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Plate Tectonics
A Scientific Revolution Unfolds
Why?
Why is Mount Everest
over 29000 feet high?
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
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
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
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
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
Gondwanaland
Gondwanaland included Antarctica, South
America, Africa, Madagascar, Australia-New
Guinea, New Zealand and Indian (and today
we include Arabia)
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
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 Earth’s 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
Rejection of Continental Drift
Talk about an idea whose time
had not arrived!
Most scientist ridiculed Alfred
Wegener’s 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?”
Accumulating Evidence
Fossil record
Earth’s crust and mantle
Magnetism and the Earth’s
magnetic field
Paleomagnetism
Magnetic reversals
The topography of the seafloor
Age of the seafloor
Seafloor spreading
Polar wandering
Fossil Record
The fossil record had revealed that the geology
and paleontology matched on opposite sides of
the Atlantic Ocean
Fossil Record
In fact, there are matching fossil records that
span across all of the continents
Without continental drift, this is hard to explain
Crust and Mantle
The Earth’s 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
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
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
Earth’s Magnetic Field
The Earth has a magnetic field
This why a compass points to the north
The simple presence of iron in the Earth’s core
is not enough to account for the Earth’s
magnetic field
The high temperatures in the Earth’s core are
far above the Curie temperature for any
magnetic mineral
Earth’s Magnetic Field
It is believed that the Earth’s 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
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
Paleomagnetism
Like tiny compass needles,
these magnetic minerals align
themselves parallel to the
lines of force of the Earth’s
magnetic field
This remnant magnetism,
which is also called
paleomagnetism, points to
the north pole like a sign post
But...
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
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
Magnetic Striping on Seafloor
In the 1950s, the Atlantic
seafloor was found to
consist of alternating
stripes of normal and
reversely magnetized
rocks
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
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
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
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
Plates that Move
Plates Move Slowly
Plates move at different
speeds:
Measured using GPS and
magnetic data
Slow spreading: 30 mm/yr
Fast spreading:100 mm/yr
Or about the thickness of
a fingernail in one day
Plates Move Slowly
Plate movement is best described as chaotic
Plates that Move
The red dots show that most major
earthquakes occur along plate boundaries
Plates that Move
The black triangles show that most volcanoes
occur along plate boundaries
When Plates Rub and Collide
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
Divergent Plate Boundaries
Usually start within continents
Can grow to become ocean basin
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
Divergent Plate Boundaries
C. A long, narrow lake or sea forms
D. Eventually, an
expansive ocean
basin and ridge
are created
Divergent Plate Boundaries
Can see the spreading
on the island of Iceland
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
Continental Rifts
Note that uprise and
decompression of the
underlying asthenosphere
results in magma
formation
The crust responds by
brittle fracture
Continental Rifts
Early rift sediments are
downfaulted into the
developing rift (graben)
Erosion takes place on
the sides of the rift valley
East African Rift Zone
Perhaps the most interesting and spectacular
plate tectonic rift zone on the land surface
Ol Doinyo Lengai
Ruwenzori
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
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
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
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
Rio Grande Rift
“Two features of the region most clearly exemplify the
Southwest's geologic-climatic-demographic 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”
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
Convergent Boundaries
There are three types of convergent plate
boundaries
Oceanic–Oceanic
Japanese Islands
Oceanic–Continent
Andes Mountains
Continent–Continent
Himalaya Mountains
We will look at each type
Oceanic - Oceanic
Under the ocean, one plate
slides underneath another at
a subduction zone
Oceanic - Oceanic
A deep trench forms along
the contact
Oceanic - Oceanic
Magma wells up along the subduction zone creating
volcanoes, which can form volcanic island arcs
such as the Japanese Islands
Oceanic - Continent
An oceanic plate slides
under a continental plate
at a subduction zone
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
Oceanic - Continent
A deep trench forms
along the subduction
zone
Oceanic - Continent
The lighter continental
crust is pushed up and
forms a mountain range
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
Andes Mountains
The Andes Mountains
stretch over 5500
miles along the Pacific
side of South America
Pacific Ring of Fire
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
Why?
Because the continent
of India collided with
the continent of Asia
Continent - Continent
Transform Plate Boundaries
A transform
fault plate
boundary
occurs when
two plate slide
past each other
in opposite
directions
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
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”
San Andreas Fault Zone
Transform Plate Boundaries
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
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
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
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
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
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
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
Hot Spot Volcanism
The Hawaiian Islands are the classic
example of hot spot volcanism
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
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
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
Hot Spot Volcanism
The volcanoes that
formed the Hawaiian
islands are classified
as shield volcanoes
Hot Spot Volcanism
Shield volcanoes are characterized by broad,
gentle slopes built up from repeated basaltic
lava flows
Hot Spot Volcanism
Not all eruption create big islands
Molokini represents one eruption that
is slowly being reclaimed by the sea
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
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 4–6 times
Convection in the Mantle
Hot mantle material rises at the spreading midoceanic ridges
Convection in the Mantle
Some of the magma erupts, but most spreads out
under the lithosphere and drags the crust along
Convection in the Mantle
Eventually, the slowly cooling material sinks
back into the mantle
Convection in the Mantle
An alternate hypothesis is that the convection
process may involve two convection layers
Convection in the Mantle
Or perhaps the convection process behaves
in a chaotic manner
Moving Continents
It is interesting to
speculate on how
the Earth looked as
the continents were
moved around...