Unit 7 Review—Forces that Change the Earth
Pangaea or Pangea—the super landmass that broke apart 225 million years ago to form all tectonic plates
(major and minor)
 Major plates
o Eurasian
o North American
o African
o South American
o Indo-Australian
o Antarctic
o Pacific
How did we get from there to here??

3 major theories
o Continental Drift—Alfred Wegener—1912
 Continents were one large landmass (Pangaea) that broke apart and moved to their
current positions
 Evidence presented to support Continental Drift Theory
1. Continental coastlines “fit” like puzzle pieces (especially Africa & S. America)
2. Fossil evidence—animal and plant fossils found on continents thousands of miles
apart
3. Climate evidence—fossils of tropical plants found in Antarctica
4. Glacial deposits (sediment, plants, etc) found on tropical continents
5. Rock structure—similar structures on different continents
 Theory rejected because Wegener didn’t know HOW the continents moved, there was no
mechanism for movement
o Seafloor Spread—Harry Hess—1960
 Navy mapped the seafloor using sound waves following WWII and found a series of
ridges and valleys (similar to surface of the continents) as well as the mid-ocean ridge
(MOR)at the “center” of most of the Earth’s ocean floor
 Hess proposed the theory of Seafloor Spread
 Hot, less dense magma from Earth’s mantle rises toward the surface at midocean ridge
 Material flows sideways when it reaches the seafloor and carries the old oceanic
curst (seafloor) away from the MOR in both directions
 Material cools as it moves away from MOR and contracts, becoming more dense,
it sinks and forms ridges = new seafloor
 Evidence to support Seafloor Spread Theory
 Oldest oceanic rock is farthest from the MOR, newest is closer to MOR
 Magnetic striping in the rocks as magnetic pole has changed over time
o Plate Tectonics—Dan McKinzie—1968
 Combination of theories of Continental Drift and Seafloor Spread
 Key components
 Crust and upper mantle (lithosphere) broken into plates that move on flowing,
plastic-like lower mantle (asthenosphere)
 Ocean floor continually moving: spreading from center  sinking along edges
 being melted and used again
 Convection currents beneath plates moves plates in different directions
Convection currents and plate movement
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Convection is the transfer of heat through a liquid
Earth’s crust is the outer most layer
o Continental crust is thicker and less dense
o Oceanic crust is thinner and more dense
Mantle is between the crust and the Earth’s core
o Lithosphere is the solid layer just below the Earth’s
crust—is part of all tectonic plates
o Asthenosphere is the flowing, plastic-like layer where
convection currents occur
Convection Current
o Asthenosphere is heated by the core  heated magma is less dense  magma rises toward
lithosphere (either at the mid-ocean ridge or below a tectonic plate)  magma cools and
becomes more dense  magma sinks toward core  REPEAT
o This current MOVES the lithosphere and crust = continental drift
o This current causes magma to rise at mid-ocean ridge = seafloor spread
Plate boundaries: Geological events and Crustal features
 DENSITY of tectonic plates determines if a plate sinks or rises
Boundary
Definition
Crust
Result
Divergent
Oceanic-Oceanic New magma at mid-ocean ridge pushes
Plates move
apart
seafloor away = seafloor spread
Continental
Rift valley forms as continent separates,
eventually becomes an ocean basin
Convergent
Plates move
ContinentalOceanic plate sinks (subduction) due to
Oceanic
toward each
being more dense below the continental plate
other
forming a deep sea trench and volcanoes due
(Subduction)
to magma chambers
Oceanic-Oceanic Older, denser oceanic crust subducts causing
volcanoes to form on ocean floor
(Subduction)
ContinentalPlates are the same density so there is no
Continental
subduction causing the plates to fold and
crumple producing mountains
Transform
Plates slide past Oceanic-Oceanic Sudden slide of one plate past the other at a
each other
OR
fault line causes Earthquakes—vibrations of
the earthquake cause rock slides, tsunamis,
Continentaletc.
Continental