Transcript PLATE TECTONICS

PLATE
TECTONICS
• Alfred Wegner
hypothesized that the
continents were once
joined into a single
continent called
Pangaea (meaning
“all land”). He further
suggested that this
supercontinent later
broke into smaller
pieces and drifted to
their present positions.
He referred to his
hypothesis as
“continental drift”.
To support his
hypotheses,
Wegner and others
collected evidence
from around the
world. Some of the
evidence to
support continental
drift include:
• The coastlines of some
of present day
continents seem to fit
together like a jigsaw
puzzle
• Identical fossil organisms
are found on continents
separated by thousands of miles
of ocean
• Rock types and
mountain ranges
match up when
the continent
puzzle is put
together
• Glacial deposits
are found in areas
which are now
located in the
tropics
• The one thing
that Wegner
could not do
was explain a
mechanism
that could
move
continents!
• Scientists now have seismic
data to support Wegner’s
hypotheses. Today’s theory is
referred to as plate tectonics.
• Plate tectonics describes plate
motion and the effects of the
motion. Plate tectonics says that
Earth’s outer shell is made of
about 20 plates. The plates are
rigid and in constant motion
relative to one another.
• The largest
plates consist of
two different
kinds of crust;
oceanic and
continental.
Many of the
smaller plates
contain only
oceanic crust.
• Continental crust is
thicker and less
dense than
oceanic crust.
Continental crust is
composed mostly
of the igneous rock
granite, while
ocean crust is
mostly the igneous
rock basalt.
The plates sit on the hot
asthenosphere. While
the rock in the
asthenosphere is solid,
it is less rigid than the
plates, and therefore
moves. The
movement within the
asthenosphere is
caused by heat from
the core of the earth.
The heat rises
slowly through
the earth
creating
convection
cells within the
asthenopsphere.
Scientists do not
fully understand
the forces that
move the plates.
• The most
accepted theory
is the idea that
the material
carried by
convection cells
in the
asthenosphere
drag the plates
along.
• The interaction between plates results
in different effects on the earth’s
surface. One of the effects of plate
motion is the Ring of Fire. The Ring
of Fire is an area of volcanoes that
form a ring around the Pacific Plate.
• Along these areas, plates are
colliding and ocean crust is
subducted, or pulled into the
mantle, and melted. This
process destroys plate material.
• Along the MidAtlantic ridge,
molten material
wells up between
plates that are
moving apart.
This magma
cools and
hardens to form
new crust.
• By destroying
and creating
plate material,
plate tectonics
constantly
recycles Earth’s
materials.
• Each plate
moves as a
distinct unit. The
borders between
plates are called
plate
boundaries.
Divergent - plates move away from
one another
•New crust material
is created
•Shallow
earthquakes along
the plate boundary
•Ocean/ocean crust
Landforms
• Creates Mid-ocean
Ridge
• Rift Valleys
Places
• Mid-Atlantic Ridge,
Atlantic Ocean
Convergent– plates move toward
one another
Ocean / Continental
• Ocean crust is pulled beneath the
continental crust into the mantle and
melts
• Old crust is destroyed
• Tight lines of volcanoes form on the
continental crust
• Deep Earthquakes on one side of
the plate boundary and shallow on
the
Landforms:
• Deep Ocean
Trenches
• High Volcanic
Mountains
Places
•Cascade Mountain Range, Oregon
and Washington
•Mt. Saint Helens
•One plate descends
beneath the other
(subduction)
•Tight lines of
Volcanoes on the
ocean floor
•Earthquakes
Ocean / Ocean Crust
Landforms
• Island and
Volcanic Arcs
• Ocean Trench
Places
• Mariana Trench
(Pacific Ocean)
• Japan
Continental / Continental Crust
•Neither plate is
subducted
•Plates crash
into one another
•No volcanic
activity
Landforms
• Mountain
Ranges
Places
• Himalaya
Mountains,
India
Transform—plates grind past one
another
•Shallow
Earthquakes
•Some
volcanoes, but
spread out
Mostly Ocean Crust
Some Continental
Landforms
• Faults (breaks in
rock)
Places
• San Andreas
Fault, California