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Oceanography
An Invitation to Marine Science, 7th
Tom Garrison
Chapter 3
Earth Structure and Plate Tectonics
Chapter 3 Study Plan
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Pieces of Earth’s Surface Look Like They Once Fit Together
Earth’s Interior Is Layered
The Study of Earthquakes Provides Evidence for Layering
Earth’s Inner Structure Was Gradually Revealed
The New Understanding of Earth Evolved Slowly
Wegener’s Idea Is Transformed
The Breakthrough: From Seafloor Spreading to Plate
Tectonics
Plates Interact at Plate Boundaries
A Summary of Plate Interactions
The Confirmation of Plate Tectonics
Scientists Still Have Much to Learn about the Tectonic
Process
Chapter 3 Main Concepts
• Earth’s interior is layered, and the layers are arranged by density. Each
deeper layer is denser than the layer above.
• Continents are not supported above sea level by resting mechanically on
a rigid base. Instead, continents rise to great height because they “float”
on a dense, deformable layer beneath them. In a sense, they are
supported at Earth’s surface in the same way a boat is supported by
water.
• The brittle surface of Earth is fractured into about a dozen tile-like
“plates.” Movement of the subterranean material on which these plates
float moves them relative to one another.
• Continents and oceans are formed and destroyed where these plates
collide, flex, and sink.
• The centers of the relatively lightweight continents are often very old. But
because plate movement often forces them back into Earth, the heavy
ocean floors are invariably young – no more than 200 million years (only
about 1/23rd the age of Earth).
• Compelling evidence for plate movement is recorded in symmetrical
remnant magnetic fields in the ocean floors.
Earth’s Interior is Layered Inside
• Density is a key concept for understanding
the structure of Earth
• 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
The Study of Earthquakes
Provides Evidence for Layering
What evidence supports the idea that Earth has
layers?
The behavior of seismic waves generated by earthquakes
give scientists some of the best evidence about the
structure of Earth. Low frequency pulses of energy
generated by the forces that cause earthquakes can spread
rapidly through Earth in all directions and then return to the
surface.
(a) Earthquake waves passing through a homogenous
planet would not be reflected or refracted (bent). The waves
would follow linear paths (arrows).
(b) In a planet that becomes gradually denser and more
rigid with depth, the waves would bend along evenly curved
paths.
(c) P Waves (compressional waves) can penetrate the liquid
outer core but are bent in transit. A P-wave shadow zone
forms between 103 and 143 from an earthquake’s source.
(d) Our earth has a liquid outer core through which the sideto-side S waves cannot penetrate, creating a large “shadow
zone” between 103 and 180 from an earthquake’s source.
Very sensitive seismographs can sometimes detect weak Pwave signals reflecting off the solid inner core.
Earth’s Inner Structure Was
Gradually Revealed
A cross section through Earth showing
the internal layers.
Note: in the expanded section the
relationship between lithosphere and
asthenosphere, and between crust and
mantle.
The thin oceanic crust is primarily basalt, a
heavy dark-colored rock.
The thicker continental crust is granite, a
speckled rock.
The mantle is thought to consist mainly of
oxygen, iron, magnesium, and silicon.
The outer and inner core consist mainly of
iron and nickel.
Earth’s Layers May Be Classified
by 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
When we examine the chemical and physical properties of Earth’s
layers, we see that a cool, rigid, less dense layer (the lithosphere) floats
on a hot, slowly-flowing, more dense layer (the asthenosphere).
Earth’s Layers May Also Be
Classified by 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
Note that Earth is density stratified; that is, each deeper layer is denser than
the layer above.
Isostatic Equilibrium Supports
Continents above Sea Level
Think for a moment about the lithosphere. Why doesn’t it sink into the asthenosphere? How are
features such as mountains supported?
Earth’s continental crust and lithosphere are supported on the on the denser asthenosphere.
Instead of buoyancy, the term isostatic equilibrium describes the way the lithosphere is
supported on the asthenosphere.
(Right) The concept of buoyancy is
illustrated by a ship on the ocean. The
ship sinks until it displaces a volume of
water equal to the weight of the ship and
its cargo.
Icebergs sink into water so that the same
proportion of their volume (about 90%) is
submerged. The more massive the
iceberg, the greater this volume is.
Isostatic Equilibrium Supports
Continents above Sea Level
Erosion and isostatic readjustment can
cause continental crust to become
thinner in mountainous regions.
(a) A mountain range soon after its
formation.
(b) As mountains are eroded over time,
isostatic uplift causes their roots to rise.
The same thing happens when a shi is
unloaded or an iceberg melts.
(c) Further erosion exposes rocks that
were once embedded deep within the
peaks. Deposition of sediments away
from the mountains often causes nearby
crust to sink.
The Age of Earth Was Controversial
and Not Easily Determined
• The age of Earth has been subject to debate.
Scientists now use an age of 4.6 billion
years.
• The principle of uniformitarianism was
introduced in 1788. This principle sates that
the forces which shaped Earth are identical
to forces working today.
• Catastrophism is the thought that Earth is
very young, and events described in the Bible
are responsible for the appearance of Earth’s
features.
The Fit Between Continental Edges
Suggested That They Might Have Drifted
The fit of all the continents
around the Atlantic at a
water depth of about 137
meters (450 feet), as
calculated by Sir Edward
Bullard at the University of
Cambridge in the early
1960s. This widely
reproduced graphic was
an effective stimulus to
the tectonic revolution.
Break up of Pangaea
The Fit Between Continental Edges
Suggested That They Might Have Drifted
• Alfred Wegener’s theory of continental drift
was out of favor with the scientific community
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
Theory of Continental Drift
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Wegener tried to support his theory with evidence:
– Accurate world maps. People noticed that the continents apparently fit
together like a jigsaw-puzzle.
– Fossils of the Glossopteris fern in South America, Africa, Australia,
India, and Antarctica. The seeds are too heavy to travel by wind and too
fragile to survive significant sea crossings.
– Distribution of other animals and fossils were studied, especially coal.
Wegener’s theories were not accepted because he could not explain how
continents could drift, also he was a meteorologist, not a geologist.
The plate tectonics theory would finally provide an explanation for how
continents move, making Wegener’s theory widely accepted.
– Additionally, it was found that including the continental shelves filled the
gaps in the jigsaw puzzle.
Theory of Seafloor Spreading
• Hess and Dietz (1960) proposed an explanation of seafloor features.
They hypothesized that the seafloor is in a constant state of creation
and destruction through a process called seafloor spreading.
• In the theory of seafloor
spreading, new crust
emerges from the rift valley
in a mid-ocean ridge. Magma
from the asthenosphere
pushes up through the rift
and solidifies into new crust.
Sea-Floor Spreading and Plate
Boundaries
The Theory of Plate Tectonics
• The ideas of continental drift and seafloor
spreading were tied together in the theory of
plate tectonics. Main points of the theory
include:
– Earth’s outer layer is divided into lithospheric
plates
– Earth’s plates float on the asthenosphere
– Plate movement is powered by convection
currents in the asthenosphere seafloor
spreading, and the downward pull of a
descending plate’s leading edge.
The Theory of Plate Tectonics
Where does the heat within Earth’s layers come from?
Heat from within Earth keeps the asthenosphere flowing. This allows the
lithosphere to keep moving. Most of the heat that drives the plates is generated
by radioactive decay, given off when nuclei of unstable elements break apart.
(Left) The tectonic system is powered by
heat. Some parts of the mantle are
warmer than others, and convection
currents form when warm mantle
material rises and cool material falls.
Above the mantle floats the cool, rigid,
lithosphere, which is fragmented into
plates. Plate movement is powered by
gravity: The plates slide down the ridges
at the places of their formation; their
dense, cool leading edges are pulled
back into the mantle.
Most Tectonic Activity Occurs at
Plate Boundaries
Seismic events worldwide, January 1977 through December 1986. The
locations of about 10,000 earthquakes are colored red, green, and blue
to represent event depths of 0-70 kilometers, 70-300 kilometers, and
below 300 kilometers.
Most Tectonic Activity Occurs at
Plate Boundaries
The major lithospheric plates, showing their directions of relative movement
and the location of the principal hot spots. Most of the million or so earthquakes
and volcanic events each year occur along plate boundaries.
Most Tectonic Activity Occurs at
Plate Boundaries
(Above) Plate boundaries in action. As Plate A moves to the left (west), a
gap forms behind it (1) and an overlap with Plate B forms in front of (2).
Sliding occurs along the top and bottom sides (3). The margins of Plate A
experience the three types of interactions: At (1), extension characteristic of
divergent boundaries; at (2), compression characteristic of convergent
boundaries; and at (3), the shear characteristic of transform plate boundaries.
Most Tectonic Activity Occurs at
Plate Boundaries
The lithospheric plates interact with the neighboring plates in several
ways. (1) Divergent, (2) Convergent, (3) Transform.
Most Tectonic Activity Occurs at
Plate Boundaries
The lithospheric plates interact with the neighboring plates in several ways.
Divergent plate boundaries – Boundaries between plates moving apart,
further classified as:
– Divergent oceanic crust – for example, the Mid-Atlantic Ridge
– Divergent continental crust - for example, the Rift Valley of East Africa.
(Right) Extension of
divergent boundaries causes
splitting and rifting.
Most Tectonic Activity Occurs at
Plate Boundaries
Convergent Plate Boundaries - Regions where plates are pushing together.
(Below) Compression at convergent boundaries produces buckling and shortening.
Most Tectonic Activity Occurs at
Plate Boundaries
Transform plate boundaries - locations where crustal plates move
past one another, for example, the San Andreas fault.
(above) Translation at transform boundaries causes shear.
Transform Faults
Ocean Basins Are Formed at
Divergent Plate Boundaries
Divergent plate boundaries – Boundaries between
plates moving apart, further classified as:
 Divergent oceanic crust – for example, the
Mid-Atlantic Ridge
 Divergent continental crust – for example,
the Rift Valley of East Africa.
(Figure to the Right)
(a) As the lithosphere began to crack, a rift formed
beneath the continent, and molten basalt from the
asthenosphere began to rise.
(b) As the rift continued to open, the two new
continents were separated by a growing ocean basin.
Volcanoes and earthquakes occur along the active rift
area, which is the mid-ocean ridge. The East African
Rift Valley currently resembles this stage.
(c) A new ocean basin (shown in green) of the
Atlantic. resembles this stage.
Ocean Basins Are Formed at
Divergent Plate Boundaries
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.
(Right) The Mid-Atlantic Ridge
showing its conformance to the
coastlines of the adjacent continents.
The first inset shows a detail of the
ridge generated from side-scan
sonar data – the central rift is clearly
visible. Red and orange colors
indicate the crest of the ridge; dark
blue, the deeper seabed on either
side. The second insert shows the
location of the ridge and associated
valley in the Atlantic.
Ocean Basins Are Formed at
Divergent Plate Boundaries
The breakup of Pangaea
shown in five stages
beginning about 225 million
years ago. (Note: Ma = megaannum, indicating millions of
years ago). Inferred motion of
lithospheric plates is indicated
by arrows. Spreading centers
(mid-ocean ridges) are shown
in red.
Convergent Plate Boundaries
Convergent Plate Boundaries Regions where plates are pushing
together can be further classified as:
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Oceanic crust toward continental
crust - for example, 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.
(Above Right) A cross section through the west coast of South America, showing the convergence of a
continental plate and an oceanic plate. The subducting oceanic plate becomes more dense as it
descends, its downward slide propelled by gravity. At a depth of about 80 kilometers (50 miles), heat
drives water and other volatile components from the subducted sediments into the overlying mantle,
lowering its melting point. Masses of the melted material, rich in water and carbon dioxide, rise to power
Andean volcanoes.
Convergent Plate Boundaries
(Above) 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.
Convergent Plate Boundaries
The distribution of shallow,
intermediate, and deep
earthquakes for part of the
Pacific Ring of Fire in the
vicinity of the Japan trench.
Note that earthquakes
occur only on one side of
the trench, the side on
which the plate subducts.
The great Indonesian
tsunami of 2005 was
caused by these forces; the
site of the catastrophic
1995 Kobe subduction
earthquake is marked.
Convergent Plate Boundaries
(Above) A cross section through southern China, showing the convergence of
two continental plates. Neither plate is dense enough to subduct; instead, their
compression and folding uplift the plate edges to form the Himalayas. Notice
the massive supporting “root” beneath the emergent mountain needed for
isostatic equilibrium.
Converging Margins: India-Asia
Collision
Crust Fractures and Slides at
Transform Plate Boundaries
(a) Transform faults
(orange) form because the
axis of seafloor spreading
on the surface of a sphere
cannot follow a smoothly
curving line. The motion of
two diverging lithospheric
plates (arrows) rotates
about an imaginary axis
extending through Earth.
(b) A long transform plate
boundary, which includes
California’s San Andreas
Fault. Note the offset plate
boundaries caused by
divergence on a sphere.
A History of Plate Movement Has Been
Captured in Residual Magnetic Fields
Paleomagnetism: strips of alternating magnetic
polarity at spreading regions.
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 bands of opposite polarity
A History of Plate Movement Has Been
Captured in Residual Magnetic Fields
The age of the ocean floors. The colors seen here represent an
expression of seafloor spreading over the last 200 million years as
revealed by paleomagnetic patterns. Note, especially the relative
symmetry of the Atlantic basin in contrast with the asymmetrical Pacific,
where the spreading center is located close to the eastern margin and
intersects the coast of California.
A History of Plate Movement Has Been
Captured in Residual Magnetic Fields
Apparent Polar wandering: plate
movement causes the apparent position
of the magnetic poles to have shifted.
(a) The magnetic properties of rocks in
North America, South America, Europe,
and Africa suggest that the north
magnetic pole apparently migrated to its
present position from much farther
south, possibly from a point in the Pacific
Ocean.
(b) The problem can be solved if the pole
remained fixed but the continents have
moved. For example, if Europe and
North America were previously joined,
the paleomagnetic fields in the rocks
would indicate a single pole until the
continents drift apart.
Plate Movement above Mantle
Plumes and Hot Spots
(above) 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.
Plate Tectonics Explains Sediment
Age, Oceanic Ridges, and Terranes
• Age and distribution of ocean sediments
– The sediment in the ocean is thinner and
younger than the age of the ocean indicates it
should be
• The Oceanic ridges: Oceanic ridges are
clear indicators of past events.
• Terranes: Oceanic plateaus that form by
uplifting and mountain building as they strike
a continent.
Plate Tectonics Explains Sediment
Age, Oceanic Ridges, and Terranes
Oceanic plateaus usually
composed of relatively lowdensity rock are not subducted
into the trench with the
oceanic plate. Instead, they
are “scraped off,” causing
uplifting and mountain building
as they strike a continent (ad).
Though rare, assemblages of
subducting oceanic
lithosphere can also be
scraped off (obducted) onto
the edges of continents.
Scientists Still Have Much to Learn
about the Tectonic Process
• Why should long lines of asthenosphere be any warmer than adjacent
areas?
• Is the increasing density of the leading edge of a subducting plate more
important than the plate’s sliding off the swollen mid-ocean ridge in
making the plate move?
• Why do mantle plumes form? What causes a superplume? How long do
they last?
• How far do most plates descend? Recent evidence suggests that much
of the material spans the entire mantle, reaching the edge of the outer
core.
• Has seafloor spreading always been a feature of Earth’s surface? Has a
previously thin crust become thicker with time, permitting plates to
function in the ways described here?
• There is evidence of tectonic movement prior to the breakup of Pangaea.
Will the process continue indefinitely, or are there cycles within cycles?
Scientists Still Have Much to Learn
about the Tectonic Process
The Wilson cycle, named in honor of John Tuzo Wilson’s synthesis of
plate tectonics. Over great spans of time, ocean floors form and are
destroyed. Mountains erode, sediments subduct, and continents rebuild.
Seawater moves from basin to basin.
Chapter 3 in Perspective
In this chapter you learned that Earth is composed of concentric spherical layers, with
the least dense layer on the outside and the most dense at the core. The layers may be
classified by chemical composition into crust, mantle, and core; or by physical properties
into lithosphere, asthenosphere, mantle, and core. Geologists have confirmed the
existence and basic properties of the layers by analysis of seismic waves, which are
generated by the forces that cause large earthquakes.
The theory of plate tectonics explains the nonrandom distribution of earthquake locations,
the curious jigsaw-puzzle fit of the continents, and the patterns of magnetism in surface
rocks. Plate tectonics theory suggests that Earth’s surface is not a static arrangement of
continents and ocean but a dynamic mosaic of jostling lithospheric plates. The plates
have converged, diverged, and slipped past one another since Earth’s crust first solidified
and cooled, driven by slow, heat-generated currents flowing in the asthenosphere.
Continental and oceanic crusts are generated by tectonic forces, and most major
continental and seafloor features are shaped by plate movement. Plate tectonics 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 the age of Earth.
In the next chapter you will learn about seabed features, which the slow process of
plate movement has made and remade on our planet. The continents are old; the ocean
floors, young. The seabed bears the marks of travels and forces only now being
understood. Hidden from our view until recent times, the contours and materials of the
seabed have their own stories to tell.