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Oceanography
An Invitation to Marine Science
6th Edition
by
Tom Garrison
http://oceanography.brookscole.com/garrison6e
Chapter 3
Earth Structure and Plate
Tectonics
Look For The Following Key Ideas
In Chapter 3
 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.
 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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Key Ideas Continued…
 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.
 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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
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
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
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, slowlyflowing, more dense layer (the asthenosphere).
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
The Fit between the Edges of
Continents 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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
The Fit between the Edges of Continents
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
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
A Synthesis of Continental Drift and
Seafloor Spreading Produced 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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
A Synthesis of Continental Drift and
Seafloor Spreading Produced 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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Most Tectonic Activity Occurs at
Plate Boundaries
The lithospheric plates interact with the neighboring plates in several
ways. (1) Divergent, (2) Convergent, (3) Transform.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Warping, stretching of East Africa
Continental crust
Formation of rift valley
Red Sea
Stepped Art
Fig. 3-17a-c, p. 76
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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Ocean Basins Are Formed at
Divergent Plate Boundaries
The breakup of Pangaea
shown in five stages
beginning about 225 million
years ago. (Note: Ma =
mega-annum, indicating
millions of years ago).
Inferred motion of
lithospheric plates is
indicated by arrows.
Spreading centers (midocean ridges) are shown in
red.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
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 - 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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Island Arcs Form, Continents Collide,
and Crust Recycles at 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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Island Arcs Form, Continents Collide,
and Crust Recycles at 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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Island Arcs Form, Continents Collide,
and Crust Recycles at 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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Crust Fractures and Slides at
Transform Plate Boundaries
(a) Transform faults
(orange) form because the
axis of seafloor spreding
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. Notethe offset plate
boundaries caused by
divergence on a sphere.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
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
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Plate Movement above Mantle Plumes
and Hot Spots Provides Evidence of
Plate Tectonics
(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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Sediment Age and Distribution, Oceanic
Ridges, and Terranes Are Explained by
Plate Tectonics
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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Sediment Age and Distribution, Oceanic
Ridges, and Terranes Are Explained by
Plate Tectonics
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 (a-d).
Though rare, assemblages of
subducting oceanic
lithosphere can also be
scraped off (obducted) onto
the edges of continents.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Scientists Still Have Much to Learn
about the Tectonic Process
In case you think that geophysicists have all the answers, consider just a few of the
theory’s unsolved problems:
 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?
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
Chapter 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 lithospere, 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.
© 2006 Brooks/Cole, a division of Thomson Learning, Inc.
End of Chapter 3