Geologic Structure of Earth - The interior of

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Chapter 3 Earth Structure

• Geologic Structure of Earth - The interior of
• the Earth is layered.
• Concentric layers crust, mantle, liquid outer
  core and solid inner core.
• Evidence (indirect) for this structure comes from
  studies of Earths dimensions, density, rotation,
  gravity, magnetic field, behavior of seismic
  waves and meteorites.

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Density is a key concept for understanding the
structure of Earth differences in density lead
to stratification (layers). Density measures
the mass per unit volume of a substance. Density
_Mass_ Volume Water has a
density of 1 g/cm3 Granite Rock is 2.7 g/cm3
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Evidence that supports the idea that Earth has
layers comes from the way seismic waves behave as
they encounter different material inside Earth
and as the material is either liquid or solid
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• Earths layers chemical composition and
  physical properties
• Core 3500 km thick, average density 13 g/cm3,
  30 of
• Earths mass and 16 of its volume
• Inner core radius of 1200 km, primarily Fe
  Ni
• _at_Temp of 4000-5500C, solid, av. den. 16 g/cm3
  
• Outer core 2260 km thick, Temp of 3200C,
• liquid (partially melted), viscous, less dense
• Mantle 70 Earths mass 80 of its volume,
  2866 km
• thick, _at_ Temp of 100-3200C, Mg-Fe silicates,
  solid but
• can flow, average density 4.5 g/cm3
• Note inner core may be rotating faster than
  mantle can be hotter than the Suns surface
  (more than 6, 500 deg C!!)
• Earths outer layer is the Crust cool, rigid,
  thin surface
• layer rocks on crust side are chemically
  different than
• rocks on mantle side separation is called
  Mohorovicic
• discontinuity

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Earths Crust cold, brittle

• thin layer, 0.4 of Earths mass and 1 of its
  volume
• Continental Crust
• Primarily granitic type rock (Na, K, Al, SiO2)
• 40 km thick on average
• Relatively light, 2.7 g/cm3
• Oceanic Crust
• Primarily basaltic (Fe, Mg, Ca, low SiO2)
• 7 km thick
• Relatively dense, 2.9 g/cm3
• cool, solid crust and upper (rigid) mantle
  float
• and move over hotter, deformable lower mantle

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Lithosphere Asthenosphere More detailed
description of Earths layered structure
according to mechanical behavior of rocks, which
ranges from very rigid to deformable
1. lithosphere rigid surface shell that includes
upper mantle and crust (here is where plate
tectonics work), cool layer 2. asthenosphere
layer below lithosphere, part of the mantle, weak
and deformable (ductile, deforms as plates move),
partial melting of material happens here, hotter
layer
(100 200 km)
(200 400 km)
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Summary Table 1 Physical Properties
Layer Chemical Properties
Continental Crust Composed primarily of granite Density 2.7 g/cm3
Oceanic Crust Composed primarily of basalt Density 2.9 g/cm3
Mantle Composed of silicon, oxygen, iron, and magnesium Density 4.5 g/cm3
Core Composed primarily of iron Density 13 g/cm3
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Summary Table 2 Composition
Layer Physical Properties
Lithosphere Cool, rigid, outer layer
Asthenosphere Hot, partially melted layer which flows slowly
Mantle Denser and more slowly flowing than the asthenosphere
Outer Core Dense, viscous liquid layer, extremely hot
Inner Core Solid, very dense and extremely hot
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Isostasy A term used to refer to the state of
gravitational equilibrium between the lithosphere
and the asthenosphere, which makes the plates
(seem like) float at an elevation that depends
on their thickness and density areas of Earths
crust get to this equilibrium after rising and
subsiding until their masses are in balance.
Less dense continental blocks float on the
denser mantle
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Buoyancy a 10 kg object can float if it lands on
a liquid (water) body large enough that the
object can displace a volume of liquid that
weighs 10 kg and there is still more liquid left
displaced water
Buoyancy depends on the mass and density of the
object and of the liquid in which object
floats Icebergs 10 of volume above water, 90
of volume below surface
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Isostatic equilibrium continental mountains
float high above sea level because the
lithosphere sinks slowly into the deformable
asthenosphere until it has displaced a volume of
asthenosphere equal to the mass of the mountains
mass. Very slow process if it goes too fast for
some reason then the rock will crack (fracture)
and a fault occurs, and cause earthquakes
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Chapter 3 on to Plate Tectonics

• Movement of the Continents Continental Drift
• Continents had once been together advanced by
• Alfred Wegener during the 1920s
• Ultimately rejected Until new technology
• provided evidence to support his ideas.
• Seismographs revealed a pattern of volcanoes and
• earthquakes.
• Radiometric dating of rocks revealed a
  surprisingly young
• oceanic crust.
• Echo sounders revealed the shape of the
  Mid-Atlantic
• Ridge

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Evidence for Seafloor Spreading

• Earthquake epicenters
• Heat flow
• Ocean Sediments
• Radiometric dating of rocks of ocean and
  continental crust
• Magnetism

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• Synthesis of Continental Drift and Seafloor
  Spreading --gt Theory of Plate Tectonics
• Main points of theory (Wilson, 1965)
• Earths outer layer is divided into lithospheric
  plate
• Earths plates float on the asthenosphere
• Plate movement is powered by convection currents
  in the
• asthenosphere seafloor spreading, and the
  downward
• pull of a descending plates leading edge.
• Hess and Dietz in 1960 proposed a model to
  explain features of ocean floor and of
  continental motion powered by heat ? mantle
  convection

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heat transfer conduction (contact)
convection (motion of an agent,
currents)
tectonic plate is the cool surface, the result of
a convection current rising from the (hot) upper
mantle (spreading center) as it cools it
becomes denser so gravity pulls it down
(subduction zone)
heated water rises, cools at the surface and
falls around the containers edge
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Model of Mantle Convection
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Divergent plate boundary marked by mid-ocean
ridge (spreading center)
Convergent plate boundary marked by trench
Transform fault
Oceanic lithosphere
Subduction fueling volcanoes
Asthenosphere
Asia
Africa
Descending plate pulled down by gravity
Mantle upwelling
Superplume
Philippine Trench
Outer core
Mariana Trench
Mantle
Mid-Atlantic Ridge
Inner core
Rapid convection at hot spots
Hot
Cold
Possible convection cells
South America
PeruChile Trench
Hawaii
East Pacific Rise
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Age and thickness of sea floor sediment heat
flow
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Earthquake Epicenters
Shallow epicenters crustal movement (less than
100 km)
Mid-deep epicenters subduction (greater than 100
km)
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The Earths Magnetic Field

• Rocks record the direction of magnetic field
  (Magnetite)
• Magnetic field direction changes through geologic
  time polar reversals recorded in rocks
• 560 C rock solidifies (Curie Point)
• Captures magnetic signature
• Particles of Magnetite align with the direction
  of Earths magnetic field at the time of rock
  formation

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Magnetites occur naturally in basaltic magma and
act as compass needles
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The patterns of paleomagnetism support plate
tectonic theory. The molten rocks at the
spreading center take on the polarity of the
planet while they are cooling. When Earths
polarity reverses, the polarity of newly formed
rock changes. (a) When scientists conducted a
magnetic survey of a spreading center, the
Mid-Atlantic Ridge, they found bands of weaker
and stronger magnetic fields frozen in the rocks.
(b) The molten rocks forming at the spreading
center take on the polarity of the planet when
they are cooling and then move slowly in both
directions from the center. When Earths magnetic
field reverses, the polarity of new-formed rocks
changes, creating symmetrical bands of opposite
polarity
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Plates ? Rigid Slabs of Rock
Seven major plates Pacific, African, Eurasian,
North American, Antarctic, South American,
Australian Minor plates Nazca, Indian,
Arabian, Philippine, Caribbean, Cocos, Scotia,
Juan de Fuca
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Plate boundaries in action (1) plates move
apart, (2) plates move toward each other, (3)
plates move past each other
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• As plates float on the deformable
  aesthenosphere, they interact among each other.
  The result of these interactions is the existence
  of 3 types of boundaries
• (a) Divergent plates move away from each other,
  examples
• Divergent oceanic crust
• the Mid-Atlantic
  Ridge
• Divergent continental crust
• the Rift Valley of East Africa
• (b) Convergent plates move toward each other.
• Three possible combinations continent-ocean,
  ocean-ocean, continent-continent
• (c) Transform
• neither (a) nor (b), plates slide
• past one another transform faults.
• Example San Andreas fault

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Fracture Zones-Transform faults
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Oceans are created along divergent boundaries

• Recall that seafloor spreading was an idea
  proposed in 1960 to explain the features of the
  ocean floor. It explained the development of the
  seafloor at the Mid-Atlantic Ridge. Convection
  currents in the mantle were proposed as the force
  that caused the ocean to grow and the continents
  to move.
• The breakdown of Pangea showing spreading centers
  and mid-ocean ridges

2 kinds of plate divergences
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Mid Atlantic Ridge South Indian Ridge
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Modern divergenceEast African Rift System
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East African Rift System
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Wilson Cycles
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Supercontinent Formation (Ma) Breakup (Ma)
Mode Ref. Pangaea 350 250 Atlantic
(4) Pannotia/Gondwanaland 650 550 Pacific
(4, 5) Rodinia 900 760 Pacific (4, 6,
39) Nuna 1800 1500
Atlantic? (4)
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Island Arcs Form, Continents Collide, and Crust
Recycles at Convergent Plate Boundaries

• Convergent Plate Boundaries - Regions where
  plates are pushing together can be further
  classified as
• Oceanic crust toward continental crust - the west
  coast of South America.
• Oceanic crust toward oceanic crust - occurring in
  the northern Pacific.
• Continental crust toward continental crust one
  example is the Himalayas.

3 kinds of plate convergences
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Convergent Plate Boundaries

• Continent Ocean
• Ocean Ocean
• Continent Continent

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Continent Ocean West Coast of South America
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• Continent Ocean
• Mount St. Helens

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Island Arcs Form, Continents Collide, and Crust
Recycles at Convergent Plate Boundaries
The formation of an island arc along a trench as
two oceanic plates converge. The volcanic islands
form as masses of magma reach the seafloor. The
Japanese islands were formed in this way.
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Convergent Plate Boundaries Ocean-Ocean Aleutian
Islands, Alaska
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Ocean Ocean Caribbean Islands
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Plate Movement above Mantle Plumes and Hot Spots
Provides Evidence of Plate Tectonics
(See also Figure 3.33 on page 89 of textbook)
Formation of a volcanic island chain as an
oceanic plate moves over a stationary mantle
plume and hot spot. In this example, showing the
formation of the Hawaiian Islands, Loihi is such
a newly forming island.
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• Chapter 3 Summary
• Keep in mind that the important points in this
  chapter are
• Internal Layers inner core, outer core, mantle,
  crust (continental and oceanic).
• P and S waves used to study Earths layered
  structure
• Lithosphere and Asthenosphere defined according
  to mechanical behavior of rocks
• Isostasy pressure balance between overlying
  crust and astheosphere deformation
• Continental drift plates/continents moving
  about surface deduced from definitive evidence
  ridges, rise, trench system, sea-floor spreading,
  spreading centers, subduction zones
• Evidence of crustal motion earthquakes
  epicenter, heat flow, radiometric dating,
  magnetism
• Plate Tectonics 7-8 major plates, 3 types of
  plate boundaries
• Convergent Plate Boundaries ocean-continent,
  ocean-ocean, continent-continent

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Chapter 3 Key Concepts Some seismic
wavesenergy associated with earthquakescan pass
through Earth. Analysis of how these waves are
changed, and the time required for their passage,
has told researchers much about conditions inside
Earth. Earth is composed of concentric spherical
layers, with the least dense layer on the outside
and the most dense as the core. The lithosphere,
the outermost solid shell that includes the
crust, floats on the hot, deformable
asthenosphere. The mantle is the largest of the
layers. Large regions of Earths continents are
held above sea level by isostatic equilibrium, a
process analogous to a ship floating in
water. Plate motion is driven by slow convection
(heat-generated) currents flowing in the mantle.
Most of the heat that drives the plates is
generated by the decay of radioactive elements
within Earth.
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Chapter 3 Key Concepts Plate tectonics theory
suggests that Earths surface is not a static
arrangement of continents and ocean, but a
dynamic mosaic of jostling segments called
lithospheric plates. The plates have collided,
moved apart, and slipped past one another since
Earths crust first solidified. The confirmation
of plate tectonics rests on diverse scientific
studies from many disciplines. Among the most
convincing is the study of paleomagnetism, the
orientation of Earths magnetic field frozen into
rock as it solidifies. Most of the
large-scale features seen at Earths surface may
be explained by the interactions of plate
tectonics. Plate tectonics also explains why our
ancient planet has surprisingly young seafloors,
the oldest of which is only as old as the
dinosaurs that is, about 1/23 of the age of
Earth.