Theory of Continental Drift
 Alfred Wegener,
German
meteorologist
 Proposed in 1912 in
articles
 Single large continent
(Pangaea – “all earth”)
started breaking apart
about 225-200 MYA
Permian 225 MYA
Jurassic 135 MYA
Triassic 200 MYA
Cretaceous
65 MYA
Evidence for Continental Drift
Fit of African and
South American
continents (1858
drawing by
geographer
Antonio SniderPellegrini)
Additional Evidence
Weakness of Theory
What kind of forces could be strong enough to
move such large masses of solid rock over such
great distances?
Wegener suggested that the continents simply
plowed through the ocean floor, but Harold
Jeffreys, a noted English geophysicist, argued
correctly that it was physically impossible for a
large mass of solid rock to plow through the
ocean floor without breaking up.
Earth’s “Inner Furnace”
Lithosphere (crust &
Crust 0-100 km
thick
uppermost solid
mantle)
Mantle
Mantle
2,900 km
Crust
Core
Outer core
5,100 km
Not to scale
Inner core
6,378 km
To scale
After World War II, new evidence from
ocean floor exploration and other
studies rekindled interest in Wegener's
theory, ultimately leading to the
development of the
theory of plate tectonics.
Evidence for Plate Tectonics
 Ruggedness & youth of ocean floor
 Confirmation of repeated reversals of Earth’s
magnetic field in geologic past
 Emergence of the seafloor-spreading hypothesis
and associated recycling of oceanic crust
 Documentation that the world's earthquake and
volcanic activity is concentrated along oceanic
trenches and submarine mountain ranges.
Magnetic Striping & Polar
Reversal
What is the nature of the forces
propelling the plates?
Conceptual
drawing of
assumed
convection cells
in the mantle.
Below a depth of
about 700 km,
the descending
slab begins to
soften and flow,
losing its form.
Faults
 Faults
 Fractures in
Earth’s crust
along which
motion may
occur
 Mark plate
boundaries
 Fault
Types of Crust
 Continental crust
 Makes up landmasses
 Granitic (felsic)
 Lighter
 Less dense
 Thicker
 Average age 2.3
billion years, oldest is
3.96 billion years
 Oceanic Crust
 Lies under oceans &
seas
 Basaltic (mafic)
 Heavier
 Denser
 Thinner
 Average age 55 million
years, totally recycled
every 150 million years
Three Types of Plate Boundaries
 Divergent: new crust forms
as plates pull away from
each other
 Convergent: crust
destroyed as plates collide,
one dives under the other
 Transform: crust neither
created nor destroyed,
plates slide past one
another
Convergent Plate Boundaries
 Continent-ocean collisions
(denser oceanic crust dives
under)
 Ocean-ocean collisions
(older denser crust dives
under younger crust, form
island arcs)
 Continent-continent
collisions (form mountain
ranges – Himalayas)
Main Types of Plate Boundaries
Earthquake Concentration
Mid-Ocean Ridges
Earth’s tectonic plates
Divergent
Boundaries
Ocean: Sea Floor Spreading
Ocean plates are being pushed apart by
convection currents and new ocean floor is
made
Crustal Features: Rift Valleys on land and
Mid-Ocean Ridge in ocean
Sea Floor Spreading
mid ocean ridge
Rift Valley on Land
Convergent
Boundaries
•Continental-Continental: huge mountains
Continental-Continental
Convergent
Boundaries
•Continental-Continental: huge mountains
•Continental-Oceanic: Subduction! The
more dense oceantic crust sinks under
the continental. Trenches, mountains, and
volcanoes occur.
Oceanic-Continental
Convergent
Boundaries
•Continental-Continental: huge mountains
•Continental-Oceanic: Subduction! The
more dense oceantic crust sinks under
the continental. Trenches, mountains, and
volcanoes occur.
•Oceanic-Oceanic: Subduction! Very deep
trenches and volcanoes
Oceanic-Oceanic
Transform
Boundaries
 A plate boundary where two plates move
past each other in opposite directions
 Makes faults and causes earthquakes
Transform Boundaries
Find the Nazca plate on the map.
direction is it moving?
What
 The
Nazca
plate is
moving
east
Find the South American plate and
describe its movement.
 The South
American
plate is
moving
west
What do you think will happen as these
plates continue to move?
As the plates
move, they will
collide
(convergent
boundary)
Since the Nazca
plate is oceanic
crust and the
South American
plate is
continental
crust, the Nazca
will plunge
beneath the
South American
plate