 250 Million Years
Plate tectonic maps and Continental drift
animations by C. R. Scotese, PALEOMAP Project
(www.scotese.com)