Download Theory of PLATE TECTONICS

CONTINENTAL DRIFT
and the
Theory of
PLATE TECTONICS
notes #14
Contracting Earth Theory early 20th century
• The Earth started out as a molten blob and gradually cooled.
• As it cooled, heavier metals (Fe) sank down and formed the core,
while lighter metals (Al) stayed up in the crust.
• Pressure produced by contraction caused some parts of the crust to
buckle upwards, forming mountains.
• Other parts buckled downwards, creating ocean basins.
• Picture in your mind a grape turning into a raisin as it dries out.
Continental drift
• In 1912, Alfred Wegener
proposed the idea that
prior to 200 mya, the
continents were all joined
together in a single
Supercontinent called
PANGAEA.
• He thought they had moved
apart over the last 200
million years by sliding
across the ocean floor.
Evidence for Continental Drift
1. The Shapes Match
• The continents look
as if they were pieces
of a giant jigsaw
puzzle.
• Because they were
one continent that
broke up.
• Don’t match exactly
due to erosion
2. The Plants and Animals Match - Wegener noted that plant
and animal fossils of late Paleozoic age found on several
different continents were quite similar. Lived on Pangaea.
3. The Rocks Match
• Broad belts of rocks in North America and Europe are the
same type. These regions match when the edges of the
continents are joined.
• Mountains formed from collision when Pangaea formed.
4. The Ice Matches
•Wegener was aware that an ice sheet covered parts of
South America, Africa, India, and Australia about 300
million years ago. Glacial evidence seems random.
The direction of
the “striations”
line up when
Pangaea
reconstructed
• His theory was not accepted at
the time because he could not
explain how the continents
moved.
New Evidence in the 1950’s
• Leads to the proposal and scientific
acceptance of the Theory of Plate Tectonics
Main Points of the Theory
1. The Earth’s crust is not one solid piece
but is rather composed of a number of
individual crustal/lithospheric/tectonic
plates.
Main Points of the Theory
2. The plates “float” on the partially molten
and denser asthenosphere
3. They move over the mantle at an average
rate of 2-5 cm/year (this is the like the growth
rate of your fingernails).
4. The movement is caused by convection
currents in the mantle.
The earth’s crust floats on the denser mantle
beneath it – pg. 10 ESRT
2.7-3.0 g/cm3
3.4-5.6 g/cm3
Glenco
McGraw-Hill
Plates and Plate Boundaries
• The crustal (lithospheric) plates
typically contain oceanic and
continental crust.
• As the plates move, they can
separate, collide, or slide past
one another.
• This results in three kinds of
plate boundaries animations
1. Divergent -apart
2. Convergent-together
3. Transform-slide side by side
• Look at page 5 in ESRT
• Where did this Theory come from?
• In the 1950’s the global mid-ocean ridge system was
discovered.
• It is an immense submarine mountain chain that zigzags
between the continents around the globe.
• It is more than 50,000 kilometers (km) long and up to 800
km across.
Mid-ocean ridges circle the globe like the seams on a baseball
Divergent Boundaries on land
1. Plates move away from each other, crust cracks.
Gap is replaced by lava (volcanoes or fissures)
-cools and creates new basaltic crust.
2. The widening region is called a rift zone or rift
valley.
• This process caused the breaking up of Pangaea.
• Short lived on land because valley becomes ocean
floor.
Ex. The Great Rift
Valley in Africa
3. Since basalt is denser and thinner than the
continents, it gets covered with sea water.
4. The ocean widens along a mid ocean ridge,
called “sea-floor spreading”, and becomes
large enough for a ridge/rift system.
link
Mid-Atlantic Ridgelongest mountain chain in the world
“New” pillow basalt from the
Mid-Atlantic Ridge
Convergent Boundaries
• When two plates come together they collide
• The edge of each plate may be continental
or oceanic crust so three types of
convergent boundaries exist.
A. Oceanic Plate - Oceanic Plate
B. Oceanic plate - Continental Plate
C. Continental Plate - Continental plate
A. Oceanic - Oceanic Convergence (common)
• When two ocean plates converge, one is a little denser and
subducts, is taken down, beneath the other.
• This creates deep trenches and basaltic volcanic island arcs
due to the partial melting of the subducting plate.
• Japan, the Philippines, and the Aleutian Islands
The “Ring of Fire” is a string of volcanic arcs & islands caused by
subduction zones that surround most of the Pacific Plate/Ocean
• These ocean trenches are the
deepest parts the world’s ocean
• The deepest is “Challenger
Deep” in the Mariana’s Trench
at 11,035 meters
B. Oceanic - Continental Convergence link
•Oceanic crust is thinner and denser than continental, so it
“subducts” underneath the continental crust.
•This creates shallow trenches that fill in with sedimentary
“clastic wedge”.
•A volcanic arc is formed on the continent.
C. Continental-Continental (pretty rare)
• When continental plates meet, neither one is subducted
because both plates are thick with low density.
• Instead, one may slide under the other and the crust buckles
upward or sideways and forms tall mountain ranges.
(animation)
The Himalayan Mountains (Everest) formed when the Indian
Plate collided with the Eurasian Plate starting 10 mya
Mt. Everest is the world’s
highest point at 29,028 ft.
Transform Boundaries
• Two plates slide past one another
• Most are found on the ocean floor as “fracture
zones” that help offset the movement of seafloor
spreading along divergent boundaries.
• A few occur on
land
• The San Andreas
Fault in CA is
about 1,300 km
long.
• The Pacific Plate
has been grinding
past the North
American Plate at
the rate of 5
cm/yr,
• resulting in often
severe earthquake
activity
Cause of Tectonics
Convection Currents
• Heated material from within the earth rises since it is
less dense
• As this heated material reaches the surface, it spreads
out, carrying the crustal plates with it.
• It then cools, increases density and sinks back into the
mantle
Mechanisms of the movement
• Ridge-push causes oceanic lithosphere to slide
down the sides of the raised oceanic ridge under the
pull of gravity - It may contribute to plate motion
• Slab-pull is when cool, dense oceanic crust sinks
into the mantle and “pulls” the trailing lithosphere
along.
The Future
• What is the predicted future position of the
continents?
• No one knows for sure but using current
plate motions, scientists can predict what
they thing the earth will look like.
Supporting Evidence for the Theory
of Plate Tectonics
1. Earthquakes and volcano patterns
Mid-Ocean Ridges/Transform
• Earthquakes and
volcanoes occur at
ridges when rock
breaks while being
uplifted and split.
• Earthquakes occur
at transform
boundaries when
rocks grind against
opposite plate.
Subduction Zones
• Earthquakes occur
in the oceanic plate
as it subducts,
bends, and breaks
• This results in
shallow and deep
focus earthquakes
• 670 km is maximum
depth due to
softening as
temperatures rise.
- It can be clearly seen that earthquakes and volcanoes
are concentrated at the ridges and trenches.
- Connecting the dots traces out the plate boundaries
The Earth’s Magnetic Field and
Paleomagnetism
• Circulation of iron
minerals in the
outer core result in
the formation of
electromagnetic
currents that extend
into space.
• These currents are a
flow of energy
between the south
to the north poles of
Earth.
• Military scientists were using magnetic instruments during
World War II to detect submarines.
• Odd magnetic patterns across the ocean floor were
recognized.
• They found rocks having “normal” and “reverse” polarity.
• These striped patterns had mirror symmetry about midocean ridges.
• The ocean floor basalt contains strongly magnetic magnetite
• When the basalt cools at the ridge, magnetite crystals record
the Earth’s magnetic field “polarity” at that time.
• Today the polarity is “normal” meaning north is north and
south is south
• Some rock shows
opposite polarity.
• Called “reverse”
• This patterns
alternates as you
leave the ridge
•Only sea floor spreading can explain this pattern.
•From the alternating pattern on the sea floor, we see that the
Earth’s magnetic field has switched numerous times, at
irregular intervals, for reasons unknown
animation
4. Hot Spots
1. are small, long-lasting, hot regions in the earth’s
interior that are NOT related to plate boundaries.
2. A stationary “fountain” of magma rises through the
mantle and crust and erupts onto the seafloor. This
forms a seamount.
3. Over time the
seamount grows
until it finally
emerges above
sea level to form
an island volcano
– like Hawaii
•The tectonic plate moves over the fixed hot spot.
•This leaves a volcanic island trail which can be used to
determine plate speed and direction link