101 chap2 intro platetectonics
DESCRIPTION
Intro to PlateTectonicsTRANSCRIPT
Introduction
Topics and Concepts
The vastness of time and scale in geology
The Scientific Method
The Law of Uniformitarianism
Age of the Solar System
Time
The rates of most geological processes are
too slow to be easily observed by humans
This is the big difference between geology
and other sciences
Rates of most geologic processes range from
micrometers per year to centimeters per year
There are certain exceptions
such as earthquakes, landslides,
and volcanic eruptions
Barringer Meteor Crater
in Arizona
This feature, which is
about 2 kilometers
across, formed in less
than one minute about
50,000 years ago
Some Geologic Events are Rapid
The Grand Canyon in Arizona
It took more than 250 million years to deposit this sequence of rocks
The rocks at the bottom of the canyon are about 2 billion years old
Most Geologic Events are Slow
Geology is a Science (Just like chemistry and physics)
However, because of
the scale of the
problems considered,
both in size and time,
geologists face the
special challenge of
not being be able to
conduct experiments
in the typical
laboratory setting
Opinion is NOT part of the
Scientific Method
The Scientific Method
The Scientific Method
All science is based on
the assumption that the
natural world behaves
in a consistent and
predictable manner that
is comprehensible
through careful,
systematic study
The Scientific Method
The collection of scientific facts
through unbiased observations
and measurements
The Scientific Method
The collection of scientific facts
through unbiased observations
and measurements
The development of one or more
working hypotheses or models
to explain these facts
The Scientific Method
The collection of scientific facts
through unbiased observations
and measurements
The development of one or more
working hypotheses or models
to explain these facts
The development of
observations and experiments
to test the hypotheses
The Scientific Method
The collection of scientific facts
through unbiased observations
and measurements
The development of one or more
working hypotheses or models
to explain these facts
The development of
observations and experiments
to test the hypotheses
The acceptance, modification or
rejection of the hypotheses or
models
Hypothesis Theory Law A hypothesis is an explanation initially offered
for a set of observations
When a hypothesis withstands many tests it
may be called a theory
A theory for which it seems there is no
sensible reasons to challenge is called a law
James Hutton
James Hutton (1726-
1797) is considered to
be the father of modern
geology
Born in Scotland,
schooled as both a
lawyer and a doctor, he
was a “gentleman
farmer” who spent most
of his free time pursuing
the study of geology
During a lifetime of research, James Hutton
set the groundwork for one of the most
important concepts in science
Law of Uniformitarianism
It can be summed up in a simple sentence:
Law of Uniformitarianism
“The present is the key to the past”
Law of Uniformitarianism
Hutton argued here that the
earth was shaped in the past
by the same forces that shape
it in the present, namely
erosion, sedimentation, and
gradual uplift
He maintained that rock formation was a cyclical
process, with mountains being eroded into the sea,
only to be consolidated and raised into new
mountains, which would in turn be eroded again
It was a view of earth-history that required a
vast expanse of geological time
The Earth could not be a mere 6,600 years old
Law of Uniformitarianism
Hutton was right
Science has
proven that the
Earth is very old
and the universe
is much older
Meteorites give us
access to debris left
over from the formation
of the Solar System
We can date meteorites
using radioactive
isotopes and their
decay products
The Age of the Earth
This radioactive dating has conclusively
proven that the Earth is just over
4,500,000,000 years old
It is difficult for the
human mind to grasp
such a large number or
to comprehend such a
staggering time span
So we will count to 4
billion 500 million at the
rate of one number per
second
The Age of the Earth
4,500,000,000 seconds
The Age of the Earth
4,500,000,000 seconds
75,000,000 minutes
The Age of the Earth
4,500,000,000 seconds
75,000,000 minutes
1,250,000 hours
The Age of the Earth
4,500,000,000 seconds
75,000,000 minutes
1,250,000 hours
52,083 days
The Age of the Earth
4,500,000,000 seconds
75,000,000 minutes
1,250,000 hours
52,083 days
142 years 7 months 4 days
The Age of the Earth
4,500,000,000 seconds
75,000,000 minutes
1,250,000 hours
52,083 days
142 years 7 months 4 days
If you start counting today
The Age of the Earth
4,500,000,000 seconds
75,000,000 minutes
1,250,000 hours
52,083 days
142 years 7 months 4 days
If you start counting today
Wednesday, August 22, 2012
The Age of the Earth
4,500,000,000 seconds
75,000,000 minutes
1,250,000 hours
52,083 days
142 years 7 months 4 days
If you start counting today
Wednesday, August 22, 2012
You will finish on
The Age of the Earth
4,500,000,000 seconds
75,000,000 minutes
1,250,000 hours
52,083 days
142 years 7 months 4 days
If you start counting today
Wednesday, August 22, 2012
You will finish on
Saturday, March 29, 2155
The Age of the Earth
Test Question
1. How old is the Earth?
A. 4.5 trillion years
B. 4.5 billion years
C. 4.5 million years
D. 450,000 years
Chapter 2
Plate Tectonics: A Scientific Revolution Unfolds
Why?
Why is Mount Everest
over 29,000 feet high?
Why is it in Asia?
Why isn’t it in
Tennessee?
The unifying concept of the Earth sciences
The outer portion of the Earth is made up of about 20 distinct “plates” which move relative to each other
Plate tectonics explains:
Volcanoes
Earthquakes
Sea floor spreading
Mountain chains
Occurrence of same fossils on different continents
Plate Tectonics
Integrates evidence from many branches of science
First suggested based on evidence from cartography, geology and paleontology
Fully embraced after evidence from geophysical measurements
However, the proof of plate tectonics was a long rambling road
Plate Tectonics
300-400 years ago
(after accurate maps
became available) it
was noticed that the
coastline on both
sides of the Atlantic
could fit together
like a jigsaw puzzle
Atlantic Coastline
The fit was even
better when you
took the continental
shelf into
consideration
In 1858, Antonio
Snider-Pellegrini, an
American published
a sketch showing
this fit
Atlantic Coastline
Gondwanaland
In the 1890s, the Austrian
Geologist Eduard Suess
suggested that the present
day southern continents
once been formed in a
super continent which he
called Gondwanaland
The name comes from the
Gondwana region in India
Gondwanaland Gondwanaland included Antarctica, South
America, Africa, Madagascar, Australia and
India
Gondwanaland
Unfortunately, Suess
did not have any
evidence other than
the jigsaw puzzle fit
He could not explain
how Gondwanaland
split up into the
modern continents
However, he got to be
on a stamp
Continental Drift
Continental drift was proposed
by the prominent German
geologist Alfred Wegner in 1912
based on his observation of
drifting sheets of ice
The concept is that large-scale
horizontal movements of the
Earth’s continents are
responsible for the major
topographical features such as
mountains and ocean basins
Continental Drift Wegner called his super-continent “Pangaea”
He spent the next two decades, until his death,
promoting continental drift
Talk about an idea whose time
had not arrived!
Most scientist ridiculed Alfred
Wegener’s idea
Not even the public believed it
possible
No proof that continents move
No “driving mechanism” that
could move the continents.
“Why should the continents
move?”
Rejection of Continental Drift
Accumulating Evidence
Fossil record
Earth’s crust and mantle
Magnetism and the Earth’s
magnetic field
Paleomagnetism
Magnetic reversals
Polar wandering
The topography of the seafloor
Age of the seafloor
Seafloor spreading
The fossil record had revealed that the geology
and paleontology matched on opposite sides of
the Atlantic Ocean
Fossil Record
In fact, there are matching fossil records that
span across all of the continents
Without continental drift, this is hard to explain
Fossil Record
The Earth’s crust and uppermost mantle are solid, somewhat brittle and elastic
This outmost solid layer is called the lithosphere
It is thinnest under the oceans, averaging 50 kilometers (km) thick
The lithosphere under the continents can be up to 100 kilometers thick
Crust and Mantle
The layer below the lithosphere is called the asthenosphere and it extends to a depth of 300 kilometers
The rock is not rigid and can flow plastically due to the high temperature and moderate pressures
The existence of the plastic asthenosphere made plate motion more feasible
Crust and Mantle
Most iron-bearing minerals are at least weakly magnetic
Each magnetic mineral has a Curie temperature, the temperature below which it remains magnetic
Above the Curie temperature the mineral is not magnetic
The Curie temperature varies from mineral to mineral, but it is always below the melting temperature of the mineral
Magnetism
Test Question
The temperature point below which iron minerals
remain magnetic and above which they do not remain
magnetic is called the?
A. Wegener temperature
B. Magnetic pole temperature
C. Hawkins temperature
D. Curie temperature
Earth’s Magnetic Field
The simple presence of iron in the Earth’s core
is not enough to account for the Earth’s
magnetic field
The high temperatures in the Earth’s core are
far above the Curie temperature for any
magnetic mineral
The Earth has a magnetic
field
This why a compass points
to the north
Earth’s Magnetic Field
It is believed that the Earth’s magnetic field
originates in a layer called the outer core
The outer core is composed of a metallic fluid
consisting mainly of iron
This metallic fluid is
in motion and the
convection currents
act like a giant
dynamo, converting
mechanical energy
into magnetic energy
A hot magma is not magnetic
As a magma cools and solidifies, the iron-bearing minerals (such as the various ferromagnesian silicates) crystallize
Eventually, the minerals cool below the Curie temperature and the iron-bearing minerals become magnetic
Paleomagnetism
Like tiny compass needles, these magnetic minerals align themselves parallel to the lines of force of the Earth’s magnetic field
This remnant magnetism, which is also called paleomagnetism, points to the north pole like a sign post
But...
Paleomagnetism
Magnetic Reversals
About a century ago, a sequence of lava flows
were found in France where some of the flows
had the north and south poles reversed
Therefore, the north pole and south pole must
have repeatedly swapped positions
Magnetic Reversals
These magnetic reversals have occurred
though out the history of the Earth
They occur on an irregular basis ranging in
time from tens of thousands of years to
millions of years
Magnetic Striping on Seafloor
In the 1950s, the Atlantic
seafloor was found to
consist of alternating
stripes of normal and
reversely magnetized
rocks
Topography of the Atlantic
Also, in the 1950s, it
was discovered that an
underwater mountain
range ran north-south
in the middle of the
north Atlantic Ocean
The Mid-Atlantic Ridge
rises as high as 2
kilometers (1.5 miles)
above the abyssal
plain
Topography of the Atlantic
It was eventually discovered that all of the oceans
have mid-oceanic ridges
In the 1960s, samples were
collected from the Atlantic
seafloor using special ships
with drill rigs
The rocks of the Atlantic
seafloor were discovered to
be basalt
Basalt contains radioactive
isotopes (such as U235)
which can be dated
Atlantic Ocean Sea Floor
Atlantic Ocean Sea Floor
It was discovered that the
youngest rocks of the
Atlantic Ocean seafloor are
found along the mid-oceanic
ridge
And that farther you move
away from the ridge, the
older the rocks become on
either side of the ridge
Seafloor Spreading The oldest oceans on Earth are only 180 million
years old, a fraction of the Earth’s total age
Therefore, the oceans are, relatively speaking,
young geologic surface features
Seafloor Spreading
By 1962, Harry Hess at Princeton
University and a Naval Reserve
Rear Admiral, and Robert S. Dietz
had coined the term seafloor
spreading
And in 1963, the team of F. J. Vine
and D. H. Matthews (and
independently L. W. Morley)
proposed that seafloor spreading
could explain the observed
magnetic reversal striping on the
Atlantic and Pacific seafloors
Seafloor Spreading
The Mid-Atlantic
Ridge is a great
fault zone where
hot magma rises
up, cools and
solidifies, forming
new basalt
Seafloor Spreading
The basalt becomes
magnetized in the
prevailing direction
of the Earth’s
magnetic field
Plates that Move
In 1965, the Canadian
Geologist, J. Tuzo Wilson
proposed the concept that
the crust of the Earth is a
mosaic of interacting plates
He called his hypothesis
plate tectonics
Plates that Move
Wilson explained that these
plates move relative to each
other
The continents ride on these
plates
Geologic features, such as
mountains, volcanoes and
earthquakes occur along the
plate boundaries
Plates that Move
There are 20 large and several micro tectonic plates
Plate movement is best described as chaotic
Each plate moves at a different speed and in
a different direction
Plates Move Slowly
Measured using GPS data
Most plates move from
about 15 to ~100
millimeters (1/2 to 4 inches)
per year
Or about the thickness of a
fingernail in one day
However, the Nazca Plate is
moving at ~150 mm (~4
inches) per year
Plates Move Slowly
Plate Boundaries The red dots show that most major earthquakes
occur along plate boundaries
Plate motion causes earthquakes
The black triangles show that volcanoes
commonly occur along plate boundaries
Plate Boundaries
3 Types of Plate Boundaries
Wilson proposed that tectonic plates interact in
three different ways along their boundaries
1. Transform 2. Divergent 3. Convergent
Test Question
The previous slide is what I call a “key slide”
The entire rest of today’s lecture is based upon
the material contained in the previous slide
So yes, this is an obvious test question
Divergent plate boundaries usually start
within continents
They can grow to become ocean basin
Divergent Plate Boundaries
Rifting occurs
where tensional
forces thin the
crust, magma
ascends and
volcanoes form
The crust is pulled
apart, forming a
valley
Divergent Plate Boundaries
A long, narrow lake or sea forms
Divergent Plate Boundaries
Eventually, an
expansive ocean
basin and ridge
are created
Divergent Plate Boundaries
Can see the spreading
on the island of Iceland,
which sits on top of the
mid-Atlantic Ridge
Note that uprise and
decompression of the
underlying asthenosphere
results in magma
formation
The crust responds by
brittle fracture
Continental Rifts
Early rift sediments are
downfaulted into the
developing rift (which is
called a graben)
Erosion takes place on
the sides of the rift valley
Continental Rifts
Two examples are the
East African and Rio
Grande Rift Valleys
This can be the beginning
of ocean formation
(although it may not get
that far)
Rifting often begins at a
triple junction
Continental Rifts
East African Rift Zone
Perhaps the most interesting and spectacular
plate tectonic rift zone on the land surface
Ruwenzori Ol Doinyo Lengai
East African Rift Zone
The East African Rift has a north-south
orientation with the African Plate moving to
the west and the Somali Subplate moving to
the east
East African Rift Zone
A triple junction,
where three plates are
pulling away from one
another: the Arabian
Plate, and the two
parts of the African
Plate (the Nubian and
the Somalian) splitting
along the East African
Rift Zone
The Rio Grande Rift Valley
extends north from Mexico,
near El Paso, Texas
through New Mexico into
central Colorado
The rift is now essentially
quiet tectonically, but
significant deformation and
faulting with major offsets
was responsible for the
formation of the rift during
the past 30 million years
Rio Grande Rift
The geology has been described as a "taffy-like"
thinning of the lower crust, with upper crust
faulting in many places, to produce the rift valley
Rio Grande Rift
“Two features of the region most clearly exemplify the
Southwest's geologic-climatic-demographic circumstances.
These features constitute two "natural geographic corridors"
or focal areas concerning the relationship between people and
the Southweses physical environment: Arizona's Mogollon
Rim Corridor and New Mexico's Rio Grande Rift Corridor”
Rio Grande Rift
NO!
The Earth is maintaining a constant diameter
New crust is created at mid-oceanic ridges
The old crust sinks back into the Earth at
subduction zones along convergent plate
boundaries
Is the Earth Expanding?
Convergent Boundaries
There are three types of convergent plate
boundaries
Oceanic–Oceanic Japanese Islands
Oceanic–Continent Andes Mountains
Continent–Continent Himalaya Mountains
Convergent Boundaries First we need to talk about the density of the rock
The continental crust is lighter and has an
average density of 2.8 g/cm3
The oceanic crust is heavier and has an average
density 3.2 g/cm3
Complicating things is that old oceanic crust is
more dense than young oceanic crust
Oceanic - Oceanic
When two oceanic plates
collide under the ocean, one
plate slides underneath the
other at a subduction zone
Oceanic - Oceanic A deep trench forms along
the subduction zone
This is caused by the plate
sinking into the interior of
the Earth
Oceanic - Oceanic
Magma rises up along the subduction zone
creating volcanoes and forms volcanic island
arcs such as the Japanese Islands
Oceanic - Oceanic What determines which oceanic plate sinks?
The older, more dense oceanic plate will sink
beneath the younger, less dense oceanic plate
Oceanic - Continent
When an oceanic plate
collides with a continental
plate, the oceanic plate
slides underneath the
continental plate
Oceanic - Continent
The heaver, more dense
oceanic plate sinks beneath
the lighter, less dense
continental plate
Oceanic - Continent
A deep trench forms along
the subduction zone
This is caused by the
oceanic plate sinking into
the interior of the Earth
Oceanic - Continent
The lighter continental
crust is pushed up and
forms a mountain range
Oceanic - Continent
Magma rises up along the subduction zone creating
volcanoes, which adds to the size of the mountains
and creates a continental volcanic arc
Andes Mountains
The Andes Mountains
stretch over 5500
miles along the Pacific
side of South America
Andes Mountains
The Andes Mountains
were and still are being
created by the Nazca
Plate colliding with the
South America Plate
Note that the continent
of South America is
essentially riding on
the South America
Plate
Most continental tectonic plates are formed of rock with essentially the same density
So, when two continental plates collide, one does not sink beneath the other
Instead, the two plates literally smash into each other
The continental crust is deformed and uplifted, creating mountains such as the Himalaya Mountains
Continent - Continent
Continent - Continent
50 to 90 million years ago the continent of
India was an island a couple thousand miles
south of Asia
Continent - Continent
The continent of India is
riding on the Indian Plate
The Indian Plate has been
moving north for tens of
millions of year
Starting around 40-50 million
years, India began to ram into
the continent of Asia
Continent - Continent
As India smashed into Asia, the continental
crust of both India and Asia were pushed up
and deformed, creating what is currently the
greatest mountain range on Earth, the
Himalayas
Why?
Why is Mount Everest
over 29,000 feet high?
Because the continent
of India collided with
the continent of Asia
By the way, Mount
Everest, the highest
mountain on Earth, is
composed of
limestone rock, which
was created at the
bottom of the ocean
Transform Plate Boundaries
A transform fault plate boundary
occurs when two plate slide past
each other in opposite directions
Transform Plate Boundaries
The most famous
example is the San
Andreas Fault Zone
in California
The portion of
California in blue is
heading northwest
to Alaska
This is the most
studied fault zone
in the world
San Andreas Fault Zone
Transform Plate Boundaries
A driving force for plate tectonics has not been definitely identified
Each plate weighs trillions upon trillions of tons
Thought to be caused by the convection of the mantle
Friction at base of the lithosphere transfers energy from the asthenosphere to the lithosphere
Convection may have overturned the asthenosphere 4 to 6 times
What Causes the Plates to Move
Convection in the Mantle
Hot mantle material rises at the spreading mid-
oceanic ridges
Convection in the Mantle
Some of the magma erupts, but most spreads out
under the lithosphere and drags the crust along
Convection in the Mantle
Eventually, the slowly cooling material sinks
back into the mantle
Convection in the Mantle
An alternate hypothesis is that the convection
process may involve two convection layers
Convection in the Mantle
Or perhaps the convection process behaves
in a chaotic manner
Moving Continents
It is interesting to
speculate on how
the Earth looked as
the continents were
moved around...
50 Million Years A.D.
Or what the Earth will look like in 50 million years
250 Million Years A.D.
Or in 250 million years...
Chapter 3
Matter and Minerals