Earth and Space Science

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Earth and Space Science
9) Solid Earth. The student knows Earths
interior is differentiated chemically,
physically, and thermally. The student is
expected to c. Explain how scientists use
geophysical methods such as seismic wave
analysis, gravity, and magnetism to interpret
Earths structure and d. Describe the
formation and structure of Earths magnetic
field, including its interaction with charged
solar particles to form the Van Allen belts and
auroras. Paleomagnetism Polar
wandering Magnetic reversals and mechanism of
sea-floor spreading Vocabulary Mohorovicic
discontinuity (Moho) Shadow Zone, P-Waves
S-Waves Magnetosphere Solar Wind Aurora
Borealis Aurora Australis
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How do we know?

• Weve spent a couple of lessons already on the
  internal structure of the Earth. Just how do we
  know what lies beneath our feet?
• Do we have direct evidence from these depths?
• 1989 Kola Superdeep Borehole in Russia 12,262 M
  
• 2008 Al Shaheen Oil Well in Qatar 12,289 M
• 2011 Sakhalin-I Odoptu Well 12,345 M
• Anyway you look at it, we havent come close to
  drilling through even the crust, or outer surface
  of the Earth yetso, how do we know?

(40,230 ft)
(40,318 ft)
(40, 502 ft)

• Scientists have used indirect measurements and
  seismic waves from earthquakes to determine the
  internal structure of the Earth. What exactly is
  a seismic wave?
• Seismic waves are those which are produced by
  movements and motions of the Earths crust along
  its tectonic boundariesWhat types of boundaries
  are there again?

Convergent, divergent, and transform
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Analysis of the Earths internal structure is
made possible because earthquakes produce
vibrations called seismic waves. These waves
travel through the interior of the Earth and can
be measured with sensitive detectors called
seismographs. Scientists have seismographs set up
all over the world to track movement of the
Earths crust.
While we may not have practical experience at
these depths, we do have ways of determining
Earths interior through extrapolation based upon
behavior of waves through various media.
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• What are the three types of tectonic boundaries?
• 2. Describe each of the following
• Convergent
• Divergent
• Transform

3. What is a seismic wave?
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Body vs. Surface Waves
Seismic waves are divided into two types Body
waves and surface waves. Body waves include P and
S waves, and these are the two types of waves
that are used to determine the internal structure
of the Earth. There are other types of seismic
waves, but they relate to surface damage and only
travel through the Earths crust, not the entire
Earth.
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P and S Wave Demo
Measuring seismic waves traveling through the
Earth allowed seismologists to determine that the
core begins at a depth of 2900 km, or in other
words, the mantle extends to 2900 km its
composition is probably fluid rock. This is based
on the velocity of the waves, mass of the Earth
and other lines of evidence.
As humans have studied, and know how these
various body waves travel through different
substances (ie solids, fluidsetc), we can
extrapolate about the structures found deep
within the Earth by determining how these waves
act as they move through it. (ie, the speed and
direction of traveletc)
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Earths Core
The shadow zone is the area of the Earth from
angular distances of 104 to 140 degrees from a
given earthquake epicenter that does not receive
any direct P waves. The shadow zone results
from S waves being stopped entirely by the liquid
core and P waves being bent (refracted) by the
liquid outer core.
Through measuring how P and S waves travel
through the Earth and out the other side, this
seismic wave shadow zone was discovered in 1910.
From the lack of S waves and a great slowing of
the P wave velocity (by about 40) it was deduced
that the outer core is made of liquid. The shadow
zone also defined the diameter of the core.
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Direct Evidence
Of Earths Interiorthere is some!
One type of direct evidence is a type of rock
called "Xenoliths." Xenoliths are foreign or
alien rocks. The word "xeno" means stranger.
Xenoliths are basically small pieces of small
fragments of rock usually enclosed in volcanic
rocks that are thought to be pieces of the
Earth's mantle, which are brought up along with
the lava during the volcanic eruption.
Another type of direct evidence is a type of rock
structure called "Ophiolites". The ophiolites
are thought to be pieces of the mantle connected
to pieces of the crust, which have been uplifted
and embedded by tectonic processes in the crust
of the Earth. They have a particular structure,
which involves a transition from one rock type to
another.
The ophiolite mantle material looks somewhat
darker than the gabbro above it
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• 4. How do scientists use seismic waves to
  determine what the Earths interior must be like?
• Mantle
• Core
• 5. What tool is used to measure the intensity of
  an Earthquake?
• 6. Describe each of the following body waves
• P Waves
• S Waves

7. What are xenoliths and ophiolites? 8. How
are xenoliths and ophiolites direct evidence of
Earths interior?
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Using Magnetism and Gravity
You can use a spring scale to measure gravity.
Notice that when this is hanging without moving,
the spring is stretched by a certain amount. If
the mass is pulled down, it causes the spring to
stretch further. A machine called a gravimeter
can be built based on this principle to measure
Earth's gravity or differences in Earth's gravity
between one place and another.
Earths rotation causes the polar regions to
flatten down slightly, making Earth an ellipsoid
instead of a sphere. As a result, the force of
gravity is slightly stronger at the poles
compared to the equator (generally). Smaller
variations in gravity across the Earths surface
are caused by differences in the thickness and
rock density of Earths crust, as well as density
differences deep in Earths interior.
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More Indirect Evidence

• The overall density of the Earth is much higher
  than the density of the rocks we find in the
  crust. This tells us that the inside must be made
  of something much denser than rock.
• Meteorites (created at the same time as the
  Earth, 4.6 billion years ago) have been analyzed.
  The most common type is called a chondrite and
  they contain iron, silicon, magnesium and oxygen
  (Others contain iron and nickel). A meteorite has
  roughly the same density as the whole Earth. A
  meteorite minus its iron has a density roughly
  the same as Mantle rock.
• Iron and Nickel are both dense and magnetic.
  Earth has a Magnetic Field. Convection currents
  in the liquid outer core force the inner core to
  spin. The inner core spins faster than the rest
  of Earth. This creates a magnetic field through
  something called the dynamo effect.
• It is Earths rotating, convecting, electrically
  conducting metallic fluid outer core that enables
  this dynamo, in turn energizing the magnetic
  field, protecting us from the Sun.

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9. Would the force of gravity be stronger at the
poles, or equatorand why? 10. Where would
gravity be stronger, on the continents or oceanic
floorand why? 11. How can the composition of
meteorites suggest the makeup of the Earths
interior? 12. What is the dynamo effect and how
does it contribute to the formation of the
magnetic field on Earth? 13. Why is Earths
magnetic field so important?
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Van Allen Belt
The Van Allen Belt is located in the inner region
of the Earth's magnetosphere. It is split into
two distinct belts, with energetic electrons
forming the outer belt and a combination of
protons and electrons forming the inner belts
The Van Allen radiation belt is a ring of
energetically charged particles (plasma) around
Earth, which is held in place by Earth's magnetic
field. It is believed that most of the particles
that form the belts originated from solar wind
and cosmic rays.
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The Auroras

• The auroras, both surrounding the north magnetic
  pole
• Aurora borealis
• and south magnetic pole
• Aurora australis
• occur when highly charged electrons from the
  solar wind interact with elements in the Earth's
  atmosphere.

Different gases give off different colors when
they are excited. Oxygen at about 60 miles up
gives off the familiar yellow-green color, Oxygen
at higher altitudes (about 200 miles above us)
gives the all red auroras. Ionic Nitrogen
produces the blue light and neutral Nitrogen
gives off the red-purple and the rippled edges.
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14. What is the Van Allen Belt, and where do the
particles that compose it come from? 15. What
composes the outer Van Allen Belt? 16. What
composes the inner Van Allen Belt? 17. Why does
Earth have a Van Allen Belt? 18. Which auroras
surround the north magnetic pole? South magnetic
pole? 19. Why are auroras composed of different
colors?
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Paleomagnetism
Paleomagnetism is possible because some magnetic
minerals common in igneous rocks, such as
magnetite, and hematite, will "acquire" the
magnetic field of their surroundings as the rock
cools. As it cools, the surrounding magnetic
field becomes "frozen," in the mineral, so at the
time of formation, these rocks preserve the
direction of the magnetic field.
Paleomagnetism is the study of the magnetic
properties of rocks over time. Paleomagnetism
aids in our understanding of plate tectonics,
minerology, petrogenesis, geochronology, and the
history of the Earth's magnetic field.

Pink Normal magnetism White Magnetic reversal
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Sea-Floor Spreading

• During World War II, geologists employed by the
  military carried out studies of the sea floor, in
  order to understand the topography of the sea
  floor to find hiding places for both allied and
  enemy submarines.
• The topographic studies involved measuring the
  depth to the sea floor. These studies revealed
  the presence of two important topographic
  features of the ocean floor
• Oceanic Ridges - long ridges that occupy the
  middle of the Atlantic Ocean and the eastern part
  of the Pacific Ocean.
• Oceanic Trenches - deep trenches along the
  margins of continents, particularly surrounding
  the Pacific Ocean.

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Apparent Polar Wander
Polar Wandering is basically the apparent
migration of the magnetic poles of the Earth
through geologic time. Scientific evidence
indicates that the magnetic poles have slowly and
erratically wandered across the surface of the
Earth. Today, magnetic north is here, and True
geographic north is here.
Calculations of pole locations over the past 20
million years show that the magnetic pole hasnt
moved greatly. Go back 30 million years,
however, and substantial deviations occur.
Calculations of polar wandering formed one of
the first important pieces of evidence for
continental drift.
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Apparent Polar Wandering and Continental Drift
Scientists have determined that the magnetic
poles do not actually wander that much they
generally stay close to the geographic poles.
Therefore the concept of apparent polar wander is
very useful in plate tectonics, since it can
retrace the relative motion of continents, as
well as the formation and break-up of
supercontinents.
As the image on the right shows, the paths of
polar wandering would be different if the pole
were fixed, compared to the path if the continent
was fixed (on the left, image a ).
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20. What is paleomagnetism, and how do we use
it? 21. What two important oceanic features
were discovered during WWII? 22. What is
polar wandering, and what did its discovery help
to prove?