Chapter 3 – Earth Structure
Waves associated with earthquakes:
P waves – Primary, compressional,
compressional, arrive first, pass
through solid, liquid and gas, oscillate in the direction
of propagation
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 Earth’
Earth’s dimensions, density,
rotation, gravity, magnetic field, behavior of
seismic waves and meteorites.
S waves – Secondary, ‘sideside-toto-side’
side’ or shear waves,
arrive second, cannot pass through liquid, pass through
solid, oscillate in the direction transverse to
propagation
Seismograph
P
S
Mantle
1
Focus
2
0
10
P
Core
S
20
Minutes
30
40
50
Surface
waves
4
5
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
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
Density is expressed as grams per cubic centimeter.
Water has a density of 1 g/cm3
Granite Rock is about 2.7 times more dense
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Earth’
Earth’s layers – chemical composition and
physical properties
Summary Table 1 – Physical Properties
¾Core:
Core: ~ 3500 km thick, average density 13 g/cm3, 30% of
Earth’
Earth’s mass and 16% of its volume
• Inner core:
core: radius of 1200 km, primarily Fe & Ni
@Temp of 40004000-5500°
5500°C, solid, av. den. 16 g/cm3
• Outer core:
3200°C,
core: 2260 km thick, Temp of 3200°
liquid (partially melted), viscous, less dense
¾Mantle:
Earth’s mass & 80% of its volume, 2866 km
Mantle: 70% Earth’
thick, @ Temp of 100100-3200°
3200°C, MgMg-Fe silicates, solid but
can flow, average density 4.5 g/cm3
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
Note: inner core may be rotating faster than mantle – can be hotter than the Sun’
Sun’s surface (more than 6, 500 deg C!!)
¾Earth’
Earth’s 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 Mohorovičić
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discontinuity
Earth’s Crust: cold, brittle
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Summary Table 2 – Composition
thin layer, 0.4% of Earth’s mass and 1% of its
volume
¾Continental Crust:
•Primarily granitic type rock (Na, K, Al, SiO2)
•40 km thick on average
•Relatively light, 2.7 g/cm3
¾Oceanic Crust
•Primarily basaltic (Fe, Mg, Ca, low SiO2)
•7 km thick
•Relatively dense, 2.9 g/cm3
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
¾cool, solid crust and upper (rigid) mantle “float”
and move over hotter, deformable lower mantle
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Lithosphere & Asthenosphere:: More detailed
description of Earth’s 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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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 Earth’s 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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2
displaced water
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
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
Earthquake epicenters
Heat flow
Ocean Sediments
Radiometric dating of rocks of ocean and
continental crust
• Magnetism
•
•
•
•
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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
mountain’s 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
Evidence for Seafloor Spreading
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Age of Earth was not
easily determined,
nor accepted as ‘that
old’!
The Fit between the Edges
of Continents Suggested That
They Might Have Drifted
The fit (first noticed by Leonardo
da Vinci) of all the continents
around the Atlantic at a water
depth of about 137 meters (450
feet), as calculated in the 1960s.
This well-known graphic was a very
effective kick-off to the ‘tectonic
revolution’.
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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 1920’
1920’s
• Ultimately rejected – Until new technology
provided evidence to support his ideas.
¾Seismographs revealed a pattern of volcanoes and
earthquakes.
¾Radiometric dating of rocks revealed a surprisingly young
oceanic crust.
¾Echo sounders revealed the shape of the Mid-Atlantic
Ridge
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Synthesis of Continental Drift and Seafloor
Spreading -->
--> Theory of Plate Tectonics
Main points of theory (Wilson, 1965):
¾Earth’s outer layer is divided into lithospheric plate
¾Earth’s plates float on the asthenosphere
¾Plate movement is powered by convection currents in the
asthenosphere seafloor spreading, and the downward
pull of a descending plate’s leading edge.
Hess and Dietz in 1960 proposed a model to
explain features of ocean floor and of continental
motion powered by heat Æ mantle convection
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heat transfer: conduction (contact)
Age of sea floor vs. distance from ridge crest
convection (motion of an agent, currents)
Figure 2.13
Lithosphere
A plate is the cooled
surface layer of a
convection current in
upper mantle.
Heat
source
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
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zone)
heated water rises,
cools at the surface
and falls around the
container’s edge
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Age and thickness of sea floor sediment
Model of Mantle Convection
spreading centers – where new sea floor and oceanic lithosphere form
subduction zones – where old oceanic lithosphere descends
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Improved Mapping, WWII
Convergent plate
boundary marked by
trench
Asthenosphere
Africa
Divergent plate boundary
marked by mid-ocean ridge Transform fault
(spreading center)
Oceanic lithosphere
Subduction fueling
volcanoes
Asia
Chapter 3 – Plate Tectonics
Descending plate
pulled down by
gravity
Philippine Trench
Mantle
upwelling
Superplume
Outer core
Mariana Trench
Mantle
Mid-Atlantic
Ridge
Inner
core
Hot
South America
Cold
Possible
convection cells
¾Plate Tectonics – a unified model with ideas
from continental drift and sea floor spreading
¾Lithosphere broken into plates
¾Plates move
Rapid
convection
at hot
spots
¾Boundaries between plates are sites of geologic
activity
Peru–Chile
Trench
Hawaii
East
Pacific Rise
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Earthquake Epicenters
Shallow epicenters –
crustal movement
(less than 100 km)
As plates float on the deformable
aesthenosphere,
aesthenosphere, they interact among
each other. The result of these
interactions is the existence of 3
types of boundaries:
Mid-deep epicenters
subduction
(greater than 100 km)
•
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Plates Æ Rigid Slabs of Rock
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
Seven major plates – Pacific, African, Eurasian, North American,
Antarctic, South American, Australian
Minor plates – Nazca,
Nazca, Indian, Arabian, Philippine, Caribbean, Cocos,
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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Oceans are created along divergent boundaries
East African Rift System
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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Island Arcs Form, Continents Collide, and Crust
Recycles at Convergent Plate Boundaries
Mid Atlantic Ridge
South Indian Ridge
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
Modern divergence
East African Rift System
• Continent – Ocean
• Ocean – Ocean
• Continent – Continent
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Convergent Plate Boundaries
Ocean-Ocean
Aleutian Islands, Alaska
Continent – Ocean
West Coast of South America
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• Continent – Ocean
Ocean – Ocean
Caribbean Islands
• 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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Many discoveries contribute to the theory of
plate tectonics but the most compelling evidence
comes from The Earth’s 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
Earth’s magnetic field at the time of rock formation
Motion of the plates:
plates:
Mechanisms – not fully understood
Rates: average 5 cm/year
MidMid-Atlantic Ridge = 2.5 – 3.0 cm/yr
EastEast-Pacific Rise = 8.0 – 13.0 cm/yr
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axis of Earth’s rotation
Chapter 3: Summary
Keep in mind that the important points in this chapter are:
angle about 11.5°
magnetic NP-SP axis
1.
‘huge’ magnet
2.
3.
4.
Magnetites occur
naturally in basaltic
magma and act as
compass needles
5.
6.
7.
8.
Internal Layers: inner core, outer core, mantle, crust (continental
(continental
and oceanic).
P and S waves – used to study Earth’
Earth’s 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
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 – oceanocean-continent, oceanocean-ocean,
continentcontinent-continent
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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 Earth’s 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 Earth’s magnetic field
reverses, the polarity of new-formed
rocks changes, creating symmetrical
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bands of opposite polarity
Plate Movement above Mantle Plumes and Hot
Spots Provides Evidence of Plate Tectonics
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Chapter 3: Key Concepts
Some seismic waves–energy associated with earthquakes–can 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 Earth’s 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 Earth’s 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 Earth’s
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 Earth’s magnetic field frozen
into rock as it solidifies.
(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 Hawai’ian Islands, Loihi is such a newly forming island.
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Most of the large-scale features seen at Earth’s 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.
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