Download Exam Block #1

Exam Block #1
• Chapter 1 – Introduction to Geology
• Chapter 2 – Plate Tectonics
How to study for this class:
1. Read the chapter and answer the Review
Questions. As you read, follow along with
the Chapter Outline (download from our
class site below) and these PowerPoint
Notes.
2. Review each chapter using the GEODe:
Earth CD-ROM in your book.
3. Go online and take the Concept Quizzes
and Chapter Test for each chapter at:
www.prenhall.com/tarbuck
4. Check your grades online at:
http://pages.sbcglobal.net/solanogeo/index.htm
BLOCK EXAM #1 - CHAPTERS 1 & 2
Page 1 of 24
TAKE ADDITIONAL NOTES HERE:
BLOCK EXAM #1 - CHAPTERS 1 & 2
Page 2 of 24
Chapter 1 – Introduction to Geology
1
ƒ Geology – From the Greek geo “Earth” and logos
“discourse”.
ƒ Physical Geology – focuses on the materials and
processes that operate on the Earth.
ƒ Historical Geology – understanding of the origin of
the Earth and life through time.
ƒ What causes earthquakes? How do mountains
form? What controls stream flow?
ƒ How did the Ice Ages start? What are the basic
minerals of the Earth?
ƒ These questions and many other topics will be
covered in this class!
Charakusa Valley, Pakistan
The Science of Geology
3
ƒ Many of the problems and issues addressed by
geology are of practical value to people.
ƒ Natural disasters are simply natural processes
when people live where these processes occur.
ƒ Earth is now gaining about 100 million people each
year.
ƒ Resources, such as water, soil, metals, non-metals,
and energy (oil, gas, coal, geothermal) all come from
the Earth.
ƒ We will cover many aspects of our relationship with
the physical environment.
Historical Notes About Geology
2
The Science of Geology
5
ƒ Aristotle believed that motion was primarily
determined by the nature of the substance that was
moving. The four primary elements were considered
to be: air, water, earth, and fire.
ƒ So for example, smoke rose to be closer to a similar
primary element: air.
ƒ Objects fell towards their primary element: earth.
ƒ Heavier objects would be even more like the earth
and fall faster.
BLOCK EXAM #1 - CHAPTERS 1 & 2
Historical Notes About Geology
4
ƒ Aristotle – (384-322 B.C.) Greek philosopher who
made observations about the natural world that were
influential throughout the Middle Ages (476-1450).
Q: What was Aristotle explaining here?:
“…a great many fishes live in the Earth
motionless and are found when excavations are
made.”
A:
ƒ His explanations were not based on observations or
experiments, but were arbitrary pronouncements. He
was regarded as the head and chief of all
philosophers and his opinion on any subject was
authoritative and final.
Historical Notes About Geology
6
ƒ The father of experimental science
is considered to be Galileo Galilei
(1564-1642). The famous experiment
at the Leaning Tower of Pisa in Italy
is attributed to him, although there is
no record in his journals.
ƒ Aristotle believed that heavier
objects fell at a faster rate. Is this
true?
ƒ Galileo correctly showed that
objects fall at the same rate,
regardless of their mass.
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Historical Notes About Geology
7
ƒ The experiment consisted of
dropping a heavy cannonball and a
lighter wooden ball. The cannonball did
strike the ground first, but the time
difference was very slight. What would
slow down the lighter ball?
ƒ Galileo correctly attributed the slight
time difference to air friction.
ƒ However, since the heavier ball did
reach the ground first (however slight),
scholars viewing the experiment
walked away still believing Aristotle’s
ideas were correct.
9
Catastrophism
ƒ Ussher’s work earned widespread acceptance
among scientific and religious leaders.
ƒ During the 1600 and 1700’s, catastrophism was
popular. The idea was that the Earth’s landscape
was shaped by great catastrophes. Mountains and
canyons were formed by sudden disasters produced
by unknowable forces that no longer operate (such as
Noah’s Flood).
ƒ The philosophy was to fit the rates of Earth
processes to the then-current ideas on the age of the
Earth.
11
Uniformitarianism
8
Idea of a Young Earth
ƒ In 1650, James Ussher
He also calculated: Adam and Eve
were driven from Paradise on
Monday November 10, 4004 BC,
and that the ark touched down on
Mt. Ararat on Wed. May 5, 2348 BC.
(1580-1655) – Archbishop of
Ireland, calculated that the
Earth was created on Sunday
October 23, 4004 BC. After
counting the Patriarchal
periods in the bible and
elsewhere, Ussher came to
this conclusion. This date was
in fact printed in the margins
of some editions of the King
James Version of the Bible.
10
Uniformitarianism
ƒ James Hutton (1726-1797) – is
considered the father of modern
geology. He published: ‘Theory of the
Earth’ in 1784 in which he showed that
the earth in fact has had a long history
- as he put it, "we find no vestige of a
beginning, no prospect of an end."
ƒ The idea was that the physical and
chemical laws that operate today have
also operated in the past is know as
uniformitarianism, which is
commonly stated as:
“The present is the key to the past.”
Uniformitarianism + Catastrophism
12
ƒ While uniformitarianism is the
d = 3000 m
Grand Tetons, Wyoming
ƒ Q: Typical erosion rates are 3 cm/1000 yrs. How
long would it take to erode a 3000 meter high
mountain to sea level? [Hint: rate = distance / time]
ƒ A: Rearrange: t = d / r =
BLOCK EXAM #1 - CHAPTERS 1 & 2
general rule in geology, there is
evidence for exceptional
catastrophic events, such as the
meteorite impact that killed the
dinosaurs. Such events cause
extinctions that are observed in the
fossil record.
ƒ Therefore, to completely explain
Earth’s geologic history, you need
both ideas: 99.9% uniformitarianism
punctuated by 0.1% catastrophes =
the Geologic Record!
Page 4 of 24
✓REVIEW QUESTIONS
13
1. Geology is traditionally divided into two broad areas
– name and describe these two subdivisions.
ƒ During the 1800’s, the geologic time scale was
developed using the principles of relative dating.
Relative dating means that events are placed in their
proper sequence or order without knowing their ages
in years. This was done using two simple principles:
ƒ Law of superposition – each horizontal layer is
younger than the one below.
ƒ Principle of fossil succession – any time
period can be recognized by its fossil content.
ƒ In the early 1900’s, radiometric dating was
developed to assign fairly accurate dates to events in
Earth history. What is the age of the Earth?
2. Briefly describe Aristotle’s influence on the science
of geology.
3. How did the proponents of catastrophism perceive
the age of Earth?
4. Describe uniformitarianism. How did the advocates
of this idea view the age of Earth?
15
Law of Superposition
14
Geologic Time
Principle of Fossil Succession
16
Extinct amphibian
from Permian age
(248-290 million
years) rocks in Texas
(left).
Younger
Fossil fish of Eocene
(33-54 million years)
age from Wyoming
(right).
Older
The Geologic Time Scale
17
The Geologic
Time Scale – was
put together using
the principles of
relative dating;
dates were added
later when
radiometric dating
was refined. We
will study the
geologic time
scale in detail in
Chapter 9.
BLOCK EXAM #1 - CHAPTERS 1 & 2
Nature of Scientific Inquiry
18
ƒ How do you know that everything in your book is
true?!
ƒ All science is based on the assumption that the
natural world behaves in a consistent and predictable
manner.
ƒ Hypothesis – a tentative, but testable explanation
for a natural phenomenon.
ƒ Theory – a well-tested and widely accepted view
that scientist agree best explains the observed facts.
Comprehensive theories are often called paradigms.
Name some theories.
ƒ Q: “The Sun rotates about the Earth.” Is this a
hypothesis or a theory? Why or why not?
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✓REVIEW QUESTIONS
19
Nature of Scientific Inquiry
20
5. What is the age of the Earth?
ƒ Scientific investigations involve the following steps:
6. The geologic time scale was established without
the aid of radiometric dating. What principles were
used to develop the time scale?
ƒ The collection of scientific facts through observation
and measurement.
ƒ The development of one or more working
hypotheses or models to explain these facts.
ƒ Development of observations and experiments to
test the hypotheses, and
ƒ The acceptance, modification, or rejection of the
model based on extensive testing.
7. How is a scientific hypothesis different from a
scientific theory?
Do Glaciers Move? Switzerland, 1874
21
A View of Earth
22
The four spheres
of the Earth.
Meteorology
Biology
Oceanography
Geology
GEOSPHERE
Give examples of how the four sphere interact with one another.
Hydrosphere & Atmosphere
23
ƒ Hydrosphere: The Earth is the blue planet. Water
more than anything else makes Earth unique. The
oceans cover 71% of the Earth’s surface and account
for 97% of Earth’s water. 3% is fresh water found in
streams, lakes, and glaciers. Running water and
groundwater are responsible for erosion that sculpts
the planet’s surface.
ƒ Atmosphere: This blanket of air is very thin
compared to Earth’s diameter. It provides life-giving
gases and well as protection from the dangerous
radiation from the Sun. The energy exchanges that
occur between the atmosphere and the Earth’s
surface produces weather and erosion.
BLOCK EXAM #1 - CHAPTERS 1 & 2
Biosphere & Geosphere
24
ƒ Biosphere: The Biosphere includes all life on
Earth. It extends from the ocean floor upward for
several kilometers into the atmosphere. The
Biosphere influences the other three spheres, which
would be very different without life.
ƒ Geosphere: This includes all of the solid Earth,
from the center to the surface. We will not only
discuss the surface features, but also the structure of
the Earth’s interior.
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25
Earth As A System
ƒ A system is any size group of interacting parts that
form a complex whole. Examples: a city’s bus
system; your car’s cooling system; our solar system.
ƒ A closed system is one that is self-contained – no
matter goes in and out of the system – like your car’s
cooling system. Energy (heat) moves in and out of
the system, but the matter (water) does not leave it.
ƒ By contrast, most natural systems are open
systems - one in which matter and energy flow into
and out of the system – like a hurricane. Water vapor
(matter) moves into and out of the system.
Global Warming Feedback Systems
Mechanism
27
ƒ Most natural systems have mechanisms that tend
to enhance change, or other mechanisms that tend to
resist change and thus stabilize the system.
ƒ Negative feedback mechanisms work to maintain
the system as it is, or maintain the status quo. For
example, when we get too hot, we perspire to cool
down – this stabilizes our body temperature.
ƒ Positive feedback mechanisms work to enhance
or drive change. For example, the warmer the ocean
water, the greater the intensity of a hurricane.
ƒ Most of Earth’s systems, such as the climate
system, contain a wide variety of negative and
positive feedback mechanisms.
Movie: The Day After Tomorrow
Positive Negative
29
Ice Age!
9 - 18° C
Warmer
X
28
What caused the
climate change?!
Higher temperatures cause the highly
reflective snow to melt - replaced by dark
soils and plants that absorb sunlight.
Higher temperatures cause greater
evaporation and formation of clouds.
Higher temperatures promote the growth
of vegetation that remove carbon dioxide
from the air.
Man cutting and clearing the tropical rain
forest.
Man burning fossil fuels putting carbon
into the atmosphere.
Global Warming May Trigger An Ice Age!
26
Earth As A System
The Earth System
30
Cycles in the
Earth System:
1) Hydrologic
Cycle –
circulation of
water, powered
by the Sun.
2) Rock Cycle –
changing of one
rock type to
another, powered
by Earth’s internal
heat.
The two cycles are linked and interact with one another.
BLOCK EXAM #1 - CHAPTERS 1 & 2
Page 7 of 24
Early Evolution of Earth – Big Bang
•
•
•
31
A. About 5 billion years ago, a
huge rotating cloud of dust
and gases (solar nebula)
begins to contract.
B. Most of material is swept to
center, to form the Sun.
Some material remains in
the flattened disk.
C. Solid particles begin to
form.
D. In time, most of the debris
was collected into the nine
planets or was swept into
space by the solar wind.
E. Final accretion of material
into the eight planets.
About 13.7 billion years ago, all mass was at
one point and exploded to form the present
day Universe (Big Bang Theory).
Before our Sun formed, there was another
larger Sun in this sector of space. Stars fuse
lighter elements into heavier ones and in the
process release energy. Hydrogen fuses to
form Helium, and then Helium fuses into
heavier elements up to Iron (Atomic No. 26).
At this point, Iron cannot fuse to heavier
elements and the star collapses and may
explode in a supernova, ejecting gases and
heavier elements into space. Some of this
material then slowly collected to form our
solar system about 5 billion years ago.
The Nebular Hypothesis
32
The Nebular Hypothesis
33
Moon Formation – The Ultimate Impact!
34
The Moon should
contain more iron
for its size – if it
formed in orbit
around the Earth
(Nebular
Hypothesis).
Where is Moon’s
missing iron? In
Earth’s core!
✓REVIEW QUESTIONS
35
9. How is an open system different from a closed
system?
9. Contrast positive and negative feedback mechanism.
10. What are the two sources of energy for the Earth
System?
11. Briefly describe the events that led to the formation of
the solar system.
BLOCK EXAM #1 - CHAPTERS 1 & 2
Earth’s Layered Structure
36
• Due to the heat of
impacts and radioactive
decay, the early Earth
became hot enough for
iron and nickel to melt
and sink to the center of
the planet. The lighter
elements rose to create
the crust and mantle.
• This created the three
compositional layers of
the earth: crust, mantle
and core.
Page 8 of 24
Earth’s Compositional Layers
37
Two Types of Crust:
1. Oceanic crust – is about 7
km thick and composed of
the dark igneous rock
basalt. It is relatively young
(less than 160 million years)
and has density of 3.0
g/cm3.
2. Continental crust – is
generally 35-70 km thick
and composed of granite
and may be 3.9 billion years
old and has density of 2.7
g/cm3.
Mantle & Core:
• The mantle is solid and the
upper mantle is composed
of peridotite, with a density
of 3.3 g/cm3. With increasing
depth, peridotite compacts
into more dense minerals.
• The core is thought to be an
iron-nickel alloy with minor
amounts of other elements
and has an average density
of 11 g/cm3.
39
Earth’s Physical Layers
• With depth in the Earth, there is a gradual
increase in temperature, pressure, and density.
• The increase in temperature and pressure with
depth affects the physical properties and hence
the mechanical behavior of the Earth materials.
• How does a pressure cooker work?
•
•
Without a lid, water boils at 100°C
(212°F). Water expands greatly in
volume as it goes from a liquid to a
gas (steam).
With a lid, the increased pressure
keeps the water liquid at a higher
temperature. At 15psi, water temp.
is now 125°C (257°F).
Earth’s Physical Layers
41
• Lithosphere: is the cool
outer layer of the earth that is
rigid and broken into tectonic
plates. Thickness averages
about 100 km, but may be up
to 250 km beneath
continents.
• Asthenosphere: is beneath
the lithosphere and to a depth
of 660 km. It is soft (ductile),
comparatively weak layer and
may have some partial melting
of rock near its top.
The lithosphere ‘floats’ on
the asthenosphere and is
free to move about: Plate
Tectonics!
BLOCK EXAM #1 - CHAPTERS 1 & 2
38
Earth’s Compositional Layers
Earth’s Physical Layers
3.0
2.7 g/cm3
3.3
11
40
• The pressure increase with depth tends to increase
rock strength. This is because, when a solid rock
melts, the volume must increase, and the pressure
keeps this from occurring.
• Even though the temperature is increasing with
depth, the pressure affect wins – in all parts of the
earth – except one: the outer core.
• Thus, depending on the physical environment
(temperature and pressure), the Earth material may
behave like a brittle solid, deform in a puttylike
manner, or even melt and become liquid.
•
Earth’s five physical layers: lithosphere, asthenosphere,
mesosphere, outer and inner core.
Earth’s Physical Layers
42
• Mesosphere: is the
lower mantle (6602900 km depth) and
rocks gradually
strengthen with
depth because
pressure increases.
• Outer core: is a
liquid layer of ironnickel alloy which
convects generating
the Earth’s magnetic • Inner core: is a solid layer
field.
of iron-nickel alloy.
Page 9 of 24
Earth’s Internal Layers
43
• The two divisions of
Earth’s surface are:
1. Continents: are less
dense, thicker and
composed of granitic
rocks. The thicker and less
dense continental rocks
float on top of the mantle
to a higher level.
2. Ocean Basins: are more
dense and composed of
basaltic rocks.
The Face of Earth - Continents
45
• The two divisions of the continents are:
1. Mountain Belts: are less dense and composed of
granitic rocks (old and young belts).
2. Stable Interior: Also called a craton – composed
of shields and stable platforms.
44
The Face of Earth
Granite
Basalt
✓REVIEW QUESTIONS
46
13. Describe Earth’s compositional divisions.
14. Contrast the asthenosphere and lithosphere.
15. Describe the general distribution of Earth’s youngest
mountains.
The Face of Earth – Ocean Floor
47
• The three divisions of the ocean floor are:
1. Continental Margins: are adjacent to continents
and include the continental shelf, the
continental slope (the boundary between the
continents and the ocean basins), and continental
rise. See Figure 13.7, Page 355.
BLOCK EXAM #1 - CHAPTERS 1 & 2
The Face of Earth – Ocean Floor
48
• The three divisions of the ocean floor are:
2. Deep-Ocean Basins: contain abyssal plains
(deep, flat regions), the deep-ocean trenches
(deep, narrow features near mountains), and
seamounts (submerged volcanic structures). See
Figure 13.9, Page 357.
Page 10 of 24
The Face of Earth – Ocean Floor
49
• The three divisions
of the ocean floor
are:
3. Oceanic Ridges:
are the most
prominent feature
on the Earth with
broad, elevated
regions over 43,000
miles long; made of
igneous rocks that
has been uplifted
and fractured.
Igneous Rocks
Igneous: form when molten rock, called magma,
cools and solidifies. When magma cools at depth, it
cools slowly and forms a course-grained (1)
Intrusive igneous rock – a good example is
granite – only exposed after uplift and erosion.
Granite in Yosemite.
51
Igneous: When lava cools at the Earth’s surface, it
cools quickly and forms a fine-grained (2)
Extrusive igneous rock – a good example is basalt
(like in Hawaii).
Sedimentary Rocks
50
Igneous Rocks – 2 Types
53
Sedimentary: (2) Chemical sedimentary rocks
form when material dissolved in water precipitates
to form a solid rock – like limestone.
Sedimentary Rocks - 2 Types
Sedimentary: Sediments are derived from
weathering of preexisting rocks and are lithified
into (1) Detrital sedimentary rocks – like
sandstone (made of medium size particles). Shale
is made of fine clay-size particles).
Metamorphic Rocks – 2 Types
54
Metamorphic: Formed by “changing” preexisting
igneous, sedimentary or other metamorphic rocks;
driving forces are increased heat and pressure;
minerals align into a (1) Foliated texture –
example is a gneiss – whose parent rock was a
granite.
Granite
BLOCK EXAM #1 - CHAPTERS 1 & 2
52
Gneiss
Page 11 of 24
55
Metamorphic Rocks
Metamorphic: when crystals grow larger without
alignment, a (2) Nonfoliated texture is produced
– example is marble – whose parent rock was
limestone.
Limestone
Marble
The Rock Cycle
56
The Rock Cycle
• The Rock Cycle: One of Earth’s subsystems.
• It is the loop that involves the processes by which
one rock changes to another.
• The Basic Cycle: Magma cools over time to
produce igneous rocks; these are uplifted and
exposed and weather into sediments, which are
transported by wind, water and ice and deposited;
they are lithified into sedimentary rocks; mountain
building processes can deeply bury the rock and
heat and pressure produce metamorphic rocks;
these can melt into magma – thus completing the
rock cycle.
✓REVIEW QUESTIONS
57
58
17. List the three major topographic units of the ocean floor.
18. Name each of the rocks described below:
✓REVIEW QUESTIONS
•
Light-colored, coarse-grained intrusive rock =
•
Detrital rock rich in fine clay-size particles =
•
A fine-grained black rock that makes up the oceanic crust =
•
Nonfoliated rock, for which limestone is its parent rock =
59
19. Indicate whether it is associated with igneous,
sedimentary, or metamorphic rocks:
•
May be intrusive or extrusive =
•
Lithified by compaction and cementation =
•
Sandstone is an example =
•
Some members of this group are foliated =
•
This group is divided into detrital and chemical
categories =
•
Gneiss is a member of this group =
20. Explain: “One rock is the raw material for another.”
BLOCK EXAM #1 - CHAPTERS 1 & 2
Page 12 of 24
Chapter 2 – Plate Tectonics
1
2
Continental Drift
ƒ This Chapter: Transformation of an old idea
ƒ
ƒ
(continental drift) Æ modern (plate tectonics in
1968). Q: What year did man land on moon?
Alfred Wegener (1915) – German geophysicist
was first to present the continental drift idea with
evidence. But there was much resistance to his
ideas.
His list of evidence included:
1. Fit of Continents – The shorelines of South
America & Africa fit together like a jigsaw puzzle.
ƒ CON: “…but the fit is not that good.”
K7, Pakistan, Himalayas
3
1) Fit of Continents
2. Fossil Evidence – Existence of identical fossils on
widely separated landmasses.
ƒ CON: “…but there were land bridges.”
ƒ Today We
Know: The best
fit is not along
the shorelines,
but along the
continental
slopes.
2) Fossil Evidence
4
2) Fossil Evidence
Mesosaurus –
aquatic reptile
with a limited
distribution.
• Glossopteris –
a fossil fern
found on
several
continents.
•
5
ƒ Today We Know: No land bridges have been found
in the oceans.
BLOCK EXAM #1 - CHAPTERS 1 & 2
3) Rock Type & Structural Similarities
6
3. Reassembling the
continents connects
mountain chains into
a single linear belt
[Appalachians in
North America &
Caledonians in
Northern Europe].
Rocks should match
in age and type.
Today We Know:
ages match from
radiometric dating.
Page 13 of 24
4) Paleoclimatic Evidence
7
Pangaea – 200 Million Years Ago
8
4. There is glacial debris on many continents located
at present-day tropical latitudes, like Africa, India,
and Australia. Glacial striations on bedrock
indicate that the direction of ice movement was
from sea to land – opposite of what you would
expect. This could only be explained if the
continents had been together during glaciation.
ƒ Wegener also correctly positioned southern
Pangaea near the south pole, since the rock
record shows large tropical swaps must have
existed in Northern Hemisphere that eventually
became the major coal fields of the eastern USA,
Europe, and Siberia.
4) Paleoclimatic Evidence
9
Evidence of
glaciation of
Pangaea.
4) Paleoclimatic Evidence
10
Location of
swaps that
eventually
formed coal
deposits.
Striations: ice moved to southwest.
Breakup of Pangaea
11
12
Plate Motions Through Time
BLOCK EXAM #1 - CHAPTERS 1 & 2
Page 14 of 24
Objections to Continental Drift
13
14
Alfred Wegener
ƒ The main objection to continental drift was
ƒ
ƒ
ƒ
Wegener’s inability to identify a mechanism that
was capable of moving continents.
Wegener proposed that the tidal forces of the
moon moved the continents. However, other
scientists correctly countered that the magnitude of
this force would halt the rotation of the Earth!
Another problem was that Wegener incorrectly
suggested that the continents broke through the
oceanic crust, much like an ice breaker cuts
through ice.
Wegener died in 1930 with few believing his
hypothesis.
✓REVIEW QUESTIONS
15
1. Who is credited with developing the continental drift
hypothesis?
2. What was probably the first evidence that led some to
suspect that the continents were once connected?
16
6. Early in the 20th century, what was the prevailing view of
how land animals migrated across vast expanses of
ocean?
3. What was Pangaea?
4. List evidence Wegener gathered to support his hypothesis.
Paleomagnetism
✓REVIEW QUESTIONS
5. Explain why the discovery of Mesosaurus in both South
America and Africa, but nowhere else, supports the
continental drift hypothesis.
17
• Little was done from 1930-1950’s on continental
drift.
• The breakthrough came by exploration of the
seafloor using paleomagnetism or fossil
magnetism.
• When lava flows cool below about 585° C (Curie
Point), the magnetic minerals align to the existing
magnetic field.
• Once cooled, the magnetism is “frozen” in place and
point to the existing magnetic poles at the time of
their cooling. This is know as paleomagnetism.
• The Earth’s magnetic field not only has direction,
but also dip or inclination.
BLOCK EXAM #1 - CHAPTERS 1 & 2
7. How did Wegener account of glaciers in the southern
landmasses, while areas in North America, Europe, and
Siberia supported lush tropical swamps?
Paleomagnetism
18
Earth’s magnetic
field produces lines
of force like a bar
magnet (but it is
not produced in
this way.)
Note: true north
and magnetic
north are not the
same.
Page 15 of 24
19
Paleomagnetism
Å Above the Curie
Point, the magnetic
directions are randomly
aligned.
The Earth’s
magnetic field
not only has
direction, but
also dip or
inclination.
Below the Curie Point,Æ
the magnetic directions
are aligned with the
Earth’s magnetic field at
the time of cooling.
Locked in are the
direction and dip of the
magnetic field.
21
Paleomagnetism
The direction and dip determine the location of the Paleopole.
Apparent Polar Wandering
X
20
Paleomagnetism
23
Apparent Polar Wandering
22
• S. K. Runcorn discovered in the 1950’s that the
apparent position of the paleopoles had wandered
from the present position of the north magnetic pole.
• The pattern of the “polar-wandering” paths were
similar for lavas in North America and Europe, except
they were separated by about 24º longitude – which
is the width of the Atlantic Ocean.
• If North America and Europe were brought back
together, the paths matched. This rekindled interest
in continental drift.
• However, because of the spinning of the Earth,
scientists believe that the magnetic pole does not
wander very far from the geographic poles. So a
better explanation is that the continents have moved.
Apparent Polar Wandering
24
✔
BLOCK EXAM #1 - CHAPTERS 1 & 2
Page 16 of 24
A Scientific Revolution Begins
25
• During the 1950’s and 1960’s, extensive mapping of
the ocean floor took place. This was the last missing
piece to the puzzle of Plate Tectonics.
• The oceanic ridge system was found to circle the
globe, have high heat flow and volcanism, and
indications of tensional stresses (divergence).
• In other parts of the oceans, scientist were
discovering and locating the many deep earthquakes
associated with trenches.
• Dredging of the oceanic crust found no rocks older
than 160 million years. How could it be so young?
Seafloor-Spreading Hypothesis
27
Seafloor-Spreading Hypothesis
26
• Harry Hess (early 1960’s) first proposed seafloor
spreading by mantle convection.
• His model is centered on the activity that happens in
the ocean basins.
• He proposed that the mantle upwells at ocean ridges
and spreads laterally, carrying the seafloor in a
conveyor-belt fashion away from the ridge crest.
• Newly formed oceanic crust fills the gap. That
explains why the oceanic crust is so young.
• Trenches are the locations where oceanic crust is
drawn back into the Earth’s mantle.
• Hess also correctly proposed the continents passively
moved by convection – unlike Wegener’s hypothesis
of continents plowing through the seafloor.
Geomagnetic Reversals
28
• The Earth’s magnetic field was found to have
reversed polarity periodically in the past, based on
lavas from around the world.
• When reversed, the north magnetic pole becomes
the south magnetic pole, and vice versa! Polarity
reversals occur frequently (in a geologic sense) on
average every 500,000 to 700,000 years.
• About 171 reversals are known since the end of the
Cretaceous period (about 65 million years ago.)
• During a reversal, the intensity or strength of the
Earth’s magnetic field decreases to almost zero, and
then flips polarity, and rebuilds strength in the
opposite direction.
Geomagnetic Reversals
29
BLOCK EXAM #1 - CHAPTERS 1 & 2
Geomagnetic Reversals
30
Page 17 of 24
Seafloor Spreading
31
ƒ Vine & Matthews in
1963 linked reversals to
simple patterns of
magnetic variations on the
ocean floor – thus proving
the seafloor spreading
hypothesis.
ƒ J. Tuzo Wilson in 1965
put the last piece of the
puzzle together by dividing
the Earth’s outer shell in
rigid plates – Plate
Tectonics was born!
Plate Tectonics
33
• Plate Tectonics – The uppermost mantle and
crust (lithosphere) behave as a rigid layer (plate)
and overlies a weaker region of the mantle
(asthenosphere). The lithosphere is effectively
detached from the layers below and can move
horizontally great distances.
• Plate boundaries are of three types:
• Divergent – plates move apart: results in
production of oceanic lithosphere.
• Convergent – plates move together: results in the
consumption of oceanic lithosphere.
• Transform Fault – plates grind past each other:
without gain/loss of lithosphere.
Rates of Plate Motion
35
✓REVIEW QUESTIONS
32
8. Explain how paleomagnetism can be used to establish
the latitude of a specific place at some distant time.
9. What is seafloor spreading? Who formulated it? Where
is it active today?
10. How did Vine & Matthews related the seafloor
spreading hypothesis to magnetic reversals.
34
Plate Tectonics
The lithospheric plates interact along their
boundaries in three different ways.
Divergent Boundaries
36
• Most divergent boundaries are located along
crests of oceanic ridges and are constructive plate
margins, since new oceanic lithosphere is generated
here – also know as spreading centers.
• The interconnected oceanic ridge system is the
longest topographic feature on Earth, exceeding
43,000 miles in length. The central portion may
have a down-faulted structure called a rift valley.
• The primary reason the oceanic ridge is elevated is
that the newly created oceanic crust is hot and the
volume expands. As it moves away from the ridge,
the new lithosphere cools and contracts and sinks,
creating greater ocean depths away from ridges.
BLOCK EXAM #1 - CHAPTERS 1 & 2
Page 18 of 24
Oceanic Ridge
37
The oceanic
lithosphere
becomes
thicker as it
moves away
from the ridge
and cools –
ocean depths
also increase.
38
Continental Rifting
When Pangaea
split apart,
continents were
torn apart by
divergence. The
process starts as a
continental rift
and may progress
to form a complete
oceanic ridge
system.
Basin & Range
Nevada
East African
Rift
Red Sea
What makes these
a true Divergent Boundary?
Atlantic
Ocean
Divergent Boundaries
39
40
Convergent Boundaries
• Since Earth is not growing is size, older portions of
the oceanic lithosphere must descend into the
mantle along convergent boundaries –
destructive plate margins.
• The deep-ocean trench is the surface expression
of the descending plate. These convergent
boundaries are also called subduction zones
because the cold oceanic lithosphere is more dense
and sinks into the mantle.
• There are three types of convergent boundaries:
1. Oceanic-Continental Convergence
2. Oceanic-Oceanic Convergence
3. Continental-Continental Convergence
Oceanic-Continental Convergence
41
Oceanic-Continental Convergence
42
• Active Examples: Andes & Cascade Range
• Inactive: Sierra Nevada
BLOCK EXAM #1 - CHAPTERS 1 & 2
Page 19 of 24
Oceanic-Oceanic Convergence
43
Oceanic-Oceanic Convergence
44
45
Continental-Continental Convergence
46
47
Transform Fault Boundaries
48
• Active Examples: Aleutian, Japan, Philippines,
Mariana & Tonga
Continental-Continental Convergence
No active volcanoes! Why?
• Active Examples: Himalayas & Alps
• Inactive: Appalachians & Urals.
Collision of India and Asia
• Where plates slide by one another, there is no
production or destruction of lithosphere – the
lithosphere is conserved – this know as a
transform fault boundary.
• Most transform faults join segments of an oceanic
ridge and produce prominent linear breaks in the
oceanic crust known as fracture zones.
• A few transform faults cut through continental crust
and two examples are the San Andreas Fault of
California and the Alpine Fault of New Zealand.
Even in these cases, the transform fault connects
oceanic ridges.
BLOCK EXAM #1 - CHAPTERS 1 & 2
Page 20 of 24
Transform Fault Boundaries
49
ƒ Notice that the
transform fault is
only active between
offsets in the
oceanic ridges.
ƒ The fracture
zones are linear
scars in the
topography of the
ocean floor.
Transform Fault Boundaries
51
Transform Fault Boundaries
50
ƒ North of Mendocino, the San
Andreas Fault ends and
subduction begins; this creates
the active volcanoes of the
Cascade Range.
✓REVIEW QUESTIONS
52
11. Where is lithosphere being formed? Consumed? Why
must the amount of production and destruction of the
lithosphere be equal?
12. Why is oceanic lithosphere subducted while
continental lithosphere is not?
13. How did the Himalaya Mountains form?
14. Differentiate between transform faults and the two
other types of plate boundaries.
Hot Spots and Mantle Plumes
53
Hawaiian Hot Spot
54
• Dating of some volcanic island chains show an age
progression away from active volcanoes. The basalt
erupted at hotspots is chemically different than that
erupted at mid-ocean ridges, indicating two
separate sources.
• Mantle plumes rise from deep in the mantle
(perhaps the core-mantle boundary) and melt
beneath the lithosphere, creating a hot spot: an
area of active volcanism. The hot spot stays
stationary for long periods of time, so as the
lithosphere moves, new younger volcanoes are
created in a chain – or hot spot track – which
record the direction of plate motion.
BLOCK EXAM #1 - CHAPTERS 1 & 2
Page 21 of 24
Hot Spots & Hot Spot Tracks
55
57
• Driving Forces: slab pull, ridge push, and slab suction.
• Resistive Forces: mantle drag and plate resistance.
Layered Mantle Convection
56
• Researchers generally agree on the following:
1. Convection in the mantle – warm, buoyant rock
rises and cooler, dense material sinks – is the
underlying driving force of plate movement.
2. Subducting oceanic plates drive the cold downwardmoving portion of convection, while the upwelling
portion consists of the shallow oceanic ridges and
the deeper mantle plumes.
3. The slow movements of Earth’s plates and mantle
are ultimately driven by the unequal distribution of
heat within Earth’s interior. Heat is transported
away from the Earth’s core and up through the
mantle.
• Active Examples: Hawaii, Yellowstone, Iceland,
Galapagos, Bermuda, Azores, and Easter Island.
Forces That Drive Plate Motion
What Drives Plate Motion?
59
• Problem: Seismic imaging of subducting slabs has
shown they penetrate the 660-km boundary, thus upper
and lower mantle would mix together.
BLOCK EXAM #1 - CHAPTERS 1 & 2
Models of Plate-Mantle Convection
58
• Researches generally agree on the following:
1. In Harry Hess’s model, the convection currents came
from deep within the mantle and spread laterally and
pull the plates apart (the plates were viewed as
being carried passively by the flow in the mantle).
2. However, the opposite appears to be true: the
horizontal movement away from ridges is causing
shallow upwelling and as the plates move, they drag
the adjacent material in the mantle to induce flow.
3. Basaltic lavas erupted along oceanic ridges are fairly
homogeneous in composition and depleted of certain
trace elements. By contrast, hot-spot volcanism have
concentrations of trace elements – so two distinct
source regions must exist within the mantle.
Whole Mantle Convection
60
• Problem: Whole mantle would mix together very
quickly and eliminate the unique deep source of hot spot
material – it would be the same as the ridge material.
Page 22 of 24
Deep Layer Model
61
62
Deep Layer Model
• Problem: The deep layer has not been observed in
seismic images. But this is the most current model.
From Louise Kellogg – UC Davis
✓REVIEW QUESTIONS
63
64
15. Some people predict California will sink into the
ocean. Is this idea consistent with the theory of plate
tectonics?
Convection and Tectonics
16. What is the oldest ages for oceanic seafloor and
continental rocks?
17. If hot spots are fixed, in which direction was the
Pacific Plate moving when the Emperor Seamounts
were produced, compared to the present Hawaiian
Islands?
✓REVIEW QUESTIONS
✓REVIEW QUESTIONS
65
66
18. Which of 3 boundaries or hotspot are these located?
18. Which of 3 boundaries or hotspot are these located?
•
Himalayas:
•
Basin & Range:
•
Aleutian Islands
•
Alps:
•
Red Sea:
•
Philippines:
•
Andes Mountains:
•
Hawaii:
•
San Andreas Fault:
•
Urals:
•
Iceland:
•
East African Rift:
•
Japan:
•
Red Sea:
•
Mount St. Helens:
•
Tonga:
•
Sierra Nevada:
•
Yellowstone:
•
Appalachians:
•
Cascade Range:
•
Galapagos:
•
Alpine Fault, New Zealand:
BLOCK EXAM #1 - CHAPTERS 1 & 2
Page 23 of 24
✓REVIEW QUESTIONS
19. Describe the
three
proposed
models for
mantle-plate
convection.
What is lacking
in each of
these
models?
67
]
If slab subducts into
the lower mantle –
the whole mantle
would eventually
mix – and basalts
from MOR and
hotspots would
have the same
composition.
]
No evidence of a
deeper layer in the
mantle.
BLOCK EXAM #1 - CHAPTERS 1 & 2
Distribution of Earthquakes (1980-1990)
68
Note the circum-Pacific belt (‘Ring of Fire’) of
earthquakes. Most earthquakes occur at plate
boundaries.
Page 24 of 24