Download U4-T2.2-Convection and a Moving Seafloor

Earth Systems 3209
Unit: 4
The Forces Within Earth
Reference:
Chapters 4, 15, 16, 19; Appendix A & B
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Unit 4:
Topic 2.2
Convection and Seafloor Spreading
Focus on . . .
 explaining mantle convection and how it relates to Hess
and Dietz's ideas behind "Seafloor Spreading.”
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Convection and Seafloor Spreading
 To fully understand the theories of Continental Drift and Plate
Tectonics, you must first have an understanding of the internal
processes within Earth.
 Convection currents in the asthenosphere, proposed by Arthur
Holmes, and the seafloor spreading idea, proposed by Harry
Hess and Robert Deitz, provides evidence for a mobile Earth.
 By combining the theories of seafloor spreading with continental
drift and earthquake information, the new theory of Plate
Tectonics became a coherent theory to explain crustal
movements.
Text Reference:
Pages 527-532
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Convection and Seafloor Spreading
 The many plates that make up Earth’s crust sit
directly on a plastic like layer within the mantle
called the Asthenosphere. A scientist named
Arthur Holmes provided evidence to prove that
tectonic plates moved on what he referred to as
convection currents.
 Convection can not take place without a source of
heat and heat within Earth comes from two main
sources:
1) radioactive decay
2) residual heat
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Convection and Seafloor Spreading
 If the asthenosphere is in fact moving as a result of
convection, then Holmes suggested that convection
could be the mechanism responsible for plate tectonics.
 Harry Hess was influenced by Holmes’ ideas, and
suggested that deep within the asthenosphere, heated
material expands, becomes less dense, rises, and
pushes its way up through ridges. It then moves along
the base of oceanic plates, pulling the plates in opposite
directions. This concept we call Seafloor Spreading.
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Convection and Seafloor Spreading
 When this slowly moving material reaches cooler
areas it contracts and sinks causing one plate to
move downward (subducting plate) beneath
another (over-riding plate). This material is then
recycled back into the mantle.
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Seafloor Studies Revealed Hidden Information:
 During World War II, geologists employed by the military
carried out studies of the sea floor, a part of the Earth that
had received little scientific study.
 The topographic studies involved measuring the depth to
the sea floor. These studies revealed the presence of two
important topographic features of the ocean floor:
1) Oceanic Ridges - long winding ridges that occupy the
middle of the Atlantic Ocean and the eastern part of the
Pacific Ocean.
2) Oceanic Trenches - deep trenches along the margins of
continents, particularly surrounding the Pacific Ocean.
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Seafloor Studies Revealed Hidden Information:
 Further studies of the ocean floor noted that as oceanic
lithosphere moves away from the ridge, it cools and
sinks deeper into the asthenosphere. Thus, the depth
to the sea floor increases with increasing age
(distance) away from the ridge.
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Seafloor Studies Revealed Hidden Information:
 Because the oceanic ridges are areas of young
crust, there is very little sediment accumulation on
the ridges. Sediment thickness increases in
both directions away from the ridge, and is
thickest where the oceanic crust is the oldest.
Younger
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Older
Sediment Seafloor and Age
 As new oceanic crust is created it is pushed aside
in two directions. Thus, the age of the oceanic
crust becomes progressively greater in both
directions away from the ridge.
Younger
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Older
Sediment Seafloor and Age
 Because oceanic lithosphere is created at rides and
destroyed at subduction zones (trenches), scientist
noted that the oceanic basins is continuously being
recycled and are relatively young. The oldest oceanic
crust occurs farthest away from a ridge. In the Atlantic
Ocean, the oldest oceanic crust is about 180 million
years old (Jurassic in age). Which corresponds to the
breakup of Pangaea.
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Sediment Seafloor and Age
In Conclusion, the following features can be
noticed the farther you move away from the ridge:
1) Seafloor gets deeper beneath sea-level.
2) Sediments gets thicker and older in age.
3) Seafloor gets older.
4) Seafloor is continuously being recycled
and is relatively young.
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Sample Problem
Explain why it is impossible for oceanic crust to be older
than 200 million years.
Answer:
A geologic process called sea floor spreading causes
the ocean floor to move. Ocean floor is created at
oceanic ridges and is destroyed or consumed within
Earth at subduction zones. This process of recycling
the ocean floor occurs within a span of 180 - 200
million years.
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Example 1:
Which Earth layer contains convection currents
that cause plate movement?
(A) asthenosphere
(B) inner core
(C) lithosphere
(D) outer core
Which scientist proposed the idea of “seafloor
spreading”?
(A) Alfred Wegener
(B) Harry Hess
(C) James Hutton
(D) Tuzo Wilson
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Example 2:
Why is the ocean floor no older than 200 million years old?
A.
B.
C.
D.
the ocean floor formed when Earth originated
approximately 200 million years ago.
the ocean floor is constantly creating new ocean floor
at subduction zones.
the ocean floor is being recycled as a result of
convection and seafloor spreading.
the ocean floor is the same age as the continental
crust, 200 million years old.
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Your Turn . . .
Take the time and complete the following questions . . .
(Solutions to follow)
Question:
In the diagram below, at which drilling site would
the oldest igneous rock most likely be found?
(A)
(B)
(C)
(D)
A
B
C
D
C
A
D
B
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Solutions . . .
Questions:
In the diagram below, at which drilling site would
the oldest igneous rock most likely be found?
(A)
(B)
(C)
(D)
A
B
C
D
C
A
D
B
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Summary . . .
Overview of Points covered:
 Harry Hess proposed the theory of seafloor spreading.
 Seafloor spreading is driven by convection in the mantle where
tectonic plates move apart at ridges and come together at trenches.
 Seafloor studies uncovered the following;
1) Seafloor gets deeper
2) Sediment gets thicker and older
3) Seafloor gets older
4) Seafloor is continuously being recycled and is relatively young.
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