Plate Tectonics
Plate Tectonics
• The Theory of Plate Tectonics is a theory that
describes the formation, movements, and
interactions of Earth’s lithospheric plates that
move on the asthenosphere.
Theory of Plate Tectonics
• The Theory of Plate Tectonics explains
why earthquakes and volcanoes are likely
to occur in particular locations and how
new crust forms along the ocean floor.
Earthquakes and volcanic activity occur
primarily at the location of plate boundaries.
– Plate boundaries are where 2 plates are pushing toward, pulling
away, or sliding past each other. The strain and friction causes
fractures in the earth, where earthquakes occur and where the
fractures allow molten rock to flow to the surface.
Evidence for plate tectonics was also
found in igneous rocks which recorded
the direction of Earth’s magnetic field.
• Mid-ocean ridge is a long chain of volcanic
mountains on the ocean floor with a deep central
valley. The mid-ocean ridge always shows the
magnetic orientation of the North and South
poles.
Mid-ocean Ridge
–A pattern of bands on either side of the mid-ocean
ridge show the changes in magnetic orientation.
Mid-ocean Ridges
– Rocks at the center of the mid-ocean ridge
are the youngest.
(so the farther the rocks are from the mid-ocean ridge the
older the rocks are).
– Mid-ocean ridges are boundaries where
plates are moving apart.
3 Main Hypothesis Which Cause
of Plate Movement:
• Mantle Convection,
• Ridge Push, and
• Slab Pull
Mantle Convection:
1. Mantle Convection - the transfer
(convection) of heat from Earth’s inner
and outer cores moves the plates (like a
convection belt)
Ridge Push
2. Ridge Push – occurs at the Mid-ocean Ridges
- The hot molten magma rises up at the mid-ocean ridges and
as it cools and hardens into a denser rock, the rock pushes out
and slides down the ridge, allowing more new hot molten material
to flow out of the mid-ocean ridge.
Slab Pull - Scientists believe this Slab Pull
is the greater factor out of the three theories.
3. Slab Pull – occurs at a subduction
boundary.
- The denser, heavier plate sinks underneath
the other less dense plate. The edge of the
subduction plate is much colder and heavier
than the mantle, so it continues to sink and
pull the rest of the plate along down with it.
CONTINENTAL DRIFT
• Alfred Wegener proposed a hypothesis
called Continental Drift which said the
continents have drifted, or moved from
one location to another over time.
Continental Drift was proven by:
• The shapes of the continents seem to
match up (fit together like puzzle pieces)
Continental Drift was proven by:
• Certain fossils are on different continents
– Example: Mesosaurus – reptile that lived about 270 mya, fossils found
only in parts of South America and Africa – could have only happened
when the 2 continents were once joined).
Continental Drift was proven by:
• Mountain chains found on 2 continents
and would have matched up if the
continents were joined.
– Example: Appalachian Mountains – run along the eastern coast of US
and through Great Britain.
Pangaea – the giant supercontinent that
existed about 200 mya.
TYPES OF PLATE BOUNDARIES
• There are 3 main types of plate
boundaries:
– Divergent Boundary,
– Convergent Boundary, and
– Transform Boundary
Divergent Boundary
Divergent Boundary
• Plates are moving apart
• Occurs at Mid-ocean Ridges and at Rift
Valleys (seafloor spreading)
– Rift valley = deep valleys at the center of a midocean ridge.
• Volcanic activity (underwater) and earthquakes
take place along this boundary.
– Hot springs also come up from
hydrothermal vents on the
ocean floor.
Divergent Boundary
• Mid-Atlantic Ridge (Atlantic Ocean)
Divergent Boundary
• East Pacific
Rise
(Pacific Ocean)
• Transform faults run
along the mid-ocean
ridges
Convergent Boundary
• Plates are moving toward each other.
• 2 Types of Convergent Boundaries
– Subduction
– Collision
Convergent Boundary
• Subduction occurs at:
– ocean to ocean plate moving toward each
other
– Ocean to continental plates moving toward
each other
– Oceanic crust is denser than continental crust, so
oceanic crust will subduct underneath the continental
plate
• Deep sea trenches are formed
• Volcanic island arcs are formed
• Earthquakes occur
Convergent Boundary - Subduction
• Islands of
Indonesia (ocean
to ocean)
• Mariana Islands
(ocean to ocean)
• Mariana Trench
(Pacific plate
moving
underneath the
Philippine Plate)
Convergent Boundary - Subduction
•Mariana
Islands –
Philippine
and Pacific
plates
colliding
(ocean to
ocean)
Convergent Boundary
• Collision Boundary occurs at:
– Continental to continental plate moving
towards each other
• Builds high mountain chains
• Earthquakes occur along these boundaries
Convergent Boundary - Collision
Convergent Boundary - Collision
• Our Appalachian Mountains were also formed this way long ago, but
are not currently being formed. (rounded mountain tops – been
weathered and eroded down)
Convergent Boundary - Collision
•Himalaya Mountains
(sharp and jagged
peaks)
Transform Boundary
• 2 plates are
sliding past each
other.
– Earthquake
activity occurs
here
Transform Boundary
• San Andreas Fault (in California)
– Movement isn’t the same along all parts of the
fault. Some parts can move up to 5cm/yr,
while others haven’t moved in over a century.
• North Anatolian Fault (in Turkey)
• Fracture zones along mid-ocean ridges