Joint Education Initiative/Joint Education Initiative.iso/programs/dos/gna/readme.gna

Text File | 1990-06-25 | 7KB | 178 lines

       The Geophysics of North America

       Introduction:  When viewed from space, Earth appears as a roughly
       spherical  body of solid material fully surrounded by gaseous and
       liquid fluids.   Indeed our planet is scientifically described as
       a  series of concentric spheres  -- core,  mantle,  crust,  lower
       atmosphere and stratoshere.

       The outwardly peaceful appearance of Earth,  evident to a  casual
       observer,  dissolves  under close scrutiny.   A close examination
       reveals a planet that is constantly in turmoil -- not just  among
       the  inhabitants,  but  the total environment.   Severe  weather,
       earthquakes,  and  volcanoes  are just a few  reminders  of  this
       perpetual restlessness.

       How  do  Earth  scientists measure,  describe and  (dare  I  say)
       predict   the  occurence  of  such  events?    Geophysicists   --
       scientists   who  study  physical  phenomena  to  solve   Earth's
       mysteries  -- do  this with instruments that  measure  elevation,
       gravity,  magnetics,  earth motion,  crustal stress,  and thermal
       properties.

       In  a  recent  project  sponsored by  the  National  Oceanic  and
       Atmospheric  Administration,  geophysical data,  compiled from  a
       large  region  covering  half of the  Northern  Hemisphere,  were
       placed on a compact disc.  This disc, known as the "Geophysics of
       North America", is enhanced with software that lets an individual
       easily browse through the data using a personal computer.

       What  type of data are present"  How do we use such  information?
       What planetary mysteries are these measurements helping us solve?

       Topography  is  a measure of how wrinkled the  solid  surface  of
       Earth  is.   On land this is referred to as elevation and in  the
       oceans  it  is called bathymetry.   Seventy-five percent  of  our
       planet  is  covered  by water;  this is why  Martians  frequently
       describe  us  as the "Water Planet".   Topographic  surveys  have
       documented  major  relief variations  -- vast  mountain  systems,
       valleys,  and plains both above and below the water surface.  Not
       until the early 1970s did scientists understand the relationships
       of  these features to natural processes.   Now they are viewed as
       the  surface reflections of crustal plates that are battling  for
       territorial  positions -- propelled by forces within  the  Earth.
       When  plates  push together mountains form;  where  plates  slide
       beneath one another trenches develop.

       Gravity is a mutual attraction between objects.  We are attracted
       to  the Earth because of our mass and the mass of the Earth  (and
       believe  me,  the  Earth is heavy).  Gravitational attraction  is
       affected  not just by mass but also by the distance  between  the
       objects.   The  closer  you are to an object,  the  greater  your
       attraction.   Earth  revolves around the sun because of  gravity;
       the moon revolves around Earth because of gravity.   Gravity data
       on  the  surface of the Earth give a clue of the distribution  of

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       masses (or density) beneath the surface.   Scientists looking for
       petroleum are happy to see gravity lows, for oil and gas have low
       densities.   Those  looking for iron or gold seek gravity  highs,
       for these ores are very dense.

       Magnetics remains one of the great unsolved scientific mysteries.
       Our magnetic field is always changing due to fluctuations of  the
       solar  winds and the planet's internal magnetic field  generator.
       Earth's  field generator does strange things.   Not only does  it
       alter its patterns slowly;  but, periodically it totally flips in
       direction  (reversing the North and South poles).   We know about
       these  reversals  by  studying volcanic  flows.   Iron  in  magma
       becomes  magnetized as it cools and orients in the  direction  of
       the  contemporary  magnetic field,  much like  a  compass.   This
       orientation  is frozen in place and does not change with  changes
       in  the  Earth's field.   Magma therefore records the history  of
       Earth's magnetic field.   Measureements of the magnetic field let
       us  monitor  its changes,  understand the  movements  of  crustal
       plates  (for the magnetic "signature" acts as a time stamp),  and
       explore for ores which have magnetic properties.

       Seismicity is the way scientists listen to Earth.   Each time the
       planet  experiences a rupture in its shell it generates  its  own
       type  of  cry  -- a  soft sigh is heard  when  small  cracks  are
       generated  and  a  scream is monitored when large  shifts  occur.
       Listening devices,  called seismometers,  measure vibrations  and
       scientists  convert these measurements to estimates of severrity.
       As an example great earthquakes,  which cause large loss of  life
       and property,  are usually greater than "8" on the Richter scale.
       These   earthquakes  most  frequently  occur  at  crustal   plate
       boundaries where pushing and shoving are common.

       Crustal  stresses  are  the results of forces  which  deform  the
       crust.   A  knowledge  of  these forces can help us  design  more
       stable  structures such as bridges,  pipelines and well  casings.
       This  knowledge also helps us understand more about  the  dynamic
       nature  of  Earth;  and,  eventually  (perhaps)  we  can  predict
       earthquakes.

       Thermal  properties  are  measures of the heat  generated  within
       Earth's  interior.   Often this heat comes to the surface as  hot
       springs (in which we bathe).   Energy from high thermal  activity
       may  be  used  to  generate electricity  (via  power  plants)  or
       captured  as  steam  for  space heating.   Some  people  in  cold
       climates tap this energy to keep their driveways free of snow and
       ice.   Why  does  Earth  have  such  a  large  variation  in  the
       distribution of interior temperatures?   Why are there hot  spots
       in  areas such as Hawaii and Yellowstone?   Is the Earth  getting
       hotter  or  colder?   These  type of  questions  generate  heated
       scientific debate.

       Satellite  imagery  is useful as a tool to study  global  change.
       Since  satellites make frequent passes around the globe they  can
       monitor  changing conditions in interrelated ecological  systems.
       Vegetation patterns are useful to monitor for they allow planners

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       time  to  provide  for  draught relief  and  other  socioeconomic
       assistance.






















































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