Transcript Plate Tectonics

The Active Earth:
Plate Tectonics
Objectives of this Chapter:
i. Describe plate tectonic theory
ii. Discuss the development of plate tectonic
theory
iii. Draw the major types of plate boundaries
and list their major features
iv. Explain the forces that drive the plates.
v. Describe how isostacy works
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The Earth evolved
from a ball of dust and
gas that formed our
Solar System about
5 billion years ago to
the relatively cool
surface that we know
today. However, the
Earth’s interior remains
hot, and drives volcanic
eruptions, earthquakes,
mountain building and other geologic processes that have created our
physical environment. These processes can be explained by Plate
Tectonics, a theory that revolutionized the Earth sciences.
Unit 2-CO, p.121
Plate Tectonics
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View from Space Shuttle Columbia. Tectonic forces are
widening the Red Sea and pushing the Sinai Peninsula further
away from Africa at a rate of about 1 cm/year.
Fig. 6-CO, p.122
Plate Tectonics Theory
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A simple, unifying theory that has revolutionized
our understanding of the way the Earth works and
how its systems interact to create our environment.
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This scientific revolution developed over many
years, building on earlier observations, hypothesis
and theories. The story shows how a scientific
theory evolves, and how scientists rely on the work
and discoveries of earlier scientists.
Development of Plate
Tectonics theory.
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Alfred Wegener
(German Scientist;
1930) noticed,
among other things,
how the continents
of South America
and Africa, now
separated by the
Atlantic Ocean, fit
together like a
jigsaw puzzle.
Fig. 6-1, p.123
Pangea:
Laurasia and
Gondwanaland
In fact, all of
the continents,
when moved
properly, fit
together like a
puzzle into a
supercontinent
he called
Pangea. He
then looked
for other evidence, and could match fossils across continents.
Fig. 6-2, p.124
Other evidence
supporting Pangea
(continued)
Along with fossil
evidence, he matched
sedimentary rock
deposits to climatic
zones. By plotting 250
million year old glacial
deposits on a map (as
found today), they would
have formed in tropical
and subtropical areas, and
crossed the oceans!
What happens if you
assemble the continents
into Pangea (lower
diagram)?
Fig. 6-3, p.125
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Glacial Evidence on modern distribution of the
continents.
Fig. 6-3a, p.125
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Glacial deposits and other climate-sensitive
sedimentary rocks plotted on Wegener’s map of
Pangea. Now deposits match climate zones. Fig. 6-3b, p.125
Rock Correlation
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Wegener also
matched certain
sequences of rock
across the
continents. He
plotted these rock
types on his Pangea
map and found
those on the east
side of the Atlantic
Ocean were
continuous with
those on the west
side.
Continental Drift
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Wegener’s concept of a single supercontinent
that broke apart to form the modern continents
is called the theory of continental drift.
However, he could not explain how the
continents moved, and it would take 30 years
after his death in 1930 before new evidence
would be discovered that would rejuvenate his
theory.
The Earth’s
Layers
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To understand
Plate Tectonics,
you must
understand that
the Earth is
layered (how do
we know this?).
Fig. 6-4, p.127
Table 6-1, p.127
The seafloor and Mid-Oceanic Ridge
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After world war II, we began mapping the seafloor, and discovered
the largest mountain chain on Earth (the Mid-Oceanic Ridge which
Fig. 6-5, p.129
circles the globe like seams of a baseball).
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We discovered
magnetic stripes:
as lava cools and
becomes basalt
(which in rich in
Fe) along the
seafloor, the Ferich minerals
become weak
magnets and align
parallel to the
Earth’s magnetic
field. With
magnetometers,
oceanographers
can record
magnetic patterns.
Fig. 6-6, p.129
We found that
rock records
“normal” and
“reversed” polarity,
forming magnetic
stripes symmetrical
about the ridge axis.
Vine, Matthews and
Morley, three
scientists, proposed
the sea floor is
spreading away from
the Mid-Oceanic Ridge (continuously and like conveyor belts). New
basalt lava rises at the ridge as the sea floor separates. When the basalt
cools, it acquires the magnetic orientation of the Earth’s field.
Fig. 6-7, p.130
Seafloor Spreading
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Reversals in the Earth’s magnetic field, which are
known to occur about every 500,000 years, are shown
by the magnetic stripes…each stripe represents new
seafloor formed over ½ million years; where is the
old seafloor?
Discovered fossils in the seafloor sediments
(overlying the basalt) were young at the ridges and
older away from the ridge axis
Discovered the layer of mud that overlies basalt at the
seafloor becomes progressively thicker away from the
ridge axis.
The Seafloor Spreading hypothesis became the
general model for the origin of all oceanic crust, and
the basis for the Theory of Plate Tectonics (along
with Continental Drift).
Tectonic Plates and Movement
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The lithosphere is a hard shell of strong rock about 100 km
thick that floats on the hot, plastic asthenosphere. The
lithosphere is broken into several large plates (and several
smaller ones) that glide slowly (1-16 cm/yr), and move
continents and oceans with them.
Fig. 6-8, p.131
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Three types of plate boundaries are known (fractures that separate one
plate from another). Plates move relative to one another in three
ways:
Fig. 6-9, p.132
Table 6-2, p.133
Divergent Plate Boundary (spreading center)
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Lithospheric plates move away from a spreading center by
gliding over the weak, plastic asthenosphere. New
lithosphere forms at the ridge, and is thin at this area, but
thickens as it becomes cooler away from the ridge. Note
subduction zones.
Fig. 6-10, p.133
Divergent Plate Boundary:
Rift Zone
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The continent of Africa
is splitting apart along
the East African Rift.
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Where else in the
world is this
occurring?
Fig. 6-11, p.134
Convergent
Plate
Boundary
One type is
continentalcontinental
convergence.
Collision
between India
and Asia
created the
Himalayan
mountain
chain.
Fig. 6-12, p.135
Transform Plate Boundary
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Continentalcontinental
transform plate
boundary; plates
slide
horizontally past
one another.
What is a plate?
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Segment of the lithosphere (uppermost rigid mantle and crust).
Can carry both oceanic and continental crust. Avg. thickness of
lithosphere covered by oceanic crust is 75 km; covered by continental
crust is 125 km (how thick at spreading centers?).
Composed of hard, strong rock
Floats and glides on underlying hot, plastic asthenosphere (like a slab
of ice on a pond).
Margins are tectonically active (EQ, Volcanoes); interiors are
generally stable.
Move at rates from 1-16 cm/yr. Continents and oceans then move
across the Earth at the rate plates move. Manhattan Island then is 9
meters farther from London than it was 225 years ago when the
Declaration of Independence was written.
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Why do plates
move?
Recall Wegener
couldn’t explain
this. Today
convection of the
mantle is thought
to drive plate
motion. Heat is
from the core and
from
radioactivity.
Fig. 6-13, p.136
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New lithosphere glides downslope away from a spreading
center. The old, cool part of the plate sinks into the mantle
at a subduction zone, pulling the rest of the plate along with
it.
Fig. 6-14, p.137
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Mantle Plumes and Hot Spots
Rising column of hot,
plastic mantle rock.
As pressure decreases
in a rising plume,
magma forms at a hot
spot in the mantle just
below the lithosphere,
and rises to erupt
from volcanoes on the
Earths surface.
Can you name a place
where this occurs, in
the middle of an
Ocean?
Isostasy:
Vertical Movement of
the lithosphere
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Add mass to the
lithosphere, it
settles and
underlying
asthenosphere
flows laterally
away to make
room (like when
you step onto a
boat).
Fig. 6-15, p.138
Isostasy (continued)
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How is mass added or subtracted from the lithosphere? The
lithosphere in floating equilibrium on the asthenosphere is
isostasy.
Fig. 6-15a, p.138
Isostasy (continued)
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Vertical movement in response to changing burden is called
isostatic adjustment.
Fig. 6-15b, p.138
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Just like a large iceberg,
continental crust extends
more deeply into the
mantle beneath high
mountain ranges than it
does under the plains.
Oceanic lithosphere is
thinner and more dense,
so it floats at a lower
level.
Fig. 6-16, p.138
How Plate Movements affect Earth’s Systems
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Generate volcanic eruptions, earthquakes, build mountains and change
global distribution of continents and oceans.
Where do volcanoes occur? Eruptions can alter the atmosphere to
change global climate. How? What are consequences of ash, sulfur
compounds and Co2 in the atmosphere? How did the eruption of
Mount Pinatubo (1991) affect the Earth’s overall temperature? What
about the Permian extinction 248 million years ago? What about 120
million years ago during the formation of a lava plateau near Peru?
Mountains strongly affect global wind and precipitation patterns.
Migration of continents and oceans affects wind and ocean currents.
Probably not much of an affect during our lifetime.
Earthquakes alter the human environment.
Eruption of Mount St. Helens, 1980
Fig. 6-17, p.139
Fig. 6-18, p.142
p.144