21 Structural Geology and Plate Tectonics.odp

7/23/2019 21 Structural Geology and Plate Tectonics.odp

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Structural Geology and PlateTectonics

By Terence Black


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Earth"uakes

%espite the potential damage earth"uakes cancause# they can !e an aid to science Scientistscan o!tain information a!out the Earth's

internal structure !y monitoring earth"uakewaves at different locations and !y applyingknowledge of wave properties


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Seismic Waves

There are two types of seismic waves thatresult from the vi!rations produced !yearth"uakes

Surface waves# which travels along the outerlayer of the Earth and !ody waves which travelthrough the Earth

Surface waves cause the most damage from anearth"uake


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Body Waves

There are two types of !ody waves& Primary orcompressional waves and Secondary or shear

waves

Primary waves are longitudinal compressional wavesthat are propagated !y particles moving longitudinally!ack and forth in the direction the wave is traveling

In secondary waves the particles move at a right

angle to the direction of the waves These aretransverse waves


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%ifferences !etween P and S Waves

S waves can only travel through solids Gasesand li"uids cannot support shear stress

P waves can travel though any medium

P waves travel faster than S waves in any solidmaterial and !ecause of this they arrive first atseismic stations These differences allowSeismologists to find out the source of

earth"uakes and find out Earth's internalstructure


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Speed of Body Waves

The speed of !ody waves depends on thedensity of the medium that it is running thoughThese densities increase with depth Because

of this the waves are curved or refracted Thewaves are also refracted when they cross a!oundary !etween different mediums

The fact that S waves can't travel through

li"uids gives Seismologists an idea a!out thestructure of Earth's interior


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Earth's Interior

rom those indirect o!servations scientists !elievethat the earth is made up of four regions The crust#the mantle# the outer core and the inner core

The density of the inner and outer cores shows thatearth may have a metallic composition It is !elievedto !e nickel and chiefly iron ()*+,

The outer core is !elieved to !e -./0 miles thick

and li"uid The inner core is !elieved to !e a solid!all with a radius of around 12. miles


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Earth's Interior (cont,

3round the outer core is the mantel# which isaround -)00 miles thick The mantle is divided intotwo parts 4 the upper mantle and the lower mantle

The temperature of the rock is near the meltingpoint in the upper mantle The molten rock that ise$pelled !y volcanoes come from the uppermantle

The lower mantle contains rocks at hightemperatures and high pressures


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Earth's Interior (cont,

The rocky# thin# outer layer of the mantle thatwe all live on is called the crust It varies inthickness from a!out . to 1 miles !eneath the

ocean !asins to a!out -5 to 5* miles The core represents **+ of the Earth's radius#

the mantle around 6*+ and the crust a!out -+


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7ones of the Earth

The lithosphere# which is comprised of the uppermantle and the crust# e$tends to a depth of a!out 6.miles and includes a thin part of the upper mantleand the whole crust

The asthenosphere is the part that lies !eneath thelithosphere It is essentially solid rock# !ut it is soclose to it's melting temperature that it is relativelyplastic and contains pockets of small molten rock It

e$tends to a depth of 6.6 miles !elow the Earth'sSurface


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8ontinental %rift and SeafloorSpreading

In the early -/00s# a German meteorologist andgeophysicist named 3lfred Wegener revived theidea of plates and their movement and compiledgeological evidence supporting the model known as

continental drift Wegener hypothesi9ed that all of the continentswere once part of a giant# single continent that hecalled Pangea :e assumed that the continent rifted

apart a!out 500 million years ago and !ecame the 1continents that we know today


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8ontinental %rift (cont,

There are three pieces of prominent evidence that supportshis claims

There are similarities in fossils and !iological species that werefound on distant continents that are separated !y oceans thatstrongly suggest that the continents were formed into Pangealong ago

The shapes of the continents strangely interlock similar to a;igsaw pu99le When the pieces# the continents# are put togetherthey fit and form one land mass

There is evidence that the southern parts of South 3merica#3frica# india and 3ustrailia were covered in a glacial ice sheetSimilar to 3ntarctica


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Seafloor Spreading

:arry : :ess# an 3merican geologist# discovered themechanism !ehind continental drift in -/20 3t thattime geologists knew that a mid


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Seafloor Spreading (cont,

In the theory of seafloor spreading the seafloorslowly spreads and moves sideways away fromthe mid


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Plate Tectonics

The lithosphere is viewed as a series of solidsegments called plates These plates areconstantly interacting with each other in a veryslow motion The surface Is the glo!e issectioned into a!out 50 plates


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Earth"uakes

3n earth"uake is a release of energy after arepositioning of underground rock and ismanifested !y the vi!rating and vigorousmovement of the Earth's surface

Earth"uakes can !e caused !y violent volcaniceruptions or e$plosions caused !y humansThe !ulk are associated with the plate

movements in the lithosphere


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Earth"uakes

The plate movements in the lithosphere form faults# or when therock on one side of the fracture moved relative to the rock on theother side of the fracture The place with the most movement areplate !oundaries The ma;or earth"uakes follow the faults inthese regions

=ovement of plates near each other e$ert force on the rockformations along the plate margins Energy in the rocks arestored until they have enough to overcome the force of frictionThen the energy of the stressed rocks is released# causing anearth"uake

3fter a ma;or earth"uake there may !e aftershocks# or additionalvi!rations


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Earth"uakes (cont,

The point of the initial energy from anearth"uake is called it's focus 3 focus can !efrom a depth of a few miles to several hundredmiles

The geologists find the location on the Earth'ssurface due to the epicenter The epicenter isthe point on the surface that takes the !runt of

the earth"uake


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Earth"uakes (cont,

The energy released at the focus of anearth"uake move outward as seismic waves

The seismic waves are monitored !y

seismographs The greater the energy of anearth"uake the greater the amplitude on thetraces on a seismogram

Earth"uakes are classified into different scales

The most common scale used to measureearth"uakes is the >ichter scale


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>ichter Scale

8harles >ichter of the 8alifornia Institute ofTechnology developed the >ichter scale in -/.* Ithas magnitudes e$pressed !y whole num!ers anddecimals They are usually !etween . and /

Each whole num!er represents a!out .- times moreenergy than the last

%amage caused !y earth"uakes is not onlydependent on the magnitude of the "uake# !ut also on

the location of the focus and epicenter and theenvironment of that particular region


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Earth"uake %amage

Earth"uake damage can indirectly withlandslides# su!sidence or directly fromvi!rational tremors

Tsunamis are huge waves generated when theenergy release of a "uake happens under inthe area or under the ocean floor


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8rustal %eformation

The two types of gradual structural deformationsare faulting and folding

aulting starts with fracturing ractures are

cracks in rock due to stress aults are fracturesthat show that one side of the fracture hasmoved relative to the other side

olding happens when e$treme pressure is

e$erted on the Earth's crust >ock can !ecompressed a certain amount !efore it folds


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Types of aults

There are three types of faults They arenormal# reverse# and transform

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21 Structural Geology and Plate Tectonics.odp

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