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Plate Tectonics
Study Guide
Alfred Wegener = Continental Drift
Continents were once part of a
single land mass called Pangaea
(all lands).
During the Mesozoic Era
Pangaea began to break up
and drift apart
Evidence to support
Continental Drift:
• Similarities
in the
coastlines of the
continents.
• Geological evidence =
age and type of rocks in
the coastal regions
match. Example = W.
Africa and E. Brazil
• Fossil
evidence = the same
fossils were found on different
continents that would have
been joined during Pangaea.
Example = Mesosaurus (S.
America and W. Africa)
• Climate
evidence =
glaciers in S. Africa and S.
America, Coal deposits in
E. United States, Europe,
and Siberia
The scientific community was not
impressed…..they insisted that Wegener
had to find evidence of some kind of force
that would have put Continental Drift into
motion.
1947 = scientists set out to map the Mid-Atlantic
Ridge (an undersea mountain range)
They discovered that
rock samples brought
up from the ocean floor
were much younger
than continental rocks.
No oceanic rocks were
older than 175 million
years old. Continental
rocks = 4 billion years
old!!!
Harry Hess and Robert Dietz
hypothesized that the valley at
the center of the Mid-Atlantic
Ridge was a break in the Earth’s
crust and that magma was welling
up through the rift. If the ocean
floor was moving outward from
the rift, then the continents might
be moving too.
Seafloor Spreading
Evidence for Seafloor
Spreading =
Paleomagnetism = As magma cools, the iron rich
minerals align themselves with the Earth’s magnetic
field and point north. However, some rocks have
magnetic orientations that point south.
HMMMMMMM……. (What does this mean?????)
Time to date some rocks!!!
All rocks with magnetic
fields pointing north
dated to the same time
period, all those with
magnetic fields pointing
south dated from the
same time period. So
what???
So, Earth’s magnetic
field must have
reversed itself at times
during Earth’s history
The same alternating
pattern of reversed
magnetism was also found
in the rocks on the ocean
floor.
This same pattern is
found on both sides of
the Mid-Atlantic Ridge.
Seafloor spreading is the “force” behind
Continental Drift
All of this evidence for
Continental Drift and
Seafloor Spreading gave rise
to a new, more
encompassing theory
Theory of Plate Tectonics:
Describes continental movement AND gives
an explanation for why and how the
continents move
So….how does this work?
There are 2 types of
Earth crust
Oceanic Crust
Continental
Crust
Oceanic crust + Continental Crust + the
Mantle = the LITHOSPHERE
Lithosphere
Aesthenosphere = solid rock that slowly
flows (like putty) when under pressure.
The Lithosphere is broken into separate plates that ride on
the Asthenosphere like blocks of wood floating on water.
The
continents
and
oceans are
along for
the ride
Lithospheric Plate Boundaries
1) Divergent Boundaries = two
plates moving away from each
other, allows the asthenosphere
to flow upward and fill in the open
space.
• Most of these are
found on the ocean
floor
• Create Rift
Valleys
Example = Red Sea
formed by the
separation of the
African and Arabian
Plates
2) Convergent Boundaries = the direct collision of
one plate with another. There are 3 types of
Convergent Boundaries.
2A) Subduction =
when oceanic
crust collides with
continental crust.
Oceanic crust is
subducted (or
forced under) the
less dense
continental crust.
Forms ocean
trenches
2B) A convergent boundary that occurs when two
continental plates collide = serious mountain
building. Example = the Himalayas
2C) A convergent
boundary that occurs
when oceanic crust
collides with oceanic
crust. A deep ocean
trench forms when one of
the plates is subducted,
the water causes mantle
rock to melt, which rises
to the surface to form
volcanic islands called an
Island Arc.
3) Third type of
Convergent Boundary
is called a Transform
Boundary. Forms
when two plates are
grinding past each
other. Does not
happen smoothly, but
in sudden spurts of
movement separated
by long periods of
inactivity. Example =
San Andreas Fault in
California
The whole system of plate movement is driven
by Convection Currents inside the Earth: hot
upwellings of Asthenosphere caused by the
sinking of lithospheric plates