Transcript 2 Review Plate Tectonics l

Earth’s Internal Structure

Earth’s internal layers defined by
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Chemical composition
Physical properties
Deduced from Seismographs of Earthquakes
Meteorites lend support
Layers defined by composition
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Crust
Mantle
Core
Iron-Nickel Meteorite
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Evidence: Density of Earth’s
Layers
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“Three centuries ago, the English scientist
Isaac Newton calculated, from his studies of
planets and the force of gravity, that the
average density of the Earth is twice that of
surface rocks and therefore that the Earth's
interior must be composed of much denser
material. “
http://pubs.usgs.gov/gip/interior/
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The S-Wave Shadow Zone
http://en.wikipedia.org/wiki/Richard_Dixon_Oldham
Since Shear (S) waves
cannot travel through
liquids, the liquid
outer core casts a
larger shadow for S
waves covering
everything past 103
degrees away from
the source.
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The P-Wave Shadow Zone
http://www.amnh.org/education/resources/rfl/web/essaybooks/earth/p_lehmann.html
P-waves through the liquid
outer core bend, leaving a
low intensity shadow zone
103 to 143 degrees away
from the source, here
shown as the north pole
HOWEVER, P-waves
traveling straight through
the center continue, and
because speeds in the
solid inner core are faster,
they arrive sooner than
expected if the core was
all liquid.
Inge Lehmann
Behavior of waves through center reveal Earth’s Interior
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Earth’s internal structure

Main layers of Earth are based on physical properties
including mechanical strength
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Outer layers mostly Silicate Minerals: Crust and Mantle
 Lithosphere (behaves like a brittle solid)
Crust and uppermost mantle
 Asthenosphere “weak sphere”
Rest of Upper Mantle
Heat softened, plastic solid
 Lower Mantle
Solid due High Pressures
• Inner Layers Core Iron and Nickel,
outer above melting point - liquid,
inner solid due to high pressures
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1_8
CRUST
(least dense)
Upper mantle
Continental crust
Oceanic crust
MANTLE
0 km
~100 km
~350 km
Lower mantle
Lithosphere
Asthenosphere
CORE
(most dense)
Outer
core
~5155 km
Inner
core
~2900 km
Earth’s center is 6371 kilometers
below the surface, 1 mi = 1.61 km.
Equals ~ 3957 miles, or about
4000 miles radius
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Harry Hess: Mid-ocean ridges are spreading apart due to flow in
the mantle. Crust moves apart as if on conveyer belts.
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Mid-ocean
1_20
Ridge
Origin of new Ocean Floor
At the Mid-Ocean Ridge
• Mantle material is moved
near the surface.
• Lithosphere (Crust + Upper
Mantle) bulges into a midocean ridge.
• It cracks, exposing the
mantle to low pressures
• Some of the Mantle minerals
are unstable at atmospheric
pressures
• The unstable minerals melt
forming lavas, and cool into
basalt, the main rock of ocean
lithosphere.
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How can we test the hypothesis?
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Fred Vine: How about geomagnetic
reversals?
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Earth's magnetic field periodically reverses
polarity – north magnetic pole becomes south
magnetic pole, and vice versa
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Dates when polarity of Earth’s magnetism
changed were determined from lava flows
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Magnetite
crystals align
with magnetic
field
Away from
equator and
poles they dip
toward the
North or south
poles
When the
Earths polarity
switches, new
lavas adjacent
older, point in
opposite
directions
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Paleomagnetic reversals recorded by
new lava rock at mid-ocean ridges
This lava rock is
called “Basalt”
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Test 1:
Princeton PostDoc Fred Vine
So, they checked. NOT FALSE
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Test 2: Oceanic Crust youngest at ridges
Hess model prediction: youngest at ridges, oldest at trenches
Also NOT FALSE
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95% of energy released by earthquakes originates in narrow zones
that wind around the Earth
These zones mark of edges of tectonic plates
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Broad are subduction zone earthquakes, narrow are MOR. Lead to recognition of plates
Earthquake depth from Trench to Arc
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Structure of
three Plates
Youngest at ridges,
oldest at trenches
NOT FALSE
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Three boundary types, divergent, convergent, and transform
Plate tectonics: The new paradigm
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Earth’s major plates
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Associated with Earth's strong, rigid outer
layer
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Known as the lithosphere
Consists of uppermost mantle and overlying
crust
Overlies a weaker region in the mantle called
the asthenosphere. The Asthenosphere is hot
and plastic, and sheds heat via convective
currents.
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Mantle circulations are an example of convection, heat transfer by moving fluids
This example shows transfer of core heat to the upper mantle and crust
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180º
90º
0º
90º
180º
Mid-Atlantic
Ridge
1_15
45º
45º
NORTH
AMERICAN
PLATE
JUAN DE
FUCA
PLATE
EURASIAN
PLATE
ARABIAN
PLATE
CARIBBEAN
PLATE
PACIFIC
PLATE
PHILIPPINE
PLATE
AFRICAN
PLATE
COCOS
PLATE
0º
PACIFIC
PLATE
FIJI
PLATE
SOUTH
AMERICAN
PLATE
NAZCA
PLATE
SCOTIA
PLATE
45º
0º
Mid-Atlantic
Ridge
INDIANAUSTRALIAN
PLATE
45º
ANTARCTIC PLATE
180º
Convergent plate
boundary
Divergent plate
boundary
Transform plate
boundary
ANTARCTIC PLATE
90º
0º
90º
180º
Plates move relative to each other at a very slow but con
Seven major lithospheric plates
Average about 5 centimeters (2 inches) per year
Seven or so smaller ones.
Cooler, denser slabs of oceanic lithosphere desce
Plates are in motion and change in shape and size
Largest plate is the Pacific plate
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Several plates include an entire continent plus a large area of seafloor
1_22a
Plate Tectonics
Concept caused revelation. Yes, revelation. Earth’s many features were
all caused by the same process.
Oceanic lithosphere
being subducted
(a)
Water driven out of
ocean lithosphere
Water hits mantle,
which partially melts.
Forms a deep basaltic
magma
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Plate boundaries
Each plate bounded by combination of all three
boundary types: divergent, convergent, transform
Edges marked by Earthquakes
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Three main plate boundaries
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Divergent boundaries are located
mainly along oceanic ridges
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The East African Rift
The rift valley
collects river and
lake sediments.
Land animals
are preserved as
fossils instead of
being eroded
away
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Continental Rift into Ocean Basin
Rift Valleys
and Oceans
are the
same thing
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Convergent plate boundaries
•On the other side of a plate, opposite the diverging
margin, a converging margin is usual.
•Three different types, formed from pushing
together of ocean floors and/or continental plate
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Types of Convergent Boundaries
Define:
Density, Buoyancy,
Gravity
Descending convective
cell this side
Ocean-Continent
Yields Continental
Volcanic Arc
Ocean-Ocean
Yields Volcanic Island
Arc
Descending convective
cell this side
Continent-Continent
Yields Collision Mtns.
Alps, Himalayas,
Appalachians
Descending convective27
cell this side
1_22a
1. Ocean - continent convergence
A volcano forms as magma
reaches the surface
Oceanic lithosphere
being subducted
(a)
A Subduction Zone
As plate descends into the Subduction Zone, partial melting of mantle rock makes magmas
(Molten Rock) These are buoyant, and rise.
Volcanic mountains associated with subduction are called volcanic arcs.
Andes and Cascades mountains are continental volcanic arcs
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Types of convergent boundaries:
2. Oceanic-oceanic convergence
When two oceanic slabs converge, one descends
beneath the other.
Often forms volcanoes on the ocean floor above the
subduction zone.
If the volcanoes emerge as islands, a volcanic island
arc is formed (Japan, Aleutian islands, Tonga islands)
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3. Continental-continental convergence
1_22b
• Continued subduction brings
continents together
• Less dense, buoyant, thick continental lithosphere does not subduct
•Result is a collision between two continental blocks. Process produces folded
mountains (Himalayas, Alps, Appalachians)
Collisional
mountains
Fault and Fold Mountains
(b)
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Rocks deformed in collision
The collision of India and Asia
produced the Himalayas
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Transform fault boundaries
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Third type of plate boundary
Plates slide past one another and no new lithosphere is
created or destroyed
Transform faults
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Most join two segments of a mid-ocean ridge
(MOR) as parts of linear breaks in the oceanic
crust known as fracture zones
Accommodate simultaneous movement of offset
ridges
Source of weak (MOR) to fairly strong (San
Andreas) earthquakes.
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Transform faults accommodate
movement on offset ridge segments
Plates are moving in
opposite directions
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Mt. Redoubt
volcano
Anchorage
Fault and Fold
mountains from earlier
Continent-Continent collisions
NORTH AMERICAN PLATE
Bering
Sea
JUAN DE FUCA PLATE
Aleutian
Islands
Ocean-to-ocean
subduction
(a)
Mt. Saint
Helens volcano
PACIFIC
PLATE
Convergent and Divergent,
Margins of Plates plus small
transform margins between
MOR segments
Small Transform Faults
Cascade Range
from dewatering
Ocean-to-continent
subduction
Mid-ocean ridge (divergent margin)
Studied by Fred Vine and Drummond Matthews
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Plate Tectonics Explains It All
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We now understand mountains, volcanoes,
and big earthquakes associated with, for
example, the San Andres fault.
We understand rift valleys and how oceans
form, deep ocean trenches, mid ocean
ridges, why fossils and mountain ranges look
alike across vast oceans.
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