Plate Tectonics Notes
Alfred Wegener and Continental Drift
German scientist who formed a hypothesis
that 300 million years ago, all continents were
joined together in a single landmass and have
since drifted apart.
He named the supercontinent Pangaea,
meaning “all lands”.
His idea came to be called continental drift.
He also used continental drift to explain
mountain formation. He stated that, as
continents collide, their edges crumple and
fold, pushing up huge chunks of rock to form
mountains.
Wegener’s Evidence for Continental Drift
Landforms-mountain ranges in S. Africa and
Argentina lined up
-European coalfields matched coalfields in
N. America
Fossils- identical fossils found in places now
separated by oceans
-Glossopteris, fossil of a fernlike plant,
was found in Africa, S. America,
Australia, India, and Antarctica but
its seeds could not have traveled across
distances that now separate the
continents.
Climate-fossils of tropical plants found on
islands with polar climates
-evidence of glaciers found on continents
with tropical climates
Wegener’s idea was rejected because he
could not explain the force that pushes and
pulls on the continents.
Sea-Floor Spreading
Harry Hess, an American geologist, studied
the mid-ocean ridge, the longest chain of
mountains on the ocean floor that extends
into all Earth’s oceans and that is mostly
completely submerged.
In 1960, Hess suggested that, beginning along
the mid-ocean ridge, the ocean floors move
apart, carrying the continents with them.
Molten material rises from a crack in the midocean ridge and spreads out, pushing older
rock to the side. As it cools, it forms a strip
of solid rock in the center, which is then
pushed apart by more molten material.
Hess called this process sea-floor
spreading.
Evidence for Sea-Floor Spreading
Molten Material-In the 1960s, scientists
studying the central valley of the midocean ridge found rocks that only form
when molten material hardens quickly
after erupting underwater, showing
that there have been repeated eruptions
there. (pillow lava)
Magnetic Stripes-Rock on the ocean floor
contains iron, which is magnetic. When
this rock formed from molten material,
the iron particles lined up in the
direction of the magnetic poles. Earth’s
poles have reversed themselves, as
shown by rock on the ocean floor that
lies in a pattern of magnetized stripes
that alternately point north and south.
The pattern is identical on both sides of
the mid-ocean ridge.
Drilling Samples-Scientists dated rocks
from both sides of the mid-ocean ridge
and found that the farther away from
the ridge, the older the rocks were
and the youngest rocks were always in
the center.
Subduction
Process by which the ocean floor sinks
beneath a deep-ocean trench (underwater
canyon at the edge of an oceanic plate) and
back into the mantle.
The Pacific Ocean is shrinking because it is
ringed by many trenches and subduction is
taking place faster than new crust is forming.
The Atlantic Ocean is expanding because it
has few trenches.
Plate Tectonics
J. Tuzo Wilson observed that the continents
have cracks similar to those on the ocean
floor. He proposed that the lithosphere is
broken into sections called plates, which fit
closely together and carry the continents,
parts of the ocean floor, or both.
He combined what is known about continental
drift, sea-floor spreading, and the plates
into a single theory called plate tectonics,
which states that pieces of the Earth’s
lithosphere are in constant, slow motion
driven by convection currents in the mantle.
Plate Boundaries
Lines where the edges of the plates meet.
Transform boundary-where two plates slip
past each other in opposite directions and
crust is neither created nor destroyed.
-Earthquakes occur here.
Divergent boundary-where two plates move
apart (diverge)
-most occur on the mid-ocean ridge
resulting in sea-floor spreading and new
crust
-on land they form a deep valley called a rift
valley
Convergent boundary-where two plates come
together (converge)
-results in a collision
-if two oceanic plates collide, subduction
results
-if oceanic and continental plates collide,
the oceanic plate subducts and volcanoes
may form on the continent
-if two continental plates collide, neither
subducts and the crust is pushed up,
forming mountains
A. Oceanic Crust(Basalt)
B. Oceanic Crust(Basalt)
C. Continental Crust(Granite)
X. Mid-Ocean Ridge
Y. Trench
Z. Subduction Zone
Changes to Earth’s Surface
Plate movements produce stress in rock, a
force that causes it to change shape or break
and move. This results in deformation, a
change in a rock’s shape or volume.
Types of Deformation
Shearing-pushes each mass of rock in
opposite directions
Tension-pulls on a rock, making it thinner in
the middle
Compression-squeezes rock, making it thicker
in the middle
Faults and Fault Movements
Faults-breaks in the crust that form along
plate boundaries.
Strike-Slip Fault-caused by shearing
-rocks on either side slip past each other
with little up or down motion
-results in a transform boundary
Normal Fault-caused by tension force
-fault is at an angle so one block of rock
lies above the fault (hanging wall) and
the other lies below (footwall)
-results in a divergent boundary
Reverse Fault-caused by compression force
-has the same structure as a normal fault
but the hanging wall slides over the
footwall
-results in a convergent boundary
Mountain Building
Folded Mountains-form when two
continental plates collide, causing layers
of rock near the edges of plates to
compress and fold,
ex: Appalachian Mts.
Fault-Block Mountains-form when blocks of
crust slide along normal faults,
ex: Teton Mts.
Land Subsidence
Occurs when land surface sinks as a result of
geologic processes or human activities.
Can be caused by plate movement along
divergent boundaries or by uplift of areas of
crust.
Volcanoes
A weak spot in the crust where molten rock
(magma) comes to the surface.
Most occur along divergent boundaries like
the mid-ocean ridge or rift valleys or at
convergent boundaries where two oceanic
plates collide (forming island arcs like Japan
and the Caribbean) or where oceanic and
continental plates collide (forming volcanic
mountain ranges like the Andes).
Hot spot volcanoes form in the middle of
plates where the mantle melts through the
crust, ex: Hawaiian Islands.