Download PLATE TECTONICS

Plate
Tectonics
The Earth
in Motion
Concept Review
Alfred Wegener - German climatologist
and geophysicist who, in 1915, was one
of the first to suggest continental drift
(which later became plate tectonics).
He suggested that a supercontinent he
called Pangaea (“all land”)had existed in
the past, broke up starting 200 million
years ago, and the pieces “drifted” to
their present positions.
Alfred Wegener
He cited the fit of South America and Africa, ancient climate
similarities, fossil evidence, and the similarity of rock structures as
proof of his proposal. The main objection to Wegener’s proposal
was its inability (at the time) to provide a mechanism for the plate
movement.
Earth
Throughout
the Ages
<video: Earth 100 MY from now>
Fossil Evidence
Rock type and structural similarities. In order for a jigsaw puzzle to make sense,
the colors and textures of adjacent pieces have to match. Rocks in northwestern
Africa match those of eastern Brazil. The Appalachian mountains match
mountains in northern Europe just as rock type and structure on a recent
spreading center, the Red Sea, match perfectly on either side .
People Didn’t Get Wegener’s “Drift” …
- So, without an explanation for how these plates were
moving, people didn’t believe Wegener’s theory. They
needed more proof for how huge continents could “drift.”
- Then World War II came, and with it, technology. After
WWII, oceanographers began to use more sophisticated
technology to explore the ocean and map the seafloor.
-What they found was very interesting! When dredging up
the seafloor, scientists could not find ANY crust older than
180 million years old.
-Plus, sediment layers were very thin, not thousands of
meters deep like they should’ve been if they had always
been there and not been disturbed.
… And along came Plate Tectonics
- Associated with Earth’s rigid outer shell (lithosphere)
•Consists of several plates
•Plates are moving slowly
•Largest plate is the Pacific plate
•Plates are mostly beneath the ocean
- Asthenosphere
•Exists beneath the lithosphere
•Hotter and weaker than lithosphere
•Allows for motion of lithosphere
- Plate boundaries
•All major interactions among plates occur
along their boundaries
Three main types of plate boundaries:
-divergent
-convergent
-transform
Divergent boundaries are located mainly along oceanic ridges
(also called constructive margins, because they tend to construct, or
make more, land). These can happen in the ocean or on a continent.
Good examples are the Mid-Atlantic Ridge and the African Rift Valley.
Divergent boundary
in the ocean
The East African Rift Valley
Divergent boundary
on a continent
Another type of plate boundary, called a convergent
boundary, is known as a destructive margin because
it tends to destroy the lithosphere during the process
of plate collision/subduction.
There are three main types of convergent
boundaries, which result in different geological
structures:
- Oceanic-continental convergence
- Oceanic-oceanic convergence
- Continental-continental convergence
Oceanic-continental convergent
plate boundary
•Denser oceanic
slab sinks into the
asthenosphere
•Pockets of magma
develop and rise
•Continental
volcanic arcs form
Oceanic-oceanic convergent plate boundary
•Two oceanic slabs
converge and one
descends beneath the
other
•Often forms volcanoes
on the ocean floor
•Volcanic island arcs
form as volcanoes
emerge from the sea
•Examples include the
Aleutian, Mariana, and
Tonga islands
Continental-continental
convergent plate boundary
•When subducting
plates contain
continental material,
two continents collide
•Can produce new
mountain ranges such
as the Himalayas
Continental-continental example
Before
After
Transform fault boundaries
•Plates slide past one
another
•No new crust is
created or destroyed
•Transform faults
•Most join two
segments of a midocean ridge
•Aid the movement of
oceanic crustal material
Hot spots and mantle plumes
•Caused by rising
plumes of mantle
material
•Volcanoes can form
over them (Hawaiian
island chain)
•Mantle plumes
•Long-lived
structures
•Some originate
at great depth,
perhaps at the
mantle-core
boundary
Evidence for the plate tectonics model
•Paleomagnetism
•Probably the most
persuasive evidence
•Ancient magnetism
preserved in rocks
•Paleomagnetic records
show
•Polar wandering
(evidence that
continents moved)
•Earth’s magnetic field
reversals
•Recorded in
rocks as they
form at oceanic
ridges
Directions and rates of plate motions
What drives plate motion?
•No one model explains
all facets of plate
tectonics
•Earth’s heat is the
driving force
•Several models have
been proposed
•Slab-pull and slabpush model
•Descending
oceanic crust
pulls the plate
•Elevated ridge
system pushes
the plate
A possible view of the world
50 million years from now
Finish Group Mapping Activity