Plate Tectonics

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Plate Tectonics A Scientific Revolution Unfolds
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Why?
Why is Mount Everest over 29000 feet high?
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Plate Tectonics

• The unifying concept of the Earth sciences
• The outer portion of the Earth is made up of
  about 20 distinct plates ( 100 km thick),
  which move relative to each other
• Plate tectonics explains
• Volcanoes
• Earthquakes
• Sea floor spreading
• Mountain chains
• Occurrence of same fossils on different continents

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Plate Tectonics

• Integrates evidence from many branches of science
• First suggested based on evidence from
  cartography, geology and paleontology
• Fully embraced after evidence from geophysical
  measurements
• However, the proof of plate tectonics was a long
  rambling road

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Atlantic Coastline

• 300-400 years ago (after accurate maps became
  available) it was noticed that the coastline on
  both sides of the Atlantic could fit together
  like a jigsaw puzzle

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Atlantic Coastline

• The fit was even better when you took the
  continental slope into consideration
• In 1858, Antonio Snider-Pellegrini, an American
  published a sketch showing this fit

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Gondwanaland
In the 1890s, the Austrian Geologist Eduard Suess
suggested that the present day southern
continents once been formed in a super continent
which he called Gondwanaland The name comes from
the Gondwana region in India
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Gondwanaland
Gondwanaland included Antarctica, South America,
Africa, Madagascar, Australia-New Guinea, New
Zealand and Indian (and today we include Arabia)
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Gondwanaland
Unfortunately, Suess did not have any evidence
other than the jigsaw puzzle fit He could not
explain how Gondwanaland split up into the modern
continents However, he got to be on a stamp
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Continental Drift

• Proposed by the prominent geologist Alfred Wegner
  in 1912 based on his observation of drifting
  sheets of ice
• The concept is that large-scale horizontal
  movements of the Earths continents are
  responsible for the major topographical features
  such as mountains and ocean basins
• Called his super-continent Pangaea
• Wegner spent the next two decades, until his
  death, promoting continental drift

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Rejection of Continental Drift

• Talk about an idea whose time had not arrived!
• Most scientist ridiculed Alfred Wegeners idea
• Not even the public believed it possible
• No proof that continents move
• No driving mechanism that could move the
  continents.
• Why should the continents move?

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Accumulating Evidence
Fossil record Earths crust and mantle Magnetism
and the Earths magnetic field Paleomagnetism Magn
etic reversals The topography of the seafloor Age
of the seafloor Seafloor spreading Polar wandering
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Fossil Record
The fossil record had revealed that the geology
and paleontology matched on opposite sides of the
Atlantic Ocean
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Fossil Record
In fact, there are matching fossil records that
span across all of the continents Without
continental drift, this is hard to explain
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Crust and Mantle

• The Earths crust and uppermost mantle are solid,
  somewhat brittle and elastic
• This outmost solid layer is called the
  lithosphere
• It is thinnest under the oceans, averaging 50
  kilometers (km) thick
• The lithosphere under the continents can be up to
  100 kilometers thick

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Crust and Mantle

• The layer below the lithosphere is called the
  asthenosphere and it extends to a depth of 300
  kilometers
• The rock is not rigid and can flow plastically
  due to the high temperature and moderate
  pressures
• The existence of the plastic asthenosphere made
  plate motion more feasible

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Magnetism

• Most iron-bearing minerals are at least weakly
  magnetic
• Each magnetic mineral has a Curie temperature,
  the temperature below which it remains magnetic
• Above the Curie temperature the mineral is not
  magnetic
• The Curie temperature varies from mineral to
  mineral, but it is always below the melting
  temperature of the mineral

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Earths Magnetic Field
The Earth has a magnetic field This why a compass
points to the north The simple presence of iron
in the Earths core is not enough to account for
the Earths magnetic field The high temperatures
in the Earths core are far above the Curie
temperature for any magnetic mineral
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Earths Magnetic Field
It is believed that the Earths magnetic field
originates in a layer called the outer core The
outer core is a metallic fluid consisting mainly
of iron
This metallic fluid is in motion and the
convection currents act like a giant dynamo,
converting mechanical energy into magnetic energy
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Paleomagnetism

• A hot magma is not magnetic
• As a magma cools and solidifies, the iron-bearing
  minerals (such as ferromagnesian silicates)
  crystallize
• Eventually, the minerals cool below the Curie
  temperature and the iron-bearing minerals become
  magnetic

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Paleomagnetism

• Like tiny compass needles, these magnetic
  minerals align themselves parallel to the lines
  of force of the Earths magnetic field
• This remnant magnetism, which is also called
  paleomagnetism, points to the north pole like a
  sign post
• But...

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Magnetic Reversals
About a century ago, a sequence of lava flows
were found in France where some of the flows had
the north and south poles reversed Therefore,
the north pole and south pole must have
repeatedly swapped positions
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Magnetic Reversals
These magnetic reversals have occurred though out
the history of the Earth They occur on an
irregular basis ranging in time from tens of
thousands of years to millions of years
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Magnetic Striping on Seafloor
In the 1950s, the Atlantic seafloor was found to
consist of alternating stripes of normal and
reversely magnetized rocks
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Topography of the Atlantic
Also, in the 1950s, it was discovered that an
underwater mountain range ran north-south in the
middle of the Atlantic Ocean The Mid-Atlantic
Ridge rises as high as 2 kilometers above the
abyssal plain
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Atlantic Ocean Sea Floor

• In the 1960s, samples were collected from the
  Atlantic seafloor using special ships with drill
  rigs
• The rocks of the Atlantic seafloor were
  discovered to be basalt
• Basalt contains radioactive isotopes (such as
  U235) which can be dated

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Atlantic Ocean Sea Floor

• It was discovered that the youngest rocks of the
  Atlantic Ocean seafloor are found along the
  mid-oceanic ridge
• And that farther you move away from the ridge,
  the older the rocks become on either side of the
  ridge
• The oldest rocks are along the continental
  boundaries

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Plates that Move

• In 1965 Wilson proposed the concept that the
  crust of the Earth is a mosaic of interacting
  plates hence plate tectonics
• These plates move relative to each other
• The continents ride on these plates
• Geologic features, such as mountains, volcanoes
  and earthquakes occur along the plate boundaries

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Plates that Move
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Plates Move Slowly
Plates move at different speeds Measured using
GPS and magnetic data Slow spreading 30
mm/yr Fast spreading100 mm/yr Or about the
thickness of a fingernail in one day
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Plates Move Slowly
Plate movement is best described as chaotic
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Plates that Move
The red dots show that most major earthquakes
occur along plate boundaries
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Plates that Move
The black triangles show that most volcanoes
occur along plate boundaries
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When Plates Rub and Collide
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3 Types of Plate Boundaries
In 1965, the Canadian Geologist, J. Tuzo Wilson,
proposes that tectonic plates interact in three
different ways along their boundaries 1.
Transform 2. Divergent 3. Convergent
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Divergent Plate Boundaries
Usually start within continents Can grow to
become ocean basin
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Divergent Plate Boundaries
A. Rifting occurs where tensional forces thin the
crust, magma ascends and volcanoes form B. The
crust is pulled apart, forming a valley
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Divergent Plate Boundaries
C. A long, narrow lake or sea forms
D. Eventually, an expansive ocean basin and ridge
are created
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Divergent Plate Boundaries
Can see the spreading on the island of Iceland
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Continental Rifts

• Two examples are the East African and Rio Grande
  Rift Valleys
• This can be the beginning of ocean formation
  (although it may not get that far)
• Rifting often begins at a triple junction

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Continental Rifts

• Note that uprise and decompression of the
  underlying asthenosphere results in magma
  formation
• The crust responds by brittle fracture

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Continental Rifts

• Early rift sediments are downfaulted into the
  developing rift (graben)
• Erosion takes place on the sides of the rift
  valley

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East African Rift Zone
Perhaps the most interesting and spectacular
plate tectonic rift zone on the land surface
Ol Doinyo Lengai
Ruwenzori
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East African Rift Zone
The East African Rift has a north-south
orientation with the African Plate moving to the
west and the Somali Subplate moving to the east
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East African Rift Zone
A triple junction, where three plates are pulling
away from one another the Arabian Plate, and the
two parts of the African Plate (the Nubian and
the Somalian) splitting along the East African
Rift Zone
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Rio Grande Rift

• The Rio Grande Rift Valley extends north from
  Mexico, near El Paso, Texas through New Mexico
  into central Colorado
• The rift is now essentially quiet tectonically,
  but significant deformation and faulting with
  major offsets was responsible for the formation
  of the rift during the past 30 million years

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Rio Grande Rift

• The geology has been described as a "taffy-like"
  thinning of the lower crust, with upper crust
  faulting in many places, to produce the rift
  valley

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Rio Grande Rift

• Two features of the region most clearly
  exemplify the Southwest's geologic-climatic-demogr
  aphic circumstances. These features constitute
  two "natural geographic corridors" or focal areas
  concerning the relationship between people and
  the Southwests physical environment Arizona's
  Mogollon Rim Corridor and New Mexico's Rio Grande
  Rift Corridor

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Is the Earth Expanding?

• NO!
• The Earth is maintaining a constant diameter
• New crust is created at mid-oceanic ridges
• The old crust sinks back into the Earth at
  subduction zones along convergent plate
  boundaries

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Convergent Boundaries

• There are three types of convergent plate
  boundaries
• OceanicOceanic Japanese Islands
• OceanicContinent Andes Mountains
• ContinentContinent Himalaya Mountains
• We will look at each type

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Oceanic - Oceanic
Under the ocean, one plate slides underneath
another at a subduction zone
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Oceanic - Oceanic
A deep trench forms along the contact
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Oceanic - Oceanic
Magma wells up along the subduction zone creating
volcanoes, which can form volcanic island arcs
such as the Japanese Islands
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Oceanic - Continent
An oceanic plate slides under a continental plate
at a subduction zone
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Oceanic - Continent

• The relative densities of the oceanic crust
  versus the continental crust are important
• The continental crust is lighter and has an
  average density of 2.8 g/cm3
• The oceanic crust is heavier and has an average
  density 3.2 g/cm3
• Therefore, the heavier oceanic crust sinks
  beneath the more buoyant, lighter continental
  crust

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Oceanic - Continent
A deep trench forms along the subduction zone
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Oceanic - Continent
The lighter continental crust is pushed up and
forms a mountain range
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Oceanic - Continent
Magma wells up along the subduction zone creating
volcanoes, which adds to the size of the
mountains and creates a continental volcanic arc
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Andes Mountains
The Andes Mountains stretch over 5500 miles along
the Pacific side of South America
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Pacific Ring of Fire
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Continent - Continent

• Both continental plates are lighter than the
  heavier, underlying asthenosphere
• So, when two continental plates collide, they are
  too buoyant to be subducted
• Instead, the continental crust is deformed and
  uplifted, creating mountains such as Mount
  Everest

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Why?
Because the continent of India collided with the
continent of Asia
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Continent - Continent
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Transform Plate Boundaries
A transform fault plate boundary occurs when two
plate slide past each other in opposite directions
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Transform Plate Boundaries
The most famous example is the San Andreas Fault
Zone in California The portion of California in
blue is heading northwest to Alaska This is the
most studied fault zone in the world
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San Andreas Fault Earthquakes
Each red dot marks an earthquake Notice how many
have occurred in California They are not kidding
when they talk about the BIG ONE
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San Andreas Fault Zone
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Transform Plate Boundaries
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Hot Spot Volcanism
All of the mountains in this map of the floor of
the Pacific Ocean are volcanoes Note that there
are several long chains of volcanoes But there
are also many individual volcanoes None of these
volcanoes are formed along plate boundaries
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Hot Spot Volcanism
These isolated areas of volcanic activity are not
associated with plate boundaries These volcanoes
are found both on continents and out in the ocean
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Hot Spot Volcanism
A plume of hot mantle material rises up to the
bottom of the lithosphere Melting occurs and
magma is created These are called hot
spots They can last for many millions of years
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Hot Spot Volcanism
The magma works its way to the surface over the
hot spot and a volcano is formed Out in the
ocean, the volcano may or may not reach the
surface and form an island In fact, few oceanic
volcanoes reach the surface
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Hot Spot Volcanism
As the moving plate carries the volcano away from
the hot spot, the volcano dies out With time,
another volcano may form This cycle may repeat
many times, forming a long chain of under sea
volcanoes and islands
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Hot Spot Volcanism
The Cortes Bank Seamount, 100 miles offshore of
San Diego, is a 17-mile long remnant of a
undersea volcanic mountain range that rises to
within 3 feet of the surface
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Hot Spot Volcanism
Fisherman and scuba divers knew about it, but it
was not on any nautical maps The U.S. Navy found
it the hard way in 1985 with the aircraft carrier
Enterprise
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Hot Spot Volcanism
The Hawaiian Islands are the classic example of
hot spot volcanism
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Hot Spot Volcanism
Every island in the Hawaiian Island chain was
created by volcanic eruptions that occurred as
the Pacific Oceanic Plate crossed over a hot spot
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Hot Spot Volcanism
In fact, the 7 main Hawaiian Islands are just the
southern most portion of two great undersea
mountain ranges, composed of hundreds of
volcanoes, that runs for 6000 kilometers (4000
miles) across the floor of the Pacific Ocean
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Hot Spot Volcanism
The plate has moved to the northwest for more
than 65 million years The big island of Hawaii is
now over the hot spot
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Hot Spot Volcanism
The volcanoes that formed the Hawaiian islands
are classified as shield volcanoes
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Hot Spot Volcanism
Shield volcanoes are characterized by broad,
gentle slopes built up from repeated basaltic
lava flows
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Hot Spot Volcanism
Not all eruption create big islands Molokini
represents one eruption that is slowly being
reclaimed by the sea
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Hot Spot Volcanism
Loihi Volcano, the youngest volcano of the
Hawaiian Island Chain, lies about 20 km off the
south coast of the Big Island Currently it rises
3500 meters above the surrounding sea floor and
its summit is about 1000 meters beneath the
surface
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What Causes the Plates to Move

• A driving force for plate tectonics has not been
  definitely identified
• Each plate weighs trillions upon trillions of
  metric tons
• Thought to be convection of the mantle
• Friction at base of the lithosphere transfers
  energy from the asthenosphere to the lithosphere
• Convection may have overturned asthenosphere 46
  times

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Convection in the Mantle
Hot mantle material rises at the spreading
mid-oceanic ridges
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Convection in the Mantle
Some of the magma erupts, but most spreads out
under the lithosphere and drags the crust along
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Convection in the Mantle
Eventually, the slowly cooling material sinks
back into the mantle
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Convection in the Mantle
An alternate hypothesis is that the convection
process may involve two convection layers
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Convection in the Mantle
Or perhaps the convection process behaves in a
chaotic manner
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Moving Continents
It is interesting to speculate on how the Earth
looked as the continents were moved around...
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