Geo 12 Chaper VI Notes.docx

7/27/2019 Geo 12 Chaper VI Notes.docx

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Geography 12 Chapter 6: Plate Tectonics

Looking at a map of the world one can ask the obvious question Did the continents once fit together The answer of course is yes they did

Over 250 million years ago, all of the continents fit together as onesupercontinent

This was called Pangaea The incredible weight and size of the

continental plates caused it to movenot as a single unit but broken up intoplates

The asthenosphere is solid but theincredible heat causes it to act almostas a liquid

The plates then 'float' on the surface They sometimes come into collision

with other plate. The Himalayas were caused by one

such collision When the Indian subcontinent collided

with the Eurasian plate, the resulting


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collision was the thrusting up of the continental boundary creating themountain range known as the Himalayas

The theory of isostasy also tells us that for as high as they are, they arealso that deep

Isostasy explains the collision in mountain forming. Collision thrusts up and

down The heat from the mantle creates convection currents The explains the movement of the plates Since the ocean plates are closer to the crust than the continental plates,

the ocean plates move sooner Ocean floor spreading is the term used to describe this movement We know this by dating the rock on the ocean floors The science that

deals with theplate movement

is called platetectonics

There areapproximately 20 major plates

These plates arein constantmotion.

The picture atthe lower right

corner shows ageneralizedmovement of theplates

There are manymore smallerdifferences inthe platemovement

When two platescome up againsteach other, theycan create seismic events

The meeting of the plates is not always only one ay Plates can collide, they can rub against each other, or the collision can be

on an angle


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These collisions of the plates are themajor tectonic processes that helpfrom the surface of the earth

Folding When plates collide and when the

two plates are a similar density, theycan causes folding

The mountain range in Iran called

the Zagros is a good example of foldmountain process

We will discuss fold mountainsfurther in a later lesson

Faulting Faulting occurs when plates "slide"

past each other Major fault lines are areas of high

seismic activity In North America, the San Andreas

Fault is a well-known fault. It is because of this fault line that

San Francisco is home toearthquakes of varying degrees

Tremors are earthquakes of lowseverity


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Vulcanism

Volcanoes areone of the earthsmost violent

examples of platetectonics

It is where wecome face to facewith the heat andinstability of theearth's interior

Volcanic activity islocated roundplate boundaries

The Pacific Oceanis surrounded bya ring of highlyactive volcanos

This area is known as the Ring of Fire

Plate boundaries Most of the earth's mountain building is due to plate tectonics The only one that is not is mountain building through vulcanism. The older mountains are the smallest

The Pyrenees in Europe are a very old mountain range This is evident from the low ,rounded tops on the mountains


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There are three main types of plate boundaries They are converging, diverging and transform fault When two plates move towards each other we say they are converging The results ofconverging plate boundaries varies When two continental plates move together we get mountain building This was shown in the creation of the Himalayan mountain range This process occurs because the plates are of the same density When one of the plates is denser than the other, the heavier plate will sink

under the lighter plate When the collision of an oceanic plate occurs with a continental plate, the

heavier oceanic plate subducts under the continental plate This is called a subduction zone

Subduction zones are also areas of

increased volcanic activity As the heavier plate is pushed under the

light plate, it opens up areas where themolten rock of the mantle can seepupwards

This erupts on the surface as volcanoes Diverging Plate Boundaries


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Sometimes the plates are moving away from each other They are diverging When this occurs, the mantle is exposed allowing hot molten rock to come

to the surface One of the most active areas for diverging plates is on the ocean floor As the plates move apart, molten rock rises to create a new floor At the fat end of the diverging plates, the plates are pushing into the

continents creating subduction zones Occasionally this occurs on land The African Rift Valley is an example of this process In the ocean, these boundaries are responsible for a large number of deep

sea vents Recently deep diving instruments and cameras have discovered a plethora

of life in depths where it was once though the life and not exist Iceland is an example of divergent plate boundary construction

Iceland is home to large thermo vents and high volcanic activity Island are constantly being created Transform Fault Lines When plats "slide" past one another they create areas called transform

faults Since the earth and the plate boundaries are not smooth, the often produce

earthquakes The San Andreas Fault is famous for its earthquakes This causes most devastating earthquake in San Francisco occurred in

1906

Sea Floor Spreading As science progresses we are able to explore further into the depths of the

oceans We are discovering more about the tectonic activity that occurs in the ocean

depths For example; did you know that there are mountains that make Everest like

a hill? The longest and highest mountain range on the earth is not the Himalayas Running down the center of the Atlantic Ocean is the longest mountain

range on earth Because it is so deep in the ocean, we never discovered it until the mid-

20th century The explanation as to why these vast ranges exist under the ocean is

understood through plate tectonics The concept of sea-floor spreading explains the existence of the mountain

ranges under the ocean and the subduction zones at the continental plateboundaries


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Due to advances in underwater exploration technology we are learningmuch more about the structure of earth's interior

We also discovered that the magnetic poles are shifting The magnetic north pole is moving. Tracking the North Magnetic Pole (Diagram:

normal/reversednorth/south pole)

Evidence of thisshift in the polesis found in theocean floor

Sea-floorspreadingcreates "bands"

as the floorspreads from thecenter

Why does theocean floorspread from thecenter

Carbon dating has shown the age of thebands

It was discovered that the youngest rock

was closer to the center The bands has also retained their

magnetic polarity Because of this we know for certain that

the earth's poles shift

The lithosphere

Magma is molten rock within the earth, lava is molten rock on the earthssurface

Magma bubbles up through openings in the ocean floor Since magma has been in a molten state or a long time, the grains of the

rock material are very small. Unlike andesitic volcanoes the eruptions of basalt are less violent. Reasons for this include the low viscosity of the magma, the pulling apart of

the plates and the thinness of the plates at the center. When these underwater volcanoes erupt, they produce pillow lava.


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Nothing on the earthcaptures the imaginationquite like a violentvolcanic eruption.

Volcanos and volcanic

eruptions are usuallyviolent, earth changingaffairs that cause deathand destruction.

These eruptionshowever also lead to anexplosion of life.

Vulcanism

There are different types ofvolcanoes.

Hawaii is home to some of the mostspectacular volcanic activity.

These one can witness volcanoes ina near constant state of eruption.

Lava flows are frequent and theresidents have learned to live sideby side with these naturaloccurrences.

In the next slide we will look at various types of volcanoes.


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Volcanoes fall into three categories1) Extinct

These are long dead volcanoes There is not fear of them erupting again in the future Reasons: the plate has moved past the hotspot that generated the volcano. Or the hotspot is no longer active (the thinning of the crust has ceased) Once a volcano becomes extinct it is open to the forces of erosion Since that magma is made of a very solid type of basalt, any remaining

magma from the extinct volcano will be the last to erode. The softer cinder and earth will erode first leaving the hard interior of basalt

remaining. This is often known as a volcanic neck Shiprock, New Mexico is an example of a volcanic neck The entrance to Morro Bay, California is an example of a volcanic plug.

2) Dormant

These types of volcanoes are considered to be sleeping A dormant volcano always has the potential to become active. It is still located in an area of tectonic activity Famous formant volcanoes include Mt.Baker in Washington state, Mt.Fuji in

Japan, and Mt. Etnain Italy.3) Active

Active volcanoesinclude almost every

volcano that has hadan eruption in recenthuman history

Remember;geological time isdifferent from humantime

The eruption ofMount Vesuvius inItaly is stillconsidered a recentevent even thoughit occurred duringthe height of theRoman Empire.

Vesuvius is anactive volcano.


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(4) Formations

Aa forms when lava flows rapidly, the gases in the lava are not releasedcausing it to form a crust. Aa lava is very "viscous". Pahoehoe lava flowsslowly. It is less viscous. Pahoehoe lava flows slowly. It is less viscous.

Convergent plate boundaries volcanoes structure

Death by volcano

There are two main ways that volcanoes extinguish large numbers of lives

The first is the pyroclastic flow The incredible heat, melted rock material and high viscosity wind can cause

instant death of large numbers of people This was the cause in Italy and the Philippines The other way is from the mud flow After the volcano explodes, all inorganic material is liquefied. It mixes with any water in the area including atmospheric moisture This mixes together like pancake batter and flows in a raging torrent down

the slopes This is called a Laha

Faulting

The movement of tectonicplates occurs along faultlines

These are areas where twodifferent plates meet


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How they meet determines the type of fault that will result A result is a fracture along which movement occurs There are different types of faults

Fault Motion

1] DIP-SLIP FAULTSa) Normal FaultIn a normal fault, the block above the fault movesdown relative to the block below the fault. This faultmotion is caused by tensional forces and results inextension. [Other names: normal-slip fault, tensionalfault or gravity fault]

b) Reverse Fault

In a reverse fault, the block above the fault moves uprelative to the block below the fault. This fault motionis caused by compressional forces and results inshortening. A reverse fault is called a thrust fault if thedip of the fault plane is small. [Other names: thrustfault, reverse-slip fault or compressional fault]

2] STRIKE-SLIP FAULTIn a strike-slip fault, the movement of blocks along afault is horizontal. If the block on the far side of thefault moves to the left, as shown in this animation, thefault is called left-lateral. If the block on the far sidemoves to the right, the fault is called right-lateral. Thefault motion of a strike-slip fault is caused by shearingforces. [Other names: transcurrent fault, lateral fault,

tear fault or wrench fault]


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3] OBLIQUE-SLIP FAULT

Oblique-slip faulting suggests both dip-slip faulting

and strike-slip faulting. It is caused by a combinationof shearing and tension of compressional forces.

When two plates are pulling awayfrom each other, the result is a riftvalley

The East African Rift Valley is one

of the most famous Located in East Africa, the plates

pulled apart to allow aGraben to form

The steep valley sidescaused many of the animalspecies to become trappedin the valley.

When the two sections of thecrust move apart and block

falls between the two parts,this creates a rift valley

The valley is formed in the graben. Through compression, when two pieces of the crust are pushed together,

the block between gets forced up This is a horst. One of the most unpredictable and most devastating types of faults are the

transform faults These are devastating because the plates can become locked When they do release, they release with such force as the cause major

earthquakes The most famous transform fault in North America is the fault along the San

Andreas region of California Known as the San Andreas Fault, this fault has caused several major and

minor earthquakes in the San Francisco Region (Picture: San Andreas Fault)

Fold Mountains


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Fold mountains are formed when two plates move together (acompressional plate margin)

The movement of two plates forces sedimentary rocks upwards into aseries of folds

There are two types of fold mountains: young fold mountains (10 to 25

million years of age, e.g. Rockies and Himalayas) and old foldmountains( over 200 million years of ages, e.g. Urals and Appalachians ofthe USA)

Because fold mountains are formed with sedimentary rocks, they can trapgreat deposits of fossil fuels

Where the rocks are floodedupwards, they are calledanticlines

Where the rocks are foldeddownwards, they are called

synclines Severely folded and faulted

rocks are called nappes

Tsunamis

All of this tectonic activity has an effect on the water that sits in our oceans Underground earthquakes can cause ripples which increase in size until

they become the deadly force known as a tsunami These massive water events cause great loss of life and properties

The December 26, 2004 Indian Ocean tsunami was caused by anearthquake that is though to have had the energy of 23000 Hiroshima-typeatomic bombs

The epicenter of the 9.0 magnitude quake was under the Indian Ocean nearthe west coast of the Indonesian Island of Sumatra.

The violent movement of sections of the Earth's crusts known as tectonicplates displaced an enormous amount of water, sending powerful shockwaves in every direction.

The tectonic plates in this area had been pushing against echo termbuilding pressure for thousands years - they continue to do so and will likelycause underwater earthquakes and tsunamis in the future

The shifting of the earth's plates in the Indian Ocean on Dec 26. 2004caused a rupture more than 600 miles long, displacing the seafloor abovethe rupture by perhaps 10 yards horizontally and several yards vertically. Asa result, trillions of tons of rocks were moved along hundreds of miles andcaused the planet to shredder with the largest magnitude earthquake in 40years.


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The earthquake-induced tsunami resulted in at least 155,000 fatalities,500,000 injuries, and damages that exceeded $10 billion. Also, it isestimated that 5 million people lost their home or access to food and water.

Plate Interiors

Not all of the earth is under constant threat from tectonic activity The further one gets from the one of plate collision, the less tectonic activity

one experience These areas are called cratons They are located deep within a plate, far from the area of collision or

subduction The theory is that these areas have long since passed their period of

movement

They are the oldest parts of the continents They go back to when there was only one large supercontinent called

Pangaea We spoke of this earlier Eventually the tectonic activity led to the break-up of this supercontinent First, it split along the equator The split created two supercontinents called Laurasia and Gondwanaland Eventually the tectonic activity continued to create the continents that we

have today

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