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Plate Tectonics and Tsunamis

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					Plate Tectonics
 and Tsunamis
Looking at a globe you can see
that most of the continents seem
to fit together like a puzzle: the
west African coastline seems to
snuggle nicely into the east coast
of South America and the
Caribbean sea; and a similar fit
appears across the Pacific.
Plate Tectonics
Alfred Wegener suggested that:
All continents were one large land
 mass called Pangaea (meaning “all
 lands”). About 200 million years ago,
 it broke apart and the continents
 moved to their current location. This
 is called continental drift.
Searching for evidence: 1. Fossil
Paleontologist observed that the same
 fossilized plants and animals from the
 same time period were found in South
 America and Africa. The same was true for
 fossils found in Europe and North America,
 and Madagascar and India. Many of these
 organisms could not have traveled across
 the vast oceans that currently exist.
 Wegener's drift theory seemed a plausible
 explanation.
2. Glacial evidence
 Another observation favoring continental
 drift was the presence of evidence for
 continental glaciation in the Pensylvanian
 period. Striae left by the scraping of
 glaciers over the land surface indicated that
 Africa and South America had been close
 together at the time of this ancient ice age.
 The same scraping patterns can be found
 along the coasts of South America and
 South Africa.
 3. Wegener's drift hypothesis also provided an
 alternate explanation for the formation of
 mountains.
Wegener's explanation was that as the
continents moved, the leading edge of the one
continent would encounter the leading edges of
another continent, compressing the land and
folding it upwards forming mountains. The
Sierra Nevada mountains on the Pacific coast
of North America and the Andes on the coast of
South America were cited. Wegener also
suggested that India drifted northward into the
Asian continent thus forming the Himalayas.
Convection moves continents?
In 1929, Arthur Holmes elaborated on one of
 Wegener's many hypotheses; the idea that the
 mantle undergoes thermal CONVECTION. This
 idea is based on the fact that as a substance is
 heated its density decreases and rises to the
 surface until it is cooled and sinks again. This
 repeated heating and cooling results in a
 current which may be enough to cause
 continents to move. Arthur Holmes suggested
 that this thermal convection was like a conveyor
 belt and that the upwelling pressure could break
 apart a continent and then force the broken
 continent in opposite directions carried by the
 convection currents.
The main features of plate tectonics
are:
   The Earth's surface is covered by a series of
    crustal plates.
   The ocean floors are continually, moving,
    spreading from the center, sinking at the edges,
    and being regenerated.
   Convection currents beneath the plates move
    the crustal plates in different directions.
   The source of heat driving the convection
    currents is radioactivity deep in the Earth’s
    mantle.
The Earth’s rigid crust and mantle are
called the lithosphere. The
lithosphere is broken into sections
called plates. Plates carry a
continent or an ocean or both. The
plates “float” and move around on the
plastic-like layer beneath the
lithosphere called the asthenosphere.
Plates move between 1-12 cm per
year.
When plates move, they can interact in
3 ways:
   Move apart----divergent boundary, crust is
    created (example: seafloor spreading on the
    Mid-Atlantic Ridge.)
   Move together----convergent boundary, crust
    is destroyed (example: Mt. St. Helens
    volcano, Andes mountains in S. America,
    Himalaya mountains in Asia, Islands of
    Japan)
   Slide past each other---transform boundary
    (earthquakes at San Andreas Fault, CA)
Earthquakes
 A crack in the earth’s crust is a
 fault. When plates slide along
 each other at the fault line,
 tremendous pressure builds up.
 When the pressure is released, it
 produces vibrations or seismic
 waves called earthquakes.
 Point on the Earth directly above
  earthquake is epicenter.
 Instrument used to measure
  earthquakes is seismograph.
 Intensity or magnitude of
  earthquake is measured on
  Richter scale.
Tsunami
 An  ocean wave caused by an
  undersea earthquake
 Japanese word tsunami means
  "harbor wave"
 Tsunamis are often mistakenly
  called "tidal waves" when, in fact,
  they have nothing to do with tidal
  action.
How an earthquake causes a tsunami
 During a major earthquake, the seafloor can
 move several meters. An enormous amount
 of water is suddenly set into motion, sloshing
 back and forth for several hours. The result
 is a series of waves that race across the
 ocean at speeds of more than 800 km/hr
 (500 mi/hr), comparable to the speed of
 commercial jets. The energy and momentum
 of these transoceanic waves can take them
 thousands of kilometers from their origin
 before slamming into far-distant islands or
 coastal areas.
Not the 1st time tsunami has hit the
lands of Southeast Asia.
 The 1883 eruption of Krakatau Volcano,
 located in the Sunda Straits between the
 islands of Sumatra and Java, Indonesia,
 provides an excellent example of an
 eruption-caused tsunami. A series of
 tsunamis washed away 165 coastal villages
 on Java and Sumatra, killing 36,000 people.
 The larger tsunamis were recorded by tide
 gauges as far away as the southern coast of
 the Arabian Peninsula-more than 7,000 km
 from Krakatau!
Tsunami of December 26, 2004
   The cause of the disastrous tsunami lies deep
    beneath the ocean, where two tectonic plates
    collided. The Indian-Australian plate slid under
    the Philippine plate in a “subduction zone” 750
    miles long and more than 300 miles wide.
   The movement is jerky, as the submerging plate
    tends to drag the upper plate down with it. But
    as pressure builds, the upper plate breaks free,
    springing back to its original position. The
    movement in the Indian Ocean was just 16½
    yards at a depth six miles below the seabed, but
    it was more than enough.
   The seabed lifted, displacing the ocean. The
    water had to find somewhere to go, so began to
    flow outwards in a huge wave, or tsunami. Dr
    Roger Musson, a seismologist with the British
    Geological Survey in Edinburgh, said that
    similar events had been taking place in the
    region for several million years and would
    happen again over the next few million years.
   “The effect of the earthquake is like throwing a
    stone in a pond, except that you are throwing it
    from below. You get the equivalent of a splash
    and water is displaced with waves spreading
    outwards,” he said.
 Over the ocean, the waves of a tsunami
  are small, probably no more than a few
  centimetres to a metre high. Fisherman 20
  miles out at sea barely notice their
  passage.
 Their speed depends on the depth of the
  water, but is typically several hundred
  miles an hour. The deeper the water, the
  faster the waves travel and at the bottom
  of the deepest ocean they can keep pace
  with a jet aircraft.
 As the Dec. 26 tsunami approached
  the coasts of Sri Lanka, Indonesia,
  India, Thailand, Malaysia and the
  Maldives, it slowed. The more it
  became compressed, the more it
  grew in height. As it reached the
  shore it grew into a monster.
 “The scale of the waves will vary from
  place to place, depending on the
  topography of the coast,” Dr Musson
  said.
map of tsunami
   http://users.powernet.co.uk/mkmarina/indo
    nesia/tsunami.html
2004 Sumatra Earthquake
   http://ioc.unesco.org/itsu/images/upload/ani
    mation.gif
To someone on a ship in the open ocean, the
passage of a tsunami wave would barely
elevate the water surface. However, when it
reaches shallower water near the coastline
and "touches bottom," the tsunami wave
increases in height, piling up into an enormous
wall of water. As a tsunami approaches the
shore, the water near shore commonly
recedes for several minutes -- long enough for
someone to be lured out to collect exposed
sea shells, fish, etc. -- before suddenly rushing
back toward land with frightening speed and
height.
Tsunami animation
   http://www.pbs.org/wnet/savageearth/animations/tsunami/index.html

   Then click Refresh and be patient!
From the Times of London (online) 1 January 2005

Girl's sea warning saved a hundred
   A GIRL aged ten saved a hundred fellow tourists from the
    tsunami because of a geography lesson about the giant
    waves. Tilly Smith urged her family to get off Maikhao beach
    in Thailand after seeing the tide rush out and boats on the
    horizon begin to bob violently.
   The youngster, recalling a recent school project on quakes,
    turned to her mother Penny and said: "Mummy, we must get
    off the beach now. I think there is going to be a tsunami."
    Penny and her husband Colin alerted others and they cleared
    the Phuket beach just in time. It was one of the few beaches
    where no one has been reported killed or seriously injured.
   Last night Tilly, from Oxshott, Surrey, told The Sun that credit
    for her quick-thinking should go to Andrew Kearney, her
    geography teacher at Danes Hill Preparatory School.
Tsunami warning system
 Because of past killer tsunamis, which
 have caused hundreds of deaths on the
 Island of Hawaii and elsewhere, the
 International Tsunami Information Center
 was created in 1965. This center issues
 tsunami warnings based on earthquake
 and wave-height information gathered
 from seismic and tide-gauge stations
 located around the Pacific Ocean basin
 and on Hawaii.
But governments have not invested in one
for the Indian Ocean, partly because events
like this are far less common.
International Coordination Group for the
Tsunami Warning System in the Pacific
   “85 percent of all tsunamis occur in the Pacific Ocean,
    generated in the regions where the main tectonic plates
    forming the floor of the Pacific collide against themselves or
    against the continental plates that surround the ocean basin,
    in an area known as the Ring of Fire. The Mediterranean
    and Caribbean Seas also have histories of some locally
    destructive tsunamis. Tsunamis in the Indian Ocean have
    been rare and far part in time. This might explain why no
    tsunami warning system has been developed in the Indian
    Ocean.”
   “The Indian Ocean tsunami is now reported to be one of the
    strongest in the world for the past 40 years. More than
    100,000 lives have been lost and material damage is
    tremendous.”
Tsunami rolls
   ashore
Aftermath in
 Indonesia
Aftermath in Sri
     Lanka
Destroyed City in
   Indonesia
Dust and smell of
   Sri Lanka
Elephants in Thailand
 used as vehicles to
 transport supplies.
Fishing pier in India
Flooding in Sri Lanka
   Cleaning up
destruction in India
Aftermath in
 Indonesia
Among the rubble in
   Indonesia
After “tidal wave”
   in Indonesia
Young boy outside
destroyed home in
    Indonesia
Missing person
   notices
 Mourning 2
 daughters in
Southern India
Cleanup effort in
 Southern India
Amid smell and
destruction in Sri
     Lanka
 Local army helps
clean up in Sumatra
Remains of Thai
   Resort
Home destroyed as
  boat is carried
ashore by tsunami
Searching for
 belongings
  Baby “Tsunami Roy”
born 3 weeks early as his
  parents headed for the
 cliffs in the wake of the
          tsunami
Among the ruins in
   Sumatra
  United States
Military aids relief
       effort
 US students help
clean up the beach
   after tsunami
Water remaining in
  Indonesia on
    January 1
Beach returns to
    normal
Where Are All the Dead Animals?
   Sri Lankan wildlife officials are stunned --
    the worst tsunami in memory has killed
    over 120,000 people along the Indian
    Ocean island's coast, but they can't find
    any dead animals.
   "The strange thing is we haven't recorded
    any dead animals," H.D. Ratnayake,
    deputy director of the national Wildlife
    Department.
 "No elephants are dead, not even a
 dead hare or rabbit," he added. "I
 think animals can sense disaster.
 They have a sixth sense. They know
 when things are happening."
Quake May Have Made Earth Wobble
 The deadly Asian earthquake may
 have permanently accelerated the
 Earth's rotation -- shortening days
 by a fraction of a second -- and
 caused the planet to wobble on its
 axis, U.S. scientists said on
 Tuesday, December 28, 2004.
 Richard  Gross, a geophysicist with
 NASA 's Jet Propulsion Laboratory in
 California, theorized that a shift of
 mass toward the Earth's center
 during the quake on Sunday caused
 the planet to spin 3 microseconds, or
 one millionth of a second, faster and
 to tilt about an inch (2.5 cm) on its
 axis.
 When one huge tectonic plate beneath the
  Indian Ocean was forced below the edge
  of another "it had the effect of making the
  Earth more compact and spinning faster,"
  Gross said.
 Gross said changes predicted by his
  model probably are too minuscule to be
  detected by a global positioning satellite
  network that routinely measures changes
  in Earth's spin, but said the data may
  reveal a slight wobble.
 The  Earth's poles travel a circular
  path that normally varies by about 33
  feet, so an added wobble of an inch
  (2.5 cm) is unlikely to cause long-
  term effects, he said.
 "That continual motion is just used to
  changing," Gross said. "The rotation
  is not actually that precise. The Earth
  does slow down and change its rate
  of rotation."
 When  those tiny variations
 accumulate, planetary scientists must
 add a "leap second" to the end of a
 year, something that has not been
 done in many years, Gross said.
 Scientists  have long theorized that
  changes on the Earth's surface such
  as tide and groundwater shifts and
  weather could affect its spin but they
  have not had precise measurements
  to prove it, Caltech seismologist
  Hiroo Kanamori said.
 "Even for a very large event, the
  effect is very small," Kanamori said.
  "It's very difficult to change the
  rotation rate substantially."