2.0 Plate Tectonics: The Unifying Theory of the Solid Earth

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2.0

Plate Tectonics:

The Unifying Theory

of the
S
olid Earth

Does the Earth’s surface look like this?

Pre
-
requisites

Learning Objectives

Table of Contents

Why Should I Care?

2.0

Plate Tectonics:

The Unifying Theory

of the
S
olid Earth

Does the Earth’s surface look like this?

Pre
-
requisites



None!


Most of the other topics


you will investigate in


this course are best


examined in the context


of Plate Tectonics.


It all starts here!

Learning Objectives:
A
fter you complete this chapter you will be able to:


1. View the Earth as Constantly Changing and Evolving


2.
Understand Why the Surface of the Earth Consists of Slowly Moving


Continents and Oceans


2.1 Understand How Mountains and Valleys are Created,




and Why they are Located in their Current Positions


3.
Describe Relative Movement at Divergent, Transform, and Convergent Margins,


and Why All Three Types of Tectonic Boundaries are Necessary on a Sphere


4. Describe How Processes in the Interior of the Earth Produce


Plate Tectonics at the Surface



2.0

Plate Tectonics:
The Unifying Theory of the
S
olid Earth

Table of Contents:


2.0 The Unifying Theory of the Solid Earth


2.1 The History Behind the Science


2.11 Early Theories for the Formation of Mountain Belts


2.12 Initial Evidence for Continental Drift and Complications


2.13 WWII and Evidence from the Oceans


2.2 Key Elements of Plate Tectonic Theory


2.21 Divergent
P
late Boundaries (Red Sea)


2.22
Convergent Plate Boundaries (Cascade Mountains)


2.23 Transform Plate Boundaries (San Andreas Fault)


2.24
Modern Evidence for Plate Movement


2.3 Complications to Plate Tectonics Theory


2.31 Mantle Plumes (Hawaii, Yellowstone)



2.4 Driving Mechanisms for Plate Tectonics


2.41 The Earth as a Convective Heat Engine


2.42 Ridge Push and Slab Pull


2.43 Models of the Earth’s Interior

Note: all items
in the TOC will
be hot links to
each particular
section.

2.0

Plate Tectonics:
The Unifying Theory of the
S
olid Earth

Why Should I Care?


Have you ever wondered
:



Why some countries have lots of
n
atural
r
esources, but others have few?



Why California has many earthquakes, but Ohio has very few?



Why the only active volcanoes in the continental US are in the Northwest?



Why most beaches are white sand, but Hawaii has black sand beaches?



Why some Himalayan mountains reach 28,000 ft. in elevation, but the



Appalachian mountains are all less than 7000 ft.?


All of these questions (and more!) can be answered with reference to the theory


of Plate Tectonics. After completing this chapter, you will be able to answer


them yourself!

Return to

Table of Contents

2.0

Plate Tectonics:
The Unifying Theory of the
S
olid Earth

USGS

2.0

Plate Tectonics:
The Unifying Theory of the
S
olid Earth

Plate
Tectonics
is the first theory
to provide
a comprehensive view
of the
processes that
produced Earth’s
major surface features
, including
the
continents and
ocean basins.
Within the framework of this theory
, geologists
have found explanations for the basic

causes and distribution of earthquakes, volcanoes,
and mountain
belts. Further, we are
now better able to
explain the
distribution of plants and animals in the geologic past,

as well as the distribution of economically significant
mineral deposits
.

Return to

Table of Contents

2.0

Plate Tectonics:
The Unifying Theory of the
S
olid Earth

Plate
Tectonics
is the first theory
to provide
a comprehensive view
of the
processes
that
produced Earth’s
major surface features
, including
the
continents and
ocean basins.
Within the framework of this theory
, geologists
have found explanations for the basic

causes and distribution of earthquakes, volcanoes,
and mountain
belts. Further, we are
now better able to
explain the
distribution of plants and animals in the geologic past,

as well as the distribution of economically significant
mineral deposits
.

Return to

Table of Contents

Tectonics


Study of large
-
scale movement & deformation of the earth’s outer
layers (mountain building)

Plate Tectonics


Relates deformation to existence & movement of rigid
“plates”
over
a weak or partly flowing
“plastic”

layer in the earth’s upper mantle

2.1

Plate Tectonics:
The Unifying Theory of the
S
olid Earth

To start our investigation of the theory of
Plate Tectonics

let’s start with two
fundamental questions:

Return to

Table of Contents

How are mountains and
mountain ranges formed?

Where are mountain
ranges located?

Text box for students to
enter their thoughts.

Text box for students to
enter their thoughts.

Early geologists had these same questions, and the next few

Pages will examine some of their hypotheses and theories.

2.11

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Eduard
Suess

Thermal Contraction

(1831
-
1914)

Wrote
Das
Antlitz

der
Erde

(1885
-
1909), translated


into English as
The Face of the Earth

(1904
-
25)


Proposed
thermal contraction

as a mechanism for


vertical height differences of
mountains vs. valleys

The

dessicated

orange”

theory…

Note: insert picture of
dessicated

orange with caption:

“When an orange dries out it shrinks, leaving ridges.
The thermal contraction theory contended that as the
Earth cooled it shrunk, leaving mountain ridges.

2.11

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Wrote “
Geological results

of
the Earth’s contraction

in
consequence of cooling”
-

1847


Supported the
thermal
cooling
theory, and
therefore concluded that
Oceans
are permanent,

continents
are stable

(1813
-
1895)

James D. Dana

Permanence of oceans

2.11

Plate Tectonics:
The History Behind the Science

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Table of Contents

Eduard
Suess

Gondwanaland

(1831
-
1914)

Documented the
Glossopteris


fern on present
-
day continents:

Proposed that Gondwanaland was a former continent where

Some of today’s continents were connected
by ancient land bridges

Glossopteris

2.11

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Albert Heim

Alpine terrain,
horizontal folds

Working
in the Alps,
Heim documented
mountain
-
scale


folds and large
horizontal

displacements. This did
no
t


support the vertical folding of thermal contraction.

Mountainside

folds in
Norway

(1849
-
1937)

2.12

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Antonio Snider
-
Pellegrini

Continental fit and “drift”

1858
-

“Creation and its Mysteries Resolved”


Noah’s flood

caused the continents to break up and drift apart

S. America and Africa break up and drift apart

2.12

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Frank
Bursley

Taylor

Drift of continents toward equator

1910
-

“Bearing of the Tertiary mountain belt on the
origin of
the Earth’s plan”


Tidal forces

induced movement of continents toward equator

2.12

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Alfred
Lothar

Wegener

Paleoclimatic

evidence

A.
Evidence
for
inland

glaciation
on several
present
-
day
continents.

B.
The distribution of
glaciation and warm,
swampy regions make
much more sense when
the continents are
positioned next to each
other.

(1880
-
1930)

2.12

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Alfred
Lothar

Wegener

Paleoclimatic

evidence

A.
Evidence
for
inland

glaciation
on several
present
-
day
continents.

B.
The distribution of
glaciation and warm,
swampy regions make
much more sense when
the continents are
positioned next to each
other.

(1880
-
1930)

2.12

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Alfred
Lothar

Wegener

Theory of
Continental
Drift

(1880
-
1930)

1915
-

“Die
Entstehung

der


Kontinente

und
Ozeane



1924
-

“The Origin
of


Continents
and Oceans”

Wegener’s continental reconstruction


(1924)

2.12

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Emile
Argand

Gondwanaland reconstruction

1916
-

“Sur
l'arc

des



Alpes

occidentales



1922
-

“La
Tectonique



de
l'Asia


(1879
-
1940)

Wegener

wasn’t the only


scientist suggesting a


continental
reconstruction

2.12

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Alexander Du
Toit

South America


Africa rock
connections

1927


“A Geological Comparison


of South America with


South Africa”


1937


“Our Wandering Continents”

(1878
-
1948)

The same fossils
and

the


same rock units
are found


in

both Africa
and
South


America

2.12

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Summary of key data supporting
Continental Drift

Jigsaw puzzle fit of the


Atlantic coast continents

Fossil evidence linking continents

Paleoclimatic

evidence

Rock evidence that links continents

But what was


the
driving


mechanism

for


Continental Drift?

2.12

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Arthur Holmes

Convection in a molten substrate

1945
-


Principles


of
Physical


Geology”

(1890
-
1965)

The best model for Continental Drift from the


first half of the 20
th

century

World War II initiated the theory of
Plate Tectonics
, so let’s again start with
some conceptual questions:

Return to

Table of Contents

How can continents plow

t
hrough oceans?

Text box for students to
enter their thoughts.

Most American geologists did not support the theory of Continental Drift,

b
ecause they thought the theory lacked a convincing mechanism or model for
the movement of continents.

Was the evidence for Continental Drift
found on continents or in the oceans?

Text box for students to enter their
thoughts.

2.12

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

2.1
Concept Check:


1. How did the theory of thermal



contraction explain the



formation of mountain ranges?



2. What was the evidence for the



theory of Continental Drift?



3. What poorly understood part of



the theory of Continental Drift



led to the lack of support by



American geologists?

Text box for students to enter their
answers.

Text box for students to enter their
answers.

2.12

Plate Tectonics:
The History Behind the Science

Text box for students to enter their
answers.

2.13

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Evidence from the oceans:

The Submarine Telegraph Cable

Samuel Morse invented the telegraph in the 1830’s, but it would
only work on land. Messages to/from Europe still required ships,
but in
1858
a transoceanic cable was
laid across the Atlantic
ocean floor.

However, the ship’s crew was surprised to discover
that the
ocean floor was not as deep as expected in the middle
.

2.13

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Evidence from the oceans:

The Challenger Expedition of 1872

HMS Challenger was the first dedicated

oceanographic expedition vessel. The

expedition of 1872 took soundings of the

ocean depth and collected samples from the

ocean floor. Shipboard scientists discovered that ocean depths,
sediments, and fossils varied widely, contrary to current thought.

2.13

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Evidence from the oceans:

SONAR


Seafloor Topography

Harry Hess, naval reserve officer called
up during World War II, used the SONAR
on his ship to collect data on shape of
the sea floor.


Not only did the depth vary, but so did
the thickness of the sediment layers on
top of the sea floor.

2.13

Plate Tectonics:
The History Behind the Science

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Table of Contents

Evidence from the oceans:

Magnetic Variations in the Seafloor

During WWII, the renewed U
-
boat threat lead to improvements in SONAR, but
other detection methods were developed:

RADAR could detect surfaced subs,
but deep
-
diving subs could escape even SONAR. So, the disruption of the
local magnetic field was sought as a way of detecting submerged U
-
boats.

Local
magnetic field

Local
magnetic field

2.13

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Evidence from the oceans:

Magnetic Variations in the Seafloor

The first magnetic detectors
were towed behind a ship,
but later were made more
compact to fit in aircraft.


More sensitive
magnetometers began to
detect reversals in the local
magnetic field preserved in
the sea floor rocks.


Magnetometer boom

2.13

Plate Tectonics:
The History Behind the Science

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Table of Contents

Evidence from the oceans:

Magnetic Variations in the Seafloor

Vine and Matthews
found linear patterns
of these variations,
with mirror images of
the patterns on
opposite sides of
ocean ridges.

These same rocks
were of similar age on
opposite sides of the
ridge.

Patterns were also
offset by apparent
faults.


Raw data from a

single ship traverse

Interpreted

data from

several

traverses

2.13

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Evidence from the oceans:

Magnetic Variations in the Seafloor

Interpreted

data from

several

traverses

Describe any patterns that you notice

Text box for students to enter their
thoughts.

2.13

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Evidence from the oceans:

Magnetic Variations in the Seafloor

Now the patterns are color coded by
age. What do you notice?

Text box for students to enter their
thoughts.

2.13

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Evidence from the oceans:

Magnetic Variations in the Seafloor

From this image and data from other oceans,
oceanographers determined that magnetic stripes of
normal (colored in the image) and reverse (white
stripes in the image) polarity were bilaterally
symmetrical around a central (red) normal polarity
stripe. In addition, after determining the age of the
basalt stripes, they realized the the youngest rocks
were along the center stripe, and the rocks got older as
one moved perpendicularly away from the center
stripe. Coupled with data that showed elevated heat
along the center stripe, researchers realized that they
had discovered Wegener’s driving mechanism for
Continental Drift!

2.13

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Evidence from the oceans:

Magnetic Variations in the Seafloor

New ocean floor (crust) was being
created at these mid
-
ocean ridges
(central stripes), which then moved
away from the ridge though time.
These
“seafloor spreading”

zones are
the mechanism by which oceans are
created/enlarged and by which
continents can
“drift”
apart from each
other!

This needs
to be an
animation
(Declan
has one)

In addition, scientists realized that the
reversing magnetic stripes meant that
the Earth’s magnetic field periodically
“flipped”
, such that sometimes
magnetic
north

was located around the
geographic
south

pole!

These were unanticipated, but fundamental discoveries!

More on Seafloor
Spreading

Links to
section
2.21

2.13

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Evidence from the oceans:

Magnetic Variations in the Seafloor

Concept question:


If new crust is being created at

mid
-
ocean ridges, what might you
deduce about the surface area of
the Earth?

This needs
to be an
animation
(Declan
has one)

Text box for students to enter
their thoughts.

2.13

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Evidence from the oceans:

Deep Sea Trenches

Scientists soon discovered ocean
“trenches”

deep regions where
ocean crust was plunging
underneath other crust. In effect
this

subducting

ocean crust was
descending
back into the interior of
the Earth and being consumed.


Problem solved! Oceanic crust is
created at mid
-
ocean ridges and
consumed at
subduction

zones
(trenches.) Therefore, the surface
of the Earth isn’t getting any larger;
it has essentially remained the
same size.

Bathymetric image of the region around the world’s deepest

trench


Challenger Deep, over 11,000 meters below sea level.

This trench is deeper than Mt. Everest is high!

(
image from http://
earthobservatory.nasa.gov
/IOTD/
view.php?id
=
77640)

More on
Subduction

Zones

Links to
section
2.22

2.13

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Evidence from the oceans:

Seafloor Spreading and
Subduction

Zones

Concept question:


Given that the Earth is a sphere
and not a cylinder, can the surface
area of the Earth stay a constant
size with only seafloor spreading
zones and
s
ubduction

zones?

Text box for students to enter
their thoughts.

2.13

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Evidence from the oceans:

Transform Boundaries

In 1965 J.
Tuzo

Wilson
published “A new Class of
Faults and their Bearing on
Continental Drift” in which he
proposed that a 3
rd

type of
boundary was necessary to
accommodate movement of
continents on a spherical Earth.
He called these boundaries
“transform faults”
-

locations
where regions of crust slid past
each other, without being
created or consumed. He also
showed how transform faults
often offset segments of mid
-
ocean ridges.

(Morgan, 1968)

More on
Transform Faults

Links to
section
2.23

These all link to
Google Earth
tour locations

2.13

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Evidence from the oceans:

Plate Boundaries

Now we have discussed all three
tectonic boundaries:


1.
Seafloor Spreading Zones


-

oceanic crust is created



-

occur along mid
-
ocean ridges



-

example:
Mid
-
Atlantic Ridge


2.
Subduction

Zones



-

oceanic crust is consumed



-

occur at deep sea trenches



-

example:
Challenger Deep


3.
Transform Faults



-

crustal blocks slide past each


other


-

offset mid
-
ocean ridges



-

example:
San Andreas Fault

Where are these
tectonic
boundaries
currently located
on the Earth?

2.13

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Evidence from the oceans:

Plate Boundaries

Now we have discussed all three
tectonic boundaries:


1.
Seafloor Spreading Zones


-

oceanic crust is created



-

occur along mid
-
ocean ridges



-

example:
Mid
-
Atlantic Ridge


2.
Subduction

Zones



-

oceanic crust is consumed



-

occur at deep sea trenches



-

example:
Challenger Deep


3.
Transform Faults



-

crustal blocks slide past each


other


-

offset mid
-
ocean ridges



-

example:
San Andreas Fault

This will be an interactive
globe where students can
explore plate boundaries,
seafloor ages, areas of
plates, volcanoes,
earthquakes, etc.

2.13

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Evidence from the oceans:

Plate Boundaries

Concept question:


Based on the tectonic boundaries
around North America, why do you
think scientists discarded the term

C
ontinental Drift”?


Text box for students to enter
their thoughts.

2.13

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Evidence from the oceans:

Plate Boundaries

The tectonic boundaries around
North America extend from the
Pacific margin of California to the
middle of the Atlantic ocean. This
area defines a crustal
“plate”,
which moves as a unit. Since this
unit encompasses both continental
crust and oceanic crust, we now
call the theory
“Plate Tectonics”
instead of Continental Drift.

2.13

Plate Tectonics:
The History Behind the Science

Return to

Table of Contents

Evidence from the oceans:

Plate Boundaries

Obviously, there is lots more to go with this chapter,
but I think this is enough to discuss tomorrow.