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Plate Tectonics
The bent and folded rock
layers shown here, high
in the mountains of
Nepal, were distorted by
the collision of the
tectonic plates that
created the Himalayan
Mountains. Movements
of the earth’s tectonic
plates create most of the
major geologic features
of the planet, including
earthquakes, volcanoes,
mountains, and major
features of the sea floor.
Summary of Important Concepts
• Plate tectonics refers to the concept that the earth’s outer rigid
rock shell, called the lithosphere, is divided up into a number of
separate pieces, called tectonic plates, that are shoved around
by the slow convection of hot rock in the underlying
asthenosphere.
• The movement of the tectonic plates causes many of the major
geologic features of the earth’s surface, including earthquakes,
volcanoes, mountain ranges, and major features of the ocean
floor like mid-ocean ridges and oceanic trenches. The
movements of these plates causes the continents to move (drift)
over time, opens up new ocean basins, and closes old ones.
Summary of Important Concepts, continued
• Alfred Wegener, in the early 1900’s, was the first person to
gather evidence that the continents had moved - a process
called continental drift. His evidence suggested that the
continents on both sides of the Atlantic Ocean had once been
joined into a single large continent he called Pangaea, that
gradually split apart over geologic time. In spite of the
evidence, his theory was not widely accepted at the time.
• Harry Hess and Robert Dietz in the 1960’s revived
Wegener’s idea with new data from the ocean floor. They
proposed that new oceanic crust is created by sea floor
spreading at mid-ocean ridges. This sea floor moves along
horizontally until it sinks into the mantle at oceanic trenches
and is destroyed: a process called subduction.
Summary of Important Concepts, continued
• The concepts of continental drift, sea floor spreading, and
subduction are all tied together into the theory of PLATE
TECTONICS, in which the lithosphere is split up into a number
of moving plates. These plates either:
1) move apart at divergent boundaries, creating new sea floor by
volcanic eruptions at mid-ocean ridges (sea floor spreading);
2) come together at convergent boundaries, destroying old sea
floor by subduction at oceanic trenches, creating volcanoes and
mountain ranges in the process;
3) slide side-by-side at transform boundaries.
• Critical evidence that proved sea floor spreading came from the
study of magnetic patterns in sea floor rocks near mid-ocean
ridges.
Summary of Important Concepts, continued
• Evidence that supports subduction includes:
- A zone of deep earthquakes at oceanic trenches -- the Benioff
zone -- shows where the sea floor is sinking down into the mantle.
- Lines of active volcanoes that run parallel to oceanic trenches,
caused by melting of subducting plates.
• Other important evidence for place movement:
- Lines of volcanoes that increase in age in one direction, showing
the movement of a plate over a hot spot - a stationary area of
rising magma in the mantle.
- Earthquakes occur in distinct zones that reflect the boundaries
between plates. Nearly all of the world’s earthquakes occur at plate
boundaries.
PLATE TECTONICS INTRODUCED
Earth’s lithosphere, which consists of the earth’s crust and
upper mantle, is cut up into roughly 20 plates that move
relative to one another atop of the asthenosphere.
Plates Interact: They converge, diverge
or slide horizontally past one another.
PLATE TECTONICS - A POWERFUL
UNIFYING THEORY
• Plate tectonics is a relatively new scientific
concept, introduced some 50 years ago, but
it has revolutionized our understanding of
the dynamic planet upon which we live.
• It has provided explanations to questions
that scientists had speculated upon for
centuries.
Alfred Wegener in the
early 1900’s proposed
that the continents
were once joined
together in a single
large land mass he
called Pangea
(meaning “all land” in
Greek). He proposed
that Pangea had split
apart and the
continents had moved
gradually to their
present positions - a
process that became
known as continental
drift.
CONTINENTAL DRIFT
CONTINENTAL DRIFT
Alfred Wegner
contended that around 200-250
million years ago the supercontinent
Pangea began splitting apart
and since then the continents have
moved to their present positions.
Pangaea about 200 million years ago, before it began breaking up.
Wegener named the southern portion of Pangaea Gondwana, and
the northern portion Laurasia.
The continents about 70 million years ago. Notice that the
breakup of Pangea formed the Atlantic Ocean. India’s eventual
collision with Eurasia would form the Himalayan Mountains.
The position of the continents today. The continents are still
slowly moving, at about the speed your fingernails grow. Satellite
measurements have confirmed that every year the Atlantic Ocean
gets a few inches wider!
Wegner’s Evidence for Continental Drift
Continents fit
together like a
puzzle….e.g. the
Atlantic coastlines of
Africa and South
America.
The Best fit includes
the continental
shelves (the
continental edges
under water.)
Wegner’s Evidence for Continental Drift
Fossils of plants and animals of the same
species found on different continents.
Wegner’s Evidence for Continental Drift
Mesosaurus
a freshwater
reptile fossil
found in
Africa and
South
America.
Glossopteris; a fern that
requires warm climates
was found on Antarctica,
Southern South America,
Australia, Southern Africa
and India.
Wegner’s Evidence for Continental Drift
The distribution of climate sensitive sedimentary rocks
on the different continents.
Coal deposits are found abundant in Pennsylvania and
Siberia. Why is this unusual?
Wegner’s Evidence for Continental
Drift
•Glacial sediment deposits
found in places where glaciers
do not exist today.
•Glacial Scratches (scratches on
rock caused by glacial
movement) line up like a jigsaw
puzzle when continents are
reassembled.
•Both show that the land masses
were all joined and partially
covered by a single large ice cap
over the ancient south pole!
Wegner’s Evidence for Continental Drift
Although today we know that Alfred Wegener was correct about
continental drift, at the time his theory was not widely accepted. In
spite of the evidence that the continents had once been joined, few
scientists could understand how the massive continents, weighing
billions of tons, could actually move. Wegener never satisfactorily
explained this problem.
Confirmation of continental drift would have to wait until the
1960’s, when a better understanding of the ocean floor lead to the
concept of sea floor spreading. Confirmation of sea floor
spreading, and additional types of evidence, would eventually
vindicate Wegener and lead to the most important unifying concept
in geology: the theory of plate tectonics.
The Revival of Continental Drift
1940’s &1950’s
Work in the 1940’s and 50’s set the stage for the
revival of Wegner’s work.
1. New studies of the sea floor as a result of
WWII technology (submarines, fathometers)
2. Geophysical research in rock magnetism.
Paleomagnetism
Throughout earth’s time the magnetic north
and south have switched RANDOMLY
and sporadically every 1000-10,000 years.
When magnetism switches, its called a
REVERSAL, where lines of magnetism
run North to South.
Paleomagnetism
Rocks record the direction of the earth’s magnetic field at the time the
rocks form. Small magnetite (Fe) crystals in cooling magma act like
compass needles that record the direction of the earth’s magnetic field
when the magma solidifies. Once the minerals solidify, the magnetism
they posses will remain.
Harry Hess and Sea Floor Spreading 1960’s
During the 1950’s, intense oceanographic research and technological
advancements provided maps of the sea floorshowing mid-ocean
ridges, deep sea trenches, and active volcanism.
Harry Hess and Sea Floor Spreading 1960’s
Harry Hess concluded in 1960 that new
sea floor was being created at midocean ridges (MOR) by volcanic
activity. But the earth is not getting
larger. Therefore he concluded that
an equal amount of oceanic crust is
probably being lost at trenches.
The driving force is convection currents
in the mantle caused by heat from
earth’s formation, radioactive decay
and gravity. Convection currents carry
the rigid crust away from the midocean ridge like a conveyor belt and
drive it into the mantle at trenches.
Sounds like ‘geopoetry’ even according to Hess, but by golly he was right.
Harry Hess and Sea Floor Spreading 1960’s
Hess suggested that the continents may be moving along with the sea floor,
not plowing through it as Wegener suggested. If the continents are not
plowing the sea floor must be moving with it.
The sea floor moves away from
the MOR as a result of mantle
convection. According to this
concept, the sea floor is moving
like a conveyor belt away from the
crest of the MOR, down the flanks
and across the deep ocean basin,
to disappear finally by plunging
into the mantle along trenches.
The ridge crest or spreading axis is stationary. The driving force pushing the sea
floor laterally away from the central axis is the result of convection currents driven
by the rising of hot material/sinking of cold material in the mantle. Locations of
spreading ridges (upwelling) and trenches (downwelling) are determined by
the convection cells.
The Theory of Plate Tectonics
Back to those paleomagnetic reversals.
Vine, Matthews,
and Dietz found
that most
magnetic
anomalies at sea
are arranged in
bands that lie
parallel to the rift
valley of the
MOR.
Alternating normal and reversely polarized
rock form a symmetrical stripe-like pattern
parallel to the ridge crest.
Plate Tectonics = Sea floor
spreading + continental drift
+ PROOF.
•
•
•
At MOR new sea floor is added and spreads laterally from the axis. As the magma cools
and the iron bearing minerals crystallize, like tiny compass needles they align
themselves parallel to the lines of force of the earth’s magnetic field.
Magnetic Reversals. The earth’s magnetic field has flipped or revered polarity
throughout earth’s history.
Therefore, the sea floor is a ticker tape recording of the earth’s magnetism through
geologic time. (Only for about ~200million years….why?)
Sediments deposited on the sea floor and radiometric dating of
basalt have ages no older than ~200 million years. Anything older
has been recycled during subduction…so there is no sea floor older
than the last Pangaea (~200-250 mya).
The rocks of the sea floor,
and the bottommost
sediments deposited on
them, are youngest close to
the MOR and become
progressively older the
farther away they are from
the ridges on either side.
The age pattern is
symmetrical across the
ridge.
The patterns measure rate
of sea floor spreading,
where larger age bands
indicate faster movement.
Other Supportive Data
A hot spot is a persistent
volcanic center thought to
be located directly above a
rising plume of hot mantle
rock. Its magma rises
through the lithosphere to
erupt and form a volcano or
volcanic island.
Hot spot mantle plumes remain stationary
while the lithosphere moves over it. This
process forms a chain of volcanic islands.
The chain of islands formed indicates the
direction of plate movement over the hot
spot.
Other Supportive Data
Interactions at Plate Boundaries
Plate boundaries
are associated with
active margins.
The distribution of earthquakes
and volcanic eruptions indicate
that these phenomena are
concentrated in belts or linear
chains at the boundaries of the
lithospheric plates.
PLATE
BOUNDARIES
Notice the three
different types of plate
boundaries.
All plate boundaries
are associated with
either volcanism,
earthquakes, or both.
PLATE BOUNDARIES
Interactions at Plate Boundaries
Divergent Plate Boundaries
Also called spreading centers and rifts; occurs where
two plates move apart horizontally and new lithosphere
is created.
Hot, plastic asthenosphere convects upward to form new
lithosphere in the gap left by the diverging plates.
Rates of sea
floor spreading
varies globally
(1-17 cm/year).
Interactions at Plate Boundaries
Continent-Continent Divergent Boundaries
Continental rifting
results from
upwelling mantle
beneath the
continent. The
continent thins out
and is eventually
torn apart producing
earthquakes and
volcanic eruption of
basaltic magma.
The upward rise of
basaltic magma
forms new oceanic
crust between the
two diverging
continents.
Continent-Continent
Divergent Boundaries
Examples include:
The break-up of
Pangea.
The East African Rift
Valley
Ocean-Ocean Divergent Boundaries
Interactions at Plate Boundaries
Sea-Floor Spreading continues along Mid-ocean ridges as rising
basaltic magma convects upward forming new ocean floor.
Examples
include:
Mid-Atlantic
Ridge spreading
~1 cm/yr;
East Pacific
Rise spreading
~6 cm/yr.
Interactions at Plate Boundaries
Convergent Plate Boundaries
Develops where two plates are moving horizontally toward each
other and therefore are colliding. Can result in orogenic events
(mountain building) or volcanism and deep ocean trenches.
Depends on the plates involved.
Convergent Plate
Boundaries
Interactions at Plate Boundaries
Continent-Ocean Convergent Boundary
Oceanic crust is denser (more Fe and Mg) than continental crust.
When they collide, the denser oceanic plate will SUBDUCT beneath that of the lower
density continental plate.
•Deep sea trench and accretionary wedge.
As the oceanic slab sinks into the mantle
along the trench, some of the sediments
carried on the subducting plate are scraped
off and plastered against the edge of the
overriding crust, forming and accretionary
wedge.
•Volcanism. Heat from friction and confining
pressure will cause water from the
subducting sediments into the overlying
mantle, inducing melting of the subducting
plate and partial melting of mantle rocks at
reduced temperatures. The mafic magma,
being less dense than the surrounding
mantle, rises up through the crust to erupt
onto the surface as a volcanic arc. Here, it
may partially melt the felsic continental crust
and subsequently produce intermediate lava.
•Earthquakes.
Examples include the Andes
Mountains in South America, the
Cascade Mountains in Western US
Interactions at Plate Boundaries
Continent-Continent Convergent Boundary
•
Orogeny. When two plates carrying
continental crust converge (after all the
oceanic crust separating them is consumed
by subduction) neither plate will subduct
because of low densities. The result is
collision and construction of large scale
high pointy mountain chains. The new
mountain ranges include faulted, deformed
and metamorphosed sedimentary rocks,
fragments of the volcanic arc, and pieces of
oceanic crust.
•
Earthquakes
Examples include the
Appalachian Mountains
(from collision of
Gondwanaland and
North America during
the formation of
Pangaea), the Himalayas
from the collision of
India with Asia.
Interactions at Plate Boundaries
Ocean-Ocean Convergent Boundary
Collision of two oceanic slabs will result in the descent of one below the other initiating
volcanic activity in a similar manner to ocean-continent collision. In the case of two
oceanic plates colliding, the older (colder, denser) oceanic crust subducts.
• Deep sea trench and accretionary wedge.
• Volcanism. Forms island arc.
• Earthquakes.
Examples include
Japan, Aleutian
islands, Carribean
islands
Interactions at Plate Boundaries
Transform Plate Boundaries
Occurs when two lithospheric plates slide past one another
horizontally, and neither is subducted.
• Not associated with volcanism or mountain building.
• Lots of shallow earthquakes.
Ocean-Ocean Transform
Boundaries. Ridge-ridge
transform boundaries.
Major offsets of mid-ocean
ridge axis.
Continent-Continent Transform Boundaries.
Example:San Andreas Fault
Driving force is not just convection.
Ridge Push and Slab Pull
Ridge Push and Slab Pull
Plates tied to
subducting
limbs spread
faster.
Examples include:
Mid-Atlantic Ridge
spreading ~1 cm/yr;
East Pacific Rise
spreading ~6 cm/yr.