Plate Tectonics, Earthquakes, and Volcanoes

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Section 17.1 and 17.2

• Plate Tectonics, Earthquakes, and Volcanoes

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Background History of the Earth

• This chapter is based on the idea that Earth is
  about 4.5 billion years ago.
• Began as a ball of small particles pulled
  together by gravity. As pressure built, the
  material heated up.

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• Possible explanations for the heat
• - collisions of particles
• - compression of the interior of the planet
• - radioactive decay of some natural elements
• The Earth began to cool and the heavier (more
  dense) elements sank while the lighter (less
  dense) materials floated to the surface.

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17.1 Earths Interior and Plate Tectonics

• The Earths Interior is divided into 4 basic
  layers

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• Crust
• - Outermost and thinnest layer of Earth 4 -
  40 km thick
• - Made of hard, solid rock
• - Can be either continental crust (thicker) or
  oceanic crust (thinner)
• - The crust (and part of the mantle) is divided
  into 7 large plates

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• Mantle
• - Thickest layer of Earth 2900 km thick
• - 80 of Earths volume
• - Scientists have never drilled to the mantle,
  so they can only guess what it is made of based
  on what we learn from earthquakes and volcanoes.
• - outer mantle is made mostly of solid rock
• - inner mantle is made of hot and melting rock
• - rich in iron, magnesium, silicon, and oxygen

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• Outer Core
• - surrounds the inner core with liquid metal
  boiling under pressure nickel and iron
• Inner Core
• - made of solid metal nickel and iron
• Temperatures increase as one gets closer to the
  center of the earth.

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• Plate Tectonics the theory that the Earths
  surface is made up of large moving plates.
• Alfred Wegner studied world maps and hypothesized
  about Pangea a supercontinent that he believed
  existed in history and split into the current
  continents.

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• Continental drift the mechanism for the
  movement of continents caused by interactions
  between
• Lithosphere
• - the earths stiff outer shell made of the
  crust and hard upper mantle
• - divided into 7 large pieces and several
  smaller pieces called tectonic plates.

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• Asthenosphere
• - the liquid inner mantle which is in constant
  slow movement because of the heating from the
  center of the earth
• - convection currents result from the liquid
  rock being heated near the core and rising toward
  the outer mantle then cooling

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• Interaction currents in asthenosphere push the
  lithosphere plates like waves push around floats

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Evidence supporting continental drift

• The shapes match
• Ex South America fits with Africa like puzzle
  pieces
• The plants and animals match
• Ex identical fossils along the coastal parts of
  Africa and South America
• The rocks match
• Ex the same type of broad belts of rocks in
  Africa and South America these bands have
  alternating magnetic polarities

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Shapes match
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Plants and Animals Match
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• The ice marks match
• Ex Glacial striations on rocks show that
  glaciers moved from Africa onto South America
  without drifting away in the Atlantic Ocean
• The current positions dont match
• Ex coal that is mined in Pennsylvania was
  actually formed from tropical plant life that
  lives near the equator.

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Basic Types of Plate movement

• Plate movement occurs at faults, which are any
  cracks in the earth where movement occurs. There
  is are forces at each fault (convection currents)
  that cause movement in a certain direction. We
  use the direction to categorize the fault into a
  boundary. Plates move about 2 cm to 10 cm a year.

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• Three types of boundaries divergent, convergent,
  transform.
• Three types of faults normal, reverse, strike
  slip
• Three types of forces tension, compression, shear

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Divergent Boundaries

• Two plates are moving apart leaving gaps the
  gaps are filled with magma rising from the mantle
  which cools into new crust

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• Occur at normal faults
• Caused by tension force

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• Movement results in volcanoes and oceanic rift
  valleys surrounded by high mountains.
• - Mid-Atlantic Ridge a submerged mountain
  range under the Atlantic Ocean

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Mid-Atlantic Ridge
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Iceland on a Divergent Boundary
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Convergent Boundaries

• Two plates are moving towards each other

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• Occur at reverse faults
• Caused by compression forces

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• Movement results in subduction zones which create
  ocean trenches, mountains, and volcanoes.
• - subduction when one plate dives beneath
  another the lower plate is often melted by the
  heat of the asthenosphere
• - Andes Mountains in South America where a
  continental plate and an oceanic plate meet
• - Mariana Trench in Pacific Ocean near Japan
  where two oceanic plates meet is more than 11km
  deep (6.8 mi)
• - Himalayan mountains are due to the collision
  of two continental plates

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Transform Boundaries

• Two plates are moving horizontally past each
  other like two cars on a road going in opposite
  directions

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• Occur at strike slip faults
• Caused by shear forces

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• Movement results in earthquakes
• - San Andreas fault runs from Mexico through
  California

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17.2 Earthquakes and Volcanoes

• What are Earthquakes?
• Vibrations from rocks sliding past one another
• Occur mostly at plate boundaries all three types
  of the boundaries.

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• What happens?
• As plates move, pressure builds and the rock
  eventually breaks at the focus. This releases
  seismic waves in all directions around the focus.

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• There are three types of energy waves
• - longitudinal waves (primary waves or P waves)
  move like a compressing spring or a slinky. They
  are the fastest and first waves that run through
  rock

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• - transverse waves (secondary waves or S waves)
  move like a shaking rope and are slower than P
  waves

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• - surface waves only move across the Earths
  surface in a rolling motion cause more
  destruction

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• The epicenter is the point on the earths surface
  directly above the focus.

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Seismology the study of earthquakes

• Seismographs are machines that record info about
  the three types of waves sent out by an
  earthquake.
• Measure ground motion in three directions (North
  to South, East to West, up and down) using
  inertia.
• 1000 seismograph stations across the world three
  are needed to triangulate an earthquakes
  movement.

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Richter Scale

• a measure of the energy released at the focus of
  an earthquake
• the magnitude is recorded from 2.0 to 10

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Volcanoes

• Any opening, or vent, through which magma reaches
  the Earths surface.

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• Magma molten rock under the surface of the
  earth
• Lava magma has reached the surface
• Tephra materials of all types that erupt from a
  vent ash, cinder
• The makeup of the magma and type of eruption
  determines the type of volcano

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Shield Volcanoes

• Magma is rich in iron and magnesium which flows
  easily and far
• Several mild eruptions
• Creates many layers of lava into a gently sloping
  mountain
• Ex Mauna Lao in Hawaii

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Composite Volcanoes

• Magma is rich in silica thicker filled with gas
  bubbles
• Gases cause alternating lava flows then an
  explosion of cinders and ash
• Creates alternating layers of ash, cinders, and
  lava
• Ex Mount St. Helens
• Seamount volcanoes are found underwater and
  resemble composites

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Cinder Cones

• Smallest and most abundant volcanoes
• Large amounts of gas in the magma which causes
  violent eruptions
• Made of layers of cinders
• Tend to be active for a short time than become
  dormant

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Volcanoes occur mainly at plate boundaries

• Convergent as plates sink under during
  subduction, the crust is melted and magma rises
  to the surface
• - Ring of Fire circle
• of volcanoes around
• the Pacific Ocean

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• Divergent magma rises between two divergent
  plates
• - Iceland is a volcanic island that is
  continually growing at its center

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Volcanoes occur at hot spots

• Mantle plumes rise from deep in the mantle and
  can break through weaker spots of the oceanic
  crusts called hot spots.
• Lava and ash build up where magma broke through
  can build up to an island like the Hawaiian
  Islands which form a trail or island chain

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Hawaiian Islands Hot spot