Planet Earth

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Planet Earth

• Chapter 17

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17.1 - Earths Interior
Notes . . . and lots of them!

• Earths interior gets hotter with depth (why?)
• Crust made of cool, hard rock
• Continental crust, average thickness of 20-40km
• Oceanic crust averages 4-7km
• 12756km diameter

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17.1 - Earths Interior

• Mantle, denser than the crust, is a layer of rock
  2900km thick
• Outer mantle rocks are mostly solid
• Inner mantle rocks exhibit plasticity because
  of heat (over 1250C)
• 80 of Earths volume is mantle!

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17.1 - Earths Interior

• Outer core is liquid iron and nickel
• Inner core is solid, where temperature exceeds
  6000C

5
review

• Without looking
• Name a layer of Earths structure and give a
  property for that layer
• Which is thicker, oceanic or continental crust?

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Hey, stars, what do you notice about 1) the edges
of the continents, 2) their shapes, and 3) their
relationship to plate boundaries?
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17.1 - Plate Tectonics

• Alfred Wegener, a German scientist, noticed that
• The coastlines of Africa and South America looked
  similar
• Several other coastlines appeared to go together,
  like pieces of a puzzle
• He pieced them together and named his
  super-continent Pangea

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• Wegener also found similar animal remains on
  continents that are now oceans apart
• He theorized that these animals did not start
  life on separate continents - that the continents
  drifted apart (have you heard of continental
  drift?)

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17.1 - Plate Tectonics

• Two key questions
• What are plates?
• How do plates (and continents) move?

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17.1 - Plate Tectonics

• The lithosphere consists of
• The crust and hard outer mantle
• Seven large pieces and many smaller pieces called
  tectonic plates
• The plates fit together like puzzle pieces
• The theory describing
• the movement of plates
• is called plate tectonics

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17.1 - Plate Tectonics

• And now, a video!
• Look out for definitions of the following, which,
  after watching the video, we will review as a
  class
• Divergent plate boundaries
• Convergent plate boundaries
• Transform fault boundaries

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review

• Without looking.
• What is the lithosphere?
• What is a plate?
• What is tectonics?
• What observations did Alfred Wegener make about
  continents?
• What evidence did Wegener find to support his
  observations?

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17.1 - Plate Tectonics
Discussion!

• From the video, do you recall how far plates move
  per year?
• Do continent boundaries line up with plate
  boundaries?
• What were the three types of boundaries?

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17.1 - Plate Tectonics
Notes!

• Plate boundaries do not always match with
  continent boundaries
• Plates move from one to 16 cm per year (about
  what your fingernails grow!)

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17.1 - Plate Tectonics
Notes!

• Why do the plates move? Not sure.
• One theory suggests that plates move because of
  the molten mantle below them. In other words,
  they float on the asthenosphere, carried by
  convection currents

However, we do not know exactly how the plates
move What is convection?
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17.1 - Plate Tectonics

• And now, a silly demonstration of divergent plate
  boundaries

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17.1 - Plate Tectonics
Notes!

• Divergent plate boundaries
• Where plates move away from each other
• Found along mid-ocean ridges (sea-floor
  spreading) or rift valleys (what is a rift?)
• As plates move apart, magma moves up and cools,
  forming new crust rock

• Examples
• mid-Atlantic
• Red Sea
• Iceland

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17.1 - Plate Tectonics

• And now, a silly demonstration of convergent
  plate boundaries
• Make a hypothesis of what will happen if you push
  the stapled ends of each booklet squarely into
  each other
• Make a hypothesis of what will happen if you push
  the un-stapled ends of each booklet squarely into
  each other

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17.1 - Plate Tectonics
Notes!

• Convergent plate boundaries
• Where plates collide
• Form subduction zones or mountain chains
• Produce volcanoes, earthquakes, and ocean
  trenches
• Crust melts when it subducts, to form magma

• Examples
• Pacific coast of South America
• Pacific Northwest
• Himalaya Mountains

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review

• Without looking
• What happens at convergent plate boundaries?
• What is the asthenosphere?
• What happens at divergent plate boundaries?
• What is a rift?
• Why do plates move?

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17.1 - Plate Tectonics
Notes!

• Convergent plate boundaries, continued
• Magma rises to the surface because it is less
  dense
• In a volcano
• Far away from the ocean trench
• Or may merely push crust upward
• The Himalayas formed from two colliding plates
  they are getting higher and further across!

• Recall from the diagrams that India was an
  island? It is now pushing into the Eurasian
  plate!

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17.1 - Plate Tectonics

• Do we need a silly demonstration of transform
  fault boundaries?

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17.1 - Plate Tectonics
Notes!

• Transform fault boundaries
• Plates slide/grind past each other, along a
  crack, or fault, causing a buildup of stress
• When stress builds too high, an earthquake
  results, as stress gets relieved

• Example
• San Andreas fault

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17.1 - Plate Tectonics
Notes!

• Evidence for plate tectonics
• Bands of rocks on either side of mid-ocean ridges
  have alternating magnetic polarities
• Iron minerals in cooling lava align with Earths
  magnetic poles this remains in place once the
  rock has solidified

http//www.uwsp.edu/geo/faculty/ritter/geog101/tex
tbook/images/lithosphere/tectonics/midsprd.gif
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17.1 - Plate Tectonics

• Worksheet!
• Go ahead . . . get it out and take a look at it

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review

• Need a volunteer to come to the board and label
  Earths structure, including the lithosphere and
  asthenosphere
• Someone else, to label a subduction zone
• What evidence do we have to support plate
  tectonics?
• What happens at transform fault boundaries?
• For the record, Mt. St. Helens is about 80 miles
  from the coast and the plate boundary
• My apologies for todays class . . . a lot of . .
  .

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17.2 - Volcanoes

• What we know about volcanoes
• Lava is red lava hardens to form igneous rocks
  they can be underwater they form from plate
  movements, above a magma chamber they blow when
  the pressure gets high three types active,
  inactive, dormant, extinct very few are active
  they formed Hawaii they have high
  temperatures theyre destructive they have
  vents they release gas, pumice, sulfur, and ash
  they are rocky and dry they form mountains
  there are supervolcanoes

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17.2 - Volcanoes

• What we want to know about volcanoes
• What causes eruptions how long are eruptions
  how do they form what is worlds largest /
  tallest how old are they what are the types
  what comes out of them name some historical
  disasters how hot are they what is their
  structure once dormant, always dormant? are
  there eruption patterns what does a smoking
  volcano mean are any in PA any active in U.S.
  where are they located nearest to Philly
  how do they become dormant how does magma form
  purpose of how fast does lava come out
  effects of can new ones appear why does only
  lava come out what have we learned by studying
  them

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17.2 - Volcanoes
Notes!

• Volcano
• Any opening in Earths crust through which magma
  reaches Earths surface
• These openings are called vents
• Volcanoes often form hills or mountains
• They release ash, molten rock, and poisonous gases

http//www.bennett.karoo.net/images/nathaz/volcano
xsec.gif
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17.2 - Volcanoes
Notes!

• When magma reaches Earths surface, its physical
  behavior changes we call it lava
• Types of volcanoes (details to follow)
• Shield
• Composite
• Cinder cones
• Supervolcanoes
• Underwater (sea-mount)

http//www.bennett.karoo.net/images/nathaz/volcano
xsec.gif
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17.2 - Volcanoes
Notes!

• Shield volcanoes
• Magma very fluid, rich in iron and magnesium
• Lava can flow great distances
• Mild eruptions
• Forms a gently sloping mountain
• Examples include Mauna Loa, in Hawaii (pictured
  below)

US Geological service
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17.2 - Volcanoes
Notes!

• Composite cone volcanoes
• Made up of alternating layers of ash, cinders,
  and lava
• Magma thicker (than shield), rich in silica
• Gases trapped in magma cause eruptions that can
  either flow or explode with cinder and ash
• Usually thousands of meters high, with steeper
  slopes than shield volcanoes
• Examples include
• Mt. Fuji (pictured),
• Mt. St. Helens, Mt.
• Ranier, Mt. Shasta,
• Mt. Hood

http//www.foxborojudo.com/MtFuji.JPG
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review

• Compare shield and composite cone volcanoes
• Height
• Mountain slopes
• Lava types
• Eruption types
• What happened seventy-five thousand years ago?
• What might happen if a supervolcano erupted
  tomorrow (a delightful quiz question!)?

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17.2 - Volcanoes
Notes!

• Cinder cone volcanoes
• Smallest and most abundant
• Made up of the cinders that fall from the cooling
  lava from their eruptions
• Eruptions last from a few weeks to a few years,
  then the volcano goes dormant
• Usually 30-700 meters high, with relatively steep
  slopes
• Examples include
• Mt. Parícutin, in Mexico (pictured)

http//www.geology.wisc.edu/courses/g112/Images/pa
ricutin.jpg
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17.2 - Volcanoes
Notes!

• Underwater volcanoes
• Called seamounts
• Look much like composite volcanoes
• Supervolcanoes
• See notes from the video

http//www.indiana.edu/g105lab/images/gaia_chapte
r_13/sfs015.gif.jpg
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17.2 - Volcanoes
Notes!

• Volcanoes are linked to plate movement
• Convergent
• 75 of active volcanoes are located where oceanic
  and continental plates collide
• The volcanoes around the Pacific Ocean lie in a
  zone known as the Ring of Fire
• Divergent
• Magma creates the volcanic mountains that form
  the ridges along a rift valley or mid-oceanic
  ridge

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17.2 - Volcanoes
Notes!

• Hot spots
• Occur in the middle of plates
• Hotter than normal mantle plume ascends toward
  the surface (may originate at core-mantle
  boundary)
• If volcano grows large enough, an island may form
• Examples
• Yellowstone
• Hawaii

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17.2 - Volcanoes

• What we want to know about volcanoes
• What causes eruptions how long are eruptions
  how do they form what is worlds largest /
  tallest how old are they what are the types
  what comes out of them name some historical
  disasters how hot are they what is their
  structure once dormant, always dormant? are
  there eruption patterns what does a smoking
  volcano mean are any in PA any active in U.S.
  where are they located nearest to Philly
  how do they become dormant how does magma form
  purpose of how fast does lava come out
  effects of can new ones appear why does only
  lava come out what have we learned by studying
  them

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review

• Describe a cinder cone volcano
• What do we call volcanoes in the middle of the
  ocean?
• What happens at a hot spot? Where do hot spots
  occur?
• How likely is it that a supervolcano will erupt
  during 271s lifetime?

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17.2 - Earthquakes

• What we know about earthquakes
• Caused by plate collision form mountains, rough
  surfaces, cracks in earth shake the earth
  underwater cause tsunamis occur mostly on plate
  edges intensity recorded on paper Richter
  scale 1-10, 1-3, 1-7 some linked to volcanoes
  occur near the Equator California will
  eventually go away most occur in California
  some are so small you cannot feel them can
  cause a lot of damage people can die there
  was a quake during a baseball game Japan has a
  lot of them occur every day detected by
  seismographs occur at an epicenter further
  from the epicenter, the less the intensity

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17.2 - Earthquakes

• What we want to know about earthquakes
• How often do they occur daily? do other
  planets have them what else causes them how
  can we avoid or stop them can we predict them
  how do we measure how long do they last where
  do they happen most why are so many in CA who
  not so many in eastern U.S. what should we do
  if the ground shakes why do they only occur at
  certain spots what causes plates to move what
  happens after a quake are there different types
  why are there aftershocks what are
  aftershocks how large is affected area what
  is the greatest damage ever done how far down
  do they occur can animals detect them can
  more than one happen in an area why do they
  fascinate us

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17.2 - Earthquakes

• Lets start with some videos of how earthquakes
  happen.
• Guess whats on the next slide? ?

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17.2 - Earthquakes
Notes!

• What are earthquakes?
• The vibrations when stress between two plates is
  relieved at a fault
• Energy is released as seismic waves
• As the waves travel through Earths layers, they
  create a shaking effect that we experience during
  an earthquake

www.uh.edu/jbutler/ physical/earthquakes1.gif
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17.2 - Earthquakes
Notes!

• What are earthquakes? (continued)
• Focus of the earthquake - the exact spot within
  the earth where rocks break
• Epicenter - the point on Earths surface directly
  above the focus (damage during an earthquake is
  usually greatest at the epicenter)

www.uh.edu/jbutler/ physical/earthquakes1.gif
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17.2 - Earthquakes
Notes!

• Energy from earthquakes travels by means of waves
• Primary waves (longitudinal)
• Originate at the focus
• Motion is compression (like a spring)
• Travel faster than other waves
• Spread in all directions from the focus
• Reach recording stations before other waves do

www.uh.edu/jbutler/ physical/earthquakes1.gif
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review

• What is an earthquake?
• Describe an earthquakes primary waves
• What is the difference between the focus and the
  epicenter of an earthquake?
• What is a tsunami?
• Name the main layers of Earth and one
  characteristic of each layer

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17.2 - Earthquakes
Notes!

• Secondary waves (transverse)
• Originate at the focus
• Motion is up and down
• Travel more slowly than primary waves
• Spread in all directions from the focus
• Reach recording stations after primary waves do

www.uh.edu/jbutler/ physical/earthquakes1.gif
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17.2 - Earthquakes
Notes!

• Surface waves
• Occur on Earths surface
• Motion is up and down and back and forth (rolling
  motion)
• Cause the most destruction because of their
  longer wavelengths
• Who can explain wavelengths?
• Demonstration Why are primary waves faster than
  secondary waves?

www.uh.edu/jbutler/ physical/earthquakes1.gif
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17.2 - Earthquakes
Notes!

• Seismology is the study of earthquakes
• Seismologists use seismographs to record
  earthquake data, including wave information
• More than 1000 seismograph stations exist around
  the world

www.uh.edu/jbutler/ physical/earthquakes1.gif
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review

• How do secondary waves differ from primary waves?
• What motion do surface waves make?
• Describe a transform fault boundary
• About how many volcanoes erupt each year?
• If we visited an erupting shield volcano, what
  would we see?

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17.2 - Earthquakes
Notes!

• Three stations are needed to pinpoint the
  epicenter of an earthquake
• Reading a seismogram
• P-waves arrive first, as we noted earlier, with
  small, zig-zag lines
• S-waves arrive later, as larger, more ragged
  lines
• Surface waves arrive last and make large lines
• Difference in time from p-wave to s-wave arrival
  enables us to calculate the distance between the
  station and the focus
• Now, put on your earthquake detective hats!

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• Read a quake
• An earthquake has just hit. Can you tell where?
• Each clock started running when the first primary
  wave arrived.
• Lines are for
• Tokyo
• Sydney
• Hawaii
• Los Angeles
• Miami
• Jamaica
• Rio de Janiero

http//cse.ssl.berkeley.edu/img/earthquakes/seismo
gram000.jpeg
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17.2 - Earthquakes

• Seismogram from an earthquake in Turkey in 1999

Come identify the P, S, and surface waves
www.okgeosurvey1.gov/ level2/ok.grams/T990817.html
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17.2 - Earthquakes
Notes!

• When studying Earths core, we asked how we know
  the outer portion is liquid
• We know that transverse waves, like s-waves,
  cannot travel through liquid
• Because s-waves do not reach seismographs on the
  side of Earths core opposite from the focus
• The velocity of seismic waves varies
• Waves change speed and direction whenever the
  density of the material through which they are
  traveling changes
• This suggests that Earths interior is made up of
  different layers of different densities, giving
  us the model we studied earlier

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17.2 - Earthquakes
Notes!

• Measuring earthquakes
• We use the Richter scale to measure the energy
  released by earthquakes
• Each step on the Richter scale represents a
  30-fold increase in energy. For example, a 4.0
  earthquake is 30 times as powerful as a 3.0
  earthquake (an 8.4 earthquake is 30 times as
  powerful as a 7.4 quake)

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17.2 - Earthquakes
Homework follow up!

• Do earthquakes occur in Philadelphia? If not,
  why not? If so, why dont we feel them?
• Who has an answer?
• What did the book say, on the chart on p. 571?
• Is a tall building more likely to be damaged by
  an earthquake if it is on a mountain of granite
  or in a valley of sediment? Explain.