Transcript Plate Tectonics #2

Plate Tectonics—part 2

Alfred Wegener did
not know the earth’s
composition, but by
the 1960’s scientists
were able to revise
Wegener’s hypothesis
into a theory now
known as Plate
Tectonics.
Post 1950’s evidence




Convection currents
Mid-ocean ridge & trenches
Age of continent rocks vs. seafloor rocks
Earthquake/volcano and satellite data
Directions and rates
of plate motions
Plate Tectonics




The lithosphere is
composed of 7 major
and 13 minor plates.
A plate can be
composed of
continental and/or
oceanic crust.
Continental crustfelsic
Oceanic crust-mafic
14.3 Differences Between
Continental & Oceanic Lithosphere

Oceanic –
 younger (180 million years or less)
 more dense
 ~7 km thick

Continental –
 older (4 billion years
old)
 less dense
 ~35-40 km thick
Plate Tectonics


The areas where plates
are in contact with
each other are called
plate boundaries.
There are three types
of plate boundaries.
Plate Boundaries (type 1)


Divergent-two plates are
moving away from each
other. New crust is
formed. Most divergent
boundaries are on the
ocean floor. These are
marked by mid-ocean
ridges.
Tensional force is applied
to rocks at a divergent
boundary. (tension is what
pulls them apart)
Geographic
Examples
The East African rift – a
divergent boundary on land
Plate Boundaries (type 2)

Convergent- two plates
are moving towards each
other. (which applies a
“compressional force”)
 Oceanic/continental
oceanic crust has a
higher density than
Continental crust,
therefore it sinks below
the continental crust.
This is called a
subuction zone.
 Volcanoes,
mountains and
Plate Boundaries (type 2)

Continental/continental
neither plate is
subducted. In this
case, both edges are
crumpled and uplifted
producing large
mountain ranges.
 Himalayan
Mountains (India is
colliding with Asia)
Plate Boundaries (type 2)

Oceanic/oceanic
These are rare and
create deep
trenches.
 Marianas Trench
deepest trench in
the world.
Plate Boundaries (type 3)

Transform Fault
Boundaries- two plates
grind past each other.
(which applies a
“shearing” force)
 They do not slide
easily-they stick and
slip.
 When these plates slip
they create
earthquakes.
 San Andreas Fault
in California
MOUNTAIN BUILDING
MOUNTAIN BUILDING

Deformation
 The bending, tilting
and breaking of
Earth’s Crust.
MOUNTAIN BUILDING
Stress
 Force applied to rock material from
Isostasy.
 Strain
 Change in shape or volume of rocks from
rocks being squeezed, twisted or pulled
apart.

MOUNTAIN BUILDING
Folding
 Permanent deformation or bending of a
rock under stress.
 Faulting
 Break in rock along which rocks on either
side of the break have moved.

MOUNTAIN BUILDING

Faulted
 Faulting breaks Earth’s
crust into large blocks
and those blocks
become tilted.
 Ex. Sierra Nevada
Mountains
MOUNTAIN BUILDING

Folded
 Plates collide
(Convergent
boundary)
 Ex. Appalachian
Mtns, Himalayas
MOUNTAIN BUILDING

Dome
 Molten rock pushes up rock
layers on Earth’s surface creating
a dome. Over time this top layer
erodes away leaving the igneous
rock exposed.
 Ex. Black Hills, South Dakota
MOUNTAIN BUILDING

Volcanic
 Molten rock that
has erupted onto
Earth’s Surface.
 Cascade Mtns.,
Mid-Atlantic
Ridge