Plate Tectonics

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Plate Tectonics
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BIG Idea

• Most geologic activity occurs at the boundaries
  between plates.

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I. Drifting Continents

• MAIN IDEA The shape and geology of the
  continents suggests that
  they were
  once joined together.

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Continental Drift

• theory proposed by Alfred Wegener
  (VAY guh nur) in 1912
• stated that the continents were once a
  single landmass
• the continents have moved...

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-Pangaea (all the earth) supercontinent
single landmass -Panthalassa (all seas) huge
ancient ocean
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A. Evidence of Continental Drift

• 1. Puzzle-Piece Fit

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2. Rock Formations

• Ages, types and layers of coastal rocks of widely
  separated regions matched

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• Western Africa and Eastern Brazil
• Appalachians (along eastern
  U.S.) fit Greenland and
  Europe

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3. Fossils
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• Mesosaurus freshwater reptile that lived 270
  million years ago
• remains (fossils) found in South America and
  Africa
• impossible for these reptiles to have crossed the
  Atlantic

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4. Climate

• (a) Glossopteris fern
  that grew in
  temperate climates
• places where fossils had been found were once
  closer to the Equator
• rocks containing these fern fossils had once been
  joined

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• (b) Coal Deposits
• Coal forms from ancient swamp plants
• Coal beds found in Antarctica indicated that this
  frozen land once had a tropical climate
• Antarctica must have
  been closer
  to the Equator

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• (c) Glacier Deposits
• found in Africa and South America
• climates are much warmer there today

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A Rejected Notion
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• Wegeners Continental Drift Hypothesis was never
  accepted by the scientific
  community
• He could not explain what forces could
  cause such massive movement
• Wegener died in 1930, on expedition in Greenland,
  while collecting evidence to further support his
  theory

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II. Seafloor Spreading

• MAIN IDEA Oceanic crust forms at ocean ridges
  and becomes part of the seafloor.

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• 1.Mid-Atlantic Ridge undersea mountain range
  with a steep narrow valley down the center
• the middle of the ocean floor is very young

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• 2. Renewal of the Ocean Floor
• seafloor spreading - floor moving away from the
  center
• Harry Hess and Robert Dietz
• 3. Paleomagnetism as magma solidifies the
  iron-rich minerals align with magnetic north like
  a compass

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reverse polarity magnetic orientation pointing
south
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III. Plate Boundaries

• MAIN IDEA Volcanoes, mountains, and deep-sea
  trenches form at the boundaries between the plates

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Theory of Plate Tectonics

• Plate Tectonics theory that the lithosphere is
  made of rigid plates that float on the
  asthenosphere
• Tectonics the study of the formation of Earths
  features

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What are the inferred properties of Earths
interior?
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Zones of Earth

• 1. Crust thin, solid outermost layer
  surrounds Earth like a shell surrounds an egg.
  

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Two Types of Crust
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• Continental Crust makes up the continents
  (land) thicker, less dense granitic rock
• Oceanic Crust under the oceans thinner, MORE
  dense basaltic rock

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• 2. Moho boundary between the crust and mantle.

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• 3. Mantle
• thickest layer
• sits below the crust
• makes up 2/3 of the Earths mass

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• Lithosphere (rocky sphere) upper part of
  mantle, cool and rigid

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• Asthenosphere (weak sphere) lower (plastic)
  mantle flows like hot tar

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• Plasticity the ability of a solid to flow

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• 4. Core center of the Earth two parts
• Inner Core ball of hot, solid metals (iron and
  nickel)

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Outer Core only true liquid zone
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If weve never been there, how have we learned
about the interior of Earth?
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By studying seismic waves

• Seismic Wave vibration that travels through the
  Earth results from earthquakes or huge
  explosions

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Types of Plate Boundaries
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• 1. Divergent Boundaries
• two plates that are moving AWAY from each other
• as plates move apart, the asthenosphere flows up
  to fill the empty space

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What forms when plates DIVERGE?
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a. Mid-Ocean Ridges Mid-Atlantic
Ridgeb. Rift Valleys (1) narrow valleys in
the middle of ocean ridges (2) where
continents separate East African Rift Valley
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2. Convergent Boundaries

• direct collision of one plate with another

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Three types of converging boundaries
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• Types of Convergent Boundaries

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Oceanic vs. Continental

• Subduction Zone oceanic crust moves (dives)
  under continental crust
• forms an oceanic trench and volcanic mountain
  range
• oceanic crust is more dense than continental,
  thus it dives down or subducts

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Andes Mountain Range
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Mount Saint Helens - Cascade Mountain Range
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Oceanic vs. Oceanic

• older, more dense oceanic crust subducts
• forms trenches and arcs of volcanic islands

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Aleutian Islands
Mariana Trench
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Continental vs. Continental

• no subduction
• two continental plates collide pushing up and
  creating folded mountains

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3. Transform Boundaries

• Plates slide horizontally past each other

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IV. Causes of Plate Motions

• MAIN IDEA Convection currents in the mantle
  cause plate motions.

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• cool material sinks as hot material rises, due
  to DENSITY DIFFERENCES.

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