Transcript Lecture 4

Visualizing
Geology
First Edition
Barbara W. Murck
Brian J. Skinner
Dana Mackenzie
Chapter 4
Plate Tectonics
Copyright © 2008 by John Wiley & Sons, Inc.
Located in the Himalayan Mountain Range, on the
border of China and Nepal, the peak of Mount Everest
is the highest point on earth. It towers 29,028 feet
(8,848 meters) above sea level.
Located in the Pacific Ocean, just southeast of the
Mariana Islands, Mariana Trench is the deepest point
on earth. At 35,798 feet (11,932 meters) below sea
level, Mariana Trench is farther below sea level than
Mt. Everest is above it. At the bottom of the Mariana
Trench, water exerts a pressure of over a thousand
times that of standard atmospheric pressure at sea
level.
Scientific revolutions challenge us to look at the world
in a new way.
Just like in 1543 when Nicolaus Copernicus argued
that the planets revolved around the Sun and he was
called mad and eccentric, and later when Galileo tried
to defend this heliocentric argument, Galileo was
tormented and arrested until the day of his death.
In the 1960’s geology had a big revolution when
geologists discovered new evidence for a 50 year old
idea called continental drift. Just like the Copernicus''
theory the hypothesis that continents move provoked a
great controversy.
Wegener’s theory of continental drift
In 1912, a German meteorologist named Alfred
Wegener had the idea that all the continents were
joined together in a single “supercontinent” called
Pangea, which means “all lands”. He also suggested
that Pangea had split into fragments like pieces of ice
floating on a pond and that the continental fragments
had slowly drifted to their present locations.
Continental drift – The slow, lateral movement of
continents across the surface.
Wegener although he had marshalled a good deal of
evidence couldn’t explain the mechanism that would
move the continents.
The puzzle piece argument
Matching geology
Matching fossils
In 1915 Alfred Wegener drew a map much like this
one, showing the distribution of continents 300 mya.
What is the “true” edge of a continent?
How well do continents fit?
How well do the ages match?
Mountain belts that are similar
What evidence did glaciers leave?
Glaciers evidence,
South Africa
Similar fossils of Glossopteris have been found in
Africa, India, Australia and South America.
Similar fossils of Glossopteris have been found in
Africa, India, Australia and South America.
Apparent polar wandering paths
Wegener gather evidence but other scientists were still
unconvinced.
In 1950, paleomagnetism emerged as a new tool.
Remember that when magma solidifies the iron
bearing minerals become magnetized and take the
polarity of the Earth at that time.
In the 1950’s geophysicists found out about a
phenomenon called apparent polar wandering.
The missing clue: sea floor spreading
In the 1960s many clues have amazed to support the
theory of continental drift.
Seafloor spreading – The processes through which the
seafloor splits and moves apart along a mid-ocean
ridge, and new oceanic crust forms along the ridge.
The symmetrical pattern of magnetic bands provided
powerful support for hypothesis, fist proposed in 1960,
called seafloor spreading.
The decisive piece of evidence for seafloor spreading
is that the ages of seafloor rocks increase with distance
from the ridge.
Recently we have access to even a more profound
piece of evidence with the use of GPS. GPS also
confirmed estimated movement velocities that vary
from 1 to 10 cm a year.
The extreme environment of the mid-ocean ridge
Measuring plate motion using GPS
Plate Tectonics and the Scientific Method
The development of the plate tectonic theory serves as an
excellent example of the scientific method.
The scientific method is a series of steps that can lead to
the discovery of new scientific principles. The steps are:
1) Gathering a set of observations or measurements (a
data set) from natural phenomena or experiments.
2) Formulating one or more hypotheses to explain the
observations or data. The hypotheses can be
conceptual, physical, or mathematical. A hypothesis
is unproved and must be testable.
Plate Tectonics and the Scientific Method
3) Making a set of predictions using the hypothesis. The
predictions are then tested. If the predictions are
correct, the hypothesis gains credibility. If the
predictions are incorrect, the hypothesis must be
modified or abandoned. The hypothesis is modified
and retested many times before they can account for
all the data.
4) Gaining the status of a theory after being tested by
new experimental advances. A theory is a generally
accepted explanation for a set of data or
observations.
Plate Tectonics and the Scientific Method
How does the development of plate tectonics follow the
scientific method?
Wegener gathered sets of observations on the
distribution of rock types, fossils, glaciation, and
climate zones. He searched for a common
explanation to relate these diverse features and
phenomena.
Wegener hypothesized that the present-day continents
were in different positions in the geologic past. By
reassembling the continents into a single
supercontinent, he explained the distribution of rock
types, glaciation, and fossils. The drift of the
continents over the surface of the Earth with time
explained the distribution of climate zones.
Plate Tectonics and the Scientific Method
Wegener also proposed that continents plowed through
oceanic crust. However, other geologists knew this
to be impossible because of the strength of rock.
Wegener's hypotheses, although satisfactory in many
respects, could not explain all the observations. It
was not until new methods of testing continental
drift (and sea floor spreading) were introduced that
plate tectonics gained acceptance. For example, it
was predicted that the age of rocks increased away
from mid-ocean ridges. Dating methods for rocks
proved this prediction true.
Plate Tectonics and the Scientific Method
Plate tectonics has been rigorously tested from
numerous subdisciplines in geology. Most geologists
accept these tests as verification of plate tectonics.
Successful testing and widespread acceptance
elevates plate tectonics from a hypothesis to a theory.
Plate Tectonics and the Scientific Method
Plate Tectonics and the Scientific Method
ATLANTIDA? LEMURIA?
LIFE ON MARS?
HOLLOW EARTH, SUBTERRANEAN
CIVILIZATIONS?
EXTRATERRESTRIALS?
GHOSTS?
What were the arguments that Wegener gave in favor of his
hypothesis of continental drift?
What further evidence for continental drift was discovered
after Wegener’s death?
What is Pangea?
What is seafloor spreading?
How does seafloor spreading create paleomagnetic bands?
How quickly do continents move, and how do we know?
The plate tectonic model
Plate tectonics – the movement and interactions of
large fragments of Earth’s lithosphere, called plates.
After the proposition of the theory of Plate Tectonics
geologist now could explain the forces behind
mountain building and the formation of ocean
trenches.
Note that we call it a theory rather than a hypothesis. A
hypothesis is a tentative explanation, whereas a theory
is an explanatory model that is supported by a lot of
scientific evidence.
Plate tectonics in a nutshell
The Earth’s lithosphere or rocky outer layer, very thin
relative to the Earth as a whole. The solid rock of the
lithosphere lies on top of a vast mantle, made of hotter
and weaker material that is constantly in a very slow
motion.
The layer directly below the lithosphere is called the
asthenosphere, the relationship between these two
layers is called isostasy, which means literally that the
lithosphere is “floating” onto the asthenosphere.
Plate tectonics in a nutshell
Driven in part by the underlying motion of the
asthenosphere, these plates collide, split apart, and
slide past one another. This generates a lot of geologic
activity, such as earthquakes and mountain building.
The boundaries of these plates are marked by huge
fractures called faults.
Fault – A fracture in Earth’s crust along which
movement has occurred.
Types of plate margin –
Divergent margin – A boundary along which two
plates move apart from one another.
Convergent margin – A boundary along which two
plates come together.
Transform fault – A fracture in the lithosphere where
two plates slide past each other.
Subduction zone – A boundary along which one
lithospheric plate descends into the mantle beneath
another plate.
Earthquakes and plate margins
Earthquakes occur along faults where huge blocks of
rocks are grinding past each other.
The movement is rarely smooth: usually blocks tick
because of friction, when the friction is overcome the
blocks slip abruptly, releasing energy with a huge snap
– an earthquake.
The location beneath the surface where the earthquake
begins is called the focus, this should not be confused
with the epicenter which is the point on the surface of
the Earth that lies directly over the focus.
In search for a mechanism
What exactly drives the plates motion?
How does the mantle interact with the crust?
What initiates subduction?
Thermal motion is at least partially responsible for the
motion of the plates.
Earth’s internal heat – Earth gives off heat for two
main reasons. First the slow cooling from its initial
formation process and second heat is being constantly
generated by the decay of radioactive elements.
Convection as a driving force –
Convection – A form of heat transfer in which hot
material circulates from hotter to colder regions, loses
its heat and then repeats the cycle.
Tectonic cycle – Movements and interactions in the
lithosphere, by which rocks are cycled from the mantle
to the crust and back, includes earthquakes, volcanism,
and plate motion, driven by convection in the mantle.
What is the motor behind the theory of plate tectonics?
Why has the Earth’s lithosphere broken into plates?
How have geologists determined the shapes of Earth’s
plates?
What are the different types of plate margins?
Explain why different kinds of plate margins are
susceptible to different kinds of earthquakes
Where is new seafloor formed? Where is old seafloor
recycled?
The Hawaiian Islands – In 1840 a geologist named J D
Dana noticed the increasing ages of the islands from
southeast to northwest.
The Hawaiian Islands – The Kilauea volcano added
new land. Just 15 km south of Hawaii a new island has
been born.
The Hawaiian Islands – Loihi seamount is an
underwater volcano that is about 1km under water and
growing everyday.
Mini version of plate tectonics form by a volcanic
eruption in Hawaii. Can you spot a convergent margin,
a divergent margin, and a transform fault that joins
two divergent margins.
Mini version of plate tectonics form by a volcanic
eruption in Hawaii. Can you spot a convergent margin,
a divergent margin, and a transform fault that joins
two divergent margins.
What is the name that Wegener gave to the super
continent?
Give some examples that support the idea of
continental drift.
What is polar wandering?
Name the three types of plate interaction.
What is the name of a recent technology that allowed
scientists to measure the movement of continental
crust?
According to this illustration, where are the oldest
rocks located and the youngest?
Draw an arrow that determines the direction of
movement of the plate over the hot spot.
What is the explanation of why do plates move?
What is convection?
What is a subduction zone?
Label the following
block diagrams.
The idea proposed by Alfred Wegener to explain the
continental shapes and positions is known as _____.
A) Pangaea
B) plate tectonics
C) continental drift
D) rift valley
E) sea floor spreading
According to Wegener's model, what evidence did glaciers
leave for the existence of Pangaea?
A) striations
B) glacial lakes
C) fossils
D) soil deposits
E) ice
In the diagram below, which two are the best examples of
different continental positions in the past?
A) North America – South America
B) North America – Africa
C) South America – Asia
D) Europe – North America
E) South America – Africa
What was the name of the floral fossils Wegener used for the
idea of continental drift?
A)
B)
C)
D)
E)
Mesosaurs
trilobites
Glossopteris
Pseudoschwagerina
oak
In the figure below, what is the age of the sea floor off of the
Bahamas?
A) younger than 9.6 million years
B) 9.6 – 33.0 million years
C) 33.0 – 83.0 million years
D) 83.0 – 141.9 million years
E) older than 141.9 million years
In the figure below, what is the approximate age of the sea
floor off the north coast of Spain?
A) younger than 9.6 million years
B) 9.6 – 33.0 million years
C) 33.0 – 83.0 million years
D) 83.0 – 141.9 million years
E) older than 141.9 million years
Which of the following terms best describes the rocky outer
layer of the Earth?
A) athenosphere
B) biosphere
C) atmosphere
D) lithosphere
E) hydrosphere
Which type of plate boundary results from two or more plates
coming together?
A) convergent
B) divergent
C) transform
D) hot spot
E) ridge
What is the depth of the newly forming Hawaiian island,
Loihi?
A) about 1000m
B) about 2000m
C) about 3000m
D) about 4000m
E) it is at the surface
According to the figure below, which of the following types of
plate boundaries produce the deepest earthquakes?
A) transform fault boundary
B) divergent boundary
C) continental collision boundary
D) subduction zone boundary
E) all the same
According to the figure below, which of the following types of
plate boundaries produce the shallowest earthquakes?
A) transform fault boundary
B) divergent boundary
C) continental collision boundary
D) subduction zone boundary
E) all the same
What term best describes to driving force behind plate
tectonics?
A) expansion
B) convergence
C) convection
D) divergence
E) transformation
The phenomenon of the Earth's magnetic pole being in
different locations in the past is known as _______.
A) convection
B) sea floor spreading
C) normal polarity
D) reverse polarity
E) apparent polar wandering
In the figure below, how many magnetic reversals occurred
between 2.0 and 1.5 million years ago?
A) 1
B) 2
C) 3
D) 4
E) 0
In the figure below, how many magnetic reversals occurred
between 1.5 million years and present?
A) 1
B) 2
C) 3
D) 4
E) 0
According to the diagram below, which of the following plates
is all oceanic crust?
A) North America
B) Africa
C) Nasca
D) South America
E) Eurasian
According to the diagram below, which of the following is the
largest plate?
A) Pacific
B) Africa
C) Nasca
D) South America
E) Eurasian
According to the figure below, most of eastern Brazil is
approximately how old?
A) older than 500 million years
B) about 550 million years
C) younger than 550 million years
D) older than 1 billion
E) none of the above
According to the figure below, the island of Molokai was
probably over the hot spot approximately how many million
years ago?
A) 0.8
B) 1.0
C) 1.30
D) 2.9
E) 3.2
True/False
Pangaea is the name of the supercontinent proposed by Harry
Hess.
The Hawaiian Islands are a result of the Pacific Plate passing
over a hot spot.
A hypothesis is a scientific theory that has been tested.
The area of the Earth's interior where rocks start to melt is
known as the asthenosphere.
Rocks closer to the spreading centers in the oceans are older
than the rocks farther away from the spreading center.
True/False
Global positioning systems (GPS) are a reliable way to
measure plate movements.
Obduction is the processes where one plate slides under
another.
“Black smokers” are chimneys on the sea floor that release
superheated gases.
Theories are able to be proved with scientific evidence.
Fill in the blank
The transform plate margin found in the western United States
is characterized by the _______ fault.
Fill in the blank
The rocky, outermost part of the Earth, comprising the upper
part of the mantle and the crust, is known as the _______.
The reptilian fossils that Wegener used to help prove his ideas
about plates are known as _______.
The process in which the sea floor separates and moves in
opposite directions is known as _______.
The exact point where an earthquake occurs below the surface
is the _______.
The rate at which plates move apart is approximately 1 to 10
cm/year. (true/false)
Fill in the blank
The point on the surface directly above an earthquake
occurrence is called the _______.
The process of one plate diving under another plate is known
as _______.
Short essay
How does a convergent plate boundary differ from a divergent
plate boundary?
How do paleomagnetic stripes on the sea-floor support seafloor spreading?
Short essay
How did the presence of Glossopteris fossils support
Wegener's idea of continental drift?
Why are earthquake foci depths associated with transform
plate boundaries fairly shallow?
How does the global distribution of earthquakes relate to the
plate boundaries?
Describe the evidences Wegener used to reconstruct Pangaea.
Describe the Plate Tectonic model.
Short essay
Why are volcanoes not present with all types of plate
boundaries?
Briefly explain how convection drives plate motion.