Plate Tectonics

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Plate Tectonics
A Unifying Theory
http//vulcan.wr.usgs.gov/
Mt. St. Helens 10/12/04
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Discovering Plate Boundaries

• Classified boundaries based on OBSERVATIONS of
  PATTERNS
• Topographic expression
• Seismicity - distribution and depth of
  earthquakes
• Volcanology - distribution of volcanoes
• Age of the sea floor
• terra.rice.edu/plateboundary/

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• Boundary Observations
• Ocean floor boundaries with positive topography
• Shallow earthquakes
• Volcanism, basalt (low silica)
• Symmetric age patterns young age
• ? Anomalies

Divergent - Plates Moving Apart
http//earthview.sdsu.edu/trees/tecqest.html
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• Boundary Observations
• Deep ocean trench near high positive relief
• Shallow, intermediate, and deep earthquakes
• Volcanism, andesite (intermediate silica)
• Asymmetric age patterns often older ages
• ? Anomalies

Convergent - Plates Moving Together
http//earthview.sdsu.edu/trees/tecqest.html
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• Boundary Observations
• High positive relief continental crust
• Shallow and intermediate earthquakes
• Volcanism, andesite (intermediate silica)
• No age information
• ? Anomalies

Convergent - Plates Moving Together
http//earthview.sdsu.edu/trees/tecqest.html
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• Boundary Observations
• No topographic markers
• Shallow earthquakes
• No volcanism
• Asymmetric age patterns
• ? Anomalies

Transform - Plates Moving Past Each Other
http//earthview.sdsu.edu/trees/tecqest.html
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Observations Explained in the Theory of Plate
Tectonics

• The upper mechanical layer of Earth (lithosphere)
  is divided into rigid plates that move away,
  toward, and along each other
• Most (!) deformation of Earths crust occurs at
  plate boundaries

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Continent-Continent Collision
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Transform Boundaries
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3 Main Boundary Types

• Convergent
• Ocean-continent
• Ocean-ocean
• Continent-continent
• Oceanic crust subducts cold, dense
• Divergent
• New oceanic crust created
• Transform
• Often difficult to recognize

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Plate Tectonics ...

• Recent
• Unifying theme of solid earth sciences
• Framework on which we hang (test!) observations
  about Earths geology and geophysics
• Is a THEORY
• What makes the plates?

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• Earth three main layers defined by composition
• Crust - Outer
• Mantle - Middle
• Core - Center

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Composition - How Do We Know?Best Guess!

• Whole Earth
• Meteorites - Fe, Ni (same age as Earth)
• Information from velocities of seismic waves
  indicate material
• Crust (5-40 Km)
• Samples (mountain building helps!)
• Mantle (5/40 to 2885 Km)
• Kimberlite pipes - intrusive igneous rock from
  the mantle
• Lava / volcanic rock
• Mountain building
• Core (2885 to 6371 Km)
• Inference
• Earths mean density 5.5 g/cm3
• Crust 2.5 to 3 g/cm3 mantle 3.3 g/cm3 to 5.5
  g/cm3
• Density of core at least 10 to 11 g/cm3 (iron and
  nickel)

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Crust

• Two types of crust
• Continental
• 30 of crust
• Granites and Diorites - rich in silicates
• and feldspars (lighter materials)
• 40 Km thick
• Oldest is 3.8 billion years (90 solar
• system age missing 700 m.y.)
• Oceanic crust
• Basalt - Mg, Fe (heavier materials)
• 5-10 Km thick
• 200 Ma oldest 100 Ma average

Our deepest hole 9 Kilometers .. we have a
long way to go!
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Mantle

• MOHO - Mohorovicic Discontinuity
• Core mantle boundary - change in mineralogy
• Density - getting heavier
• 3.3 - 5.5 g/cm3
• Probably material such as peridotite (lots of
  heavy olivine - Fe, Mg)
• Samples from kimberlites, xenoliths in volcanic
  eruptions, basalt composition lab experiments

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Core

• Outer core
• Molten, near solid point (does not transmit
• certain seismic waves)
• Density of pure iron or nickel/iron
• Includes half of diameter of Earth
• 2x density of mantle
• Inner core
• Solid (higher pressure than outer core)
• Density of pure iron or nickel/iron
• Size of moon

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• Crust
• Mantle
• Core
• (Composition)
• Earth three main layers defined by mechanical
  properties (strength)
• Lithosphere
• Asthenosphere
• Mesosphere

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• Lithosphere
• PLATES in Plate Tectonics
• Upper 100 km
• Crust and upper mantle
• Rigid
• Asthenosphere
• 100 km to 700 Km
• Upper mantle
• Near melting point little strength ductile -
  NOT A LIQUID!
• Plates moving on this
• Magma generation
• Mesosphere
• Extends to core
• Also hot strong due to pressure

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Why Do the Plates Move?
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Got Heat?
Earth - 3 Heat Sources

• Loss of original heat of formation (geothermal /
  core is cooling)
• Radioactive decay of elements in Earths
  materials
• The Sun - external not important to plate
  tectonics

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Convection Driving Force of Plate Tectonics

• Interior of Earth has sluggish convection in some
  regions
• Heat from core rises, creates convection cells in
  the mantle
• NOT LIQUID

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• Rising hot material at mid-ocean ridges and
  mid-ocean volcanic islands
• Descending cooler material at trenches
• Lithospheric plates carried with the convection
  cells

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Accumulation of Observations - Evidence
Plate Tectonics as the Unifying Concept of Earth
Science

• Patterns of continents
• Paleontology
• Geology
• Patterns of sea floor ages
• Patterns of seafloor depth
• Patterns of volcanoes
• Patterns of earthquakes

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• 1912 Continental Drift
• Observations
• Fit of Continents
• Geology
• Paleontology
• Climate belts
• Pangaea 200 Ma
• Breakup 180 Ma
• Rigid bodies moving through
• yielding seafloor
• No mechanism of movement

Alfred Wegener
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Jigsaw-Puzzle Fit of Continents

• Matching mountain ranges

• Matching glacial evidence

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(No Transcript)
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Matching Fossils
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Mechanism for Plate Movement!

• Author Holmes (Early 20s)
• Interior of Earth has sluggish convection
  (transport of heat from core)
• New ocean crust injected into ocean floor (where?)

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Harry Hess and Seafloor Spreading

• Crust moves with convection currents
• New ocean crust at MORs
• Ocean crust dragged down at trenches mountains
  form here
• Continental crust too light remains at surface
• Earthquakes occur where crust descends

• It explains everything.

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Mapping Ocean Basins

• Ocean mapping revealed
• a ridge system 65,000 km long
• the most extensive mountain range in the world
• The Mid-Atlantic Ridge
• is the best known
• divides Atlantic Ocean basin in two nearly equal
  parts

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Atlantic Ocean Basin

• Mid-Atlantic Ridge

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Seafloor Spreading - Observations

• Fit of continents - new material pushes them
  apart
• Topography of ocean floors - hot ridges,
  trenches
• Volcanism at ridge axes - hot mantle material
• Seismic zones near margins - descending plates

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Magnetism The Final Piece

• Earth has magnetic field
• Similar to a giant dipole magnet
• magnetic poles essentially coincide with the
  geographic poles
• may result from different rotation of outer core
  and mantle

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Rocks and Magnetism - Tools

• When magma cools, takes on signature of Earths
  prevailing magnetic field
• Three magnetic measurements can be taken from
  rocks
• Inclination - latitude
• Declination - direction to the pole
• Positive (normal) or negative (reversed) -
  depending on what Earths field is doing
• Add age and you can track changes in Earths
  fieldand a few other important things.

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Inclination - latitude (lines of force relative
horizontal - surface) Declination - angle
between magnetic and geographic poles Positive
(normal) or negative (reversed) - depending on
what Earths field is doing
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Earths Magnetic Field Through Time
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Magnetic Reversals

• Earths present magnetic field is called normal
• magnetic north near the north geographic pole
• magnetic south near the south geographic pole
• At various times in the past, Earths magnetic
  field has completely reversed
• magnetic south near the north geographic pole
• magnetic north near the south geographic pole

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Magnetic Reversals

• Measuring paleomagnetism and dating continental
  lava flows lead to
• the realization that magnetic reversals existed
• the establishment of a magnetic reversal time
  scale

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When magma cools, takes on signature of Earths
prevailing magnetic field
magnetic iron-bearing minerals align with Earths
magnetic field
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How would you test this?
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Confirmation of Hesss Hypothesis
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Confirmation of Hesss Hypothesis

• The magnetic anomalies were discovered to be
  striped, parallel to the oceanic ridges and
  symmetrical with the ridges

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How Do Magnetic Reversals Relate to Seafloor
Spreading?
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Oceanic Crust Is Young

• Seafloor spreading theory indicates that
• oceanic crust is geologically young
• forms during spreading
• destroyed during subduction
• Radiometric dating confirms young age
• youngest oceanic crust occurs at mid-ocean ridges
  
• and the oldest oceanic crust is less than 180
  million years old
• oldest continental crust is 3.96 billion yeas old

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Age of Ocean Basins
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Building Continental Material Observable Trends?
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How Fast Do Plates Move?What Absolute
Direction?How Do We Know?
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Intra-Plate Volcanism

• Provides absolute rates and motions
• Fixed hot spot in the mantle deep, long-lived
  magma chamber
• Plume?
• If hot spot is fixed then plates are moving

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Hot Spots
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• Hawaiian Ridge-Emperor Seamounts chain -
  6,000-km-long
• Bend at 43 Ma indicates motion of Pacific Plate
  abruptly changed from north to west (??related
  to collision of India and Eurasia??)

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Theory of Plate Tectonics
Fit of continents Patterns of heat flow Ocean
floor topography/Sediment patterns Age patterns
of seafloor
Volcanism at ridge axes / hot spots Magnetic
stripes Polar wander Seismic zones Patterns of
mountains
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How Does Plate Tectonics Affect the Distribution
of Life?

• Present distribution of plants and animals
• climate
• geographic barriers
• Barriers create biotic provinces
• distinctive assemblage of plants and animals
• Plate movements largely control barriers
• when continents break up, new provinces form
• when continents come together, fewer provinces
  result
• as continents move north or south they move
  across temperature barriers

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How Does Plate Tectonics Affect the Distribution
of Life?

• Physical barriers caused by plate movements
  include
• intraplate volcanoes
• island arcs
• mid-ocean ridges
• mountain ranges
• subduction zones

• Example Isthmus of Panama creates a barrier to
  marine organisms

Caribbean
Pacific
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Summary

• Plate tectonic theory
• widely accepted by the 1970s
• overwhelming evidence supporting it
• It helps explain
• volcanism
• earthquake activity
• mountain building
• global climate changes
• distribution of biota and resources