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Inside the Earth
Layers of Composition
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Inside the Earth
Inside the Earth – Apple Analogy
The usual introduction to the inside of the earth is to use an
apple. An apple gives you a good sense of the scale.
If the earth was an apple, our deepest drills haven’t even
punctured through the “skin”!
We have drilled 3km and soon hope to drill 7km under the
ocean.
The Crust
The Mantle
Inner Core
Outer Core
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The Crust
The earth’s crust makes up less than 1% of the total volume of the
earth. It is made of metamorphic (extreme heat and pressure),
igneous (once molten) and sedimentary rocks (made by
weathering / erosion and compression). The temperature can be as
hot as 6000C. The crust itself is a relatively thin solid layer and is in
two categories: continental and oceanic.
 Continental Crust (mainly Granite – density ~ 2.7 g/cm3)
The continents form large land masses higher than the oceans.
The average thickness of continental crust is 35km and is made of
three forms:
 Shields – Ancient igneous land masses that form the core of the
continents (contain the oldest rocks on earth)
 Mountains – “Young” folded rocks (usually from plated tectonics)
Sedimentary Basins – Depressions on continents (become filled
with sedimentary rocks or become seas)
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Oceanic Crust
(mainly Basalt which is denser than granite at ~3.2 g/cm3)
The land under the oceans makes up the oceanic crust. The
average thickness of oceanic crust is only 7km and has three
layers:
The top layer (1km)
Mud, Sand, and dead organisms washed to see from the
continents (recall that during landslides these can form
“turbidity currents”)
The middle layer (1km)
Volcanically formed layer of fine grain basalt rock
The bottom layer (5km)
Slow cooling magma forms course grained “Gabbro” rock
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Key Concept:
• Since oceanic crust is denser and thinner than
continental crust, oceanic crust is subducted when
continental and oceanic crusts collide. The continents
contain the oldest rocks since they essentially “float”
over the denser ocean crusts.
Note:
• Do you remember growing crystals in an earlier grade?
Slow cooling forms large crystals. If you forgot (or didn’t):
try making “rock candy” out of supersaturated sugar
solutions at home.
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The Mantle (density around 3.8 g/cm3)
The mantle makes up about 80% of the total volume of the
earth. It is primarily made of silicon oxide SiO2,
magnesium oxide MgO, and iron oxide FeO. Recall
that the mantle also contains radioactive uranium which
releases tremendous amounts of thermal energy.
The temperature at the boundary of the crust varies
around 9000C to over 20000C near the outer core. Even
at these extreme temperatures the mantle remains
mainly solid because of the great pressure exerted on it
from the crust (but it is ductile enough for convection
currents).
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Based on the way earthquake waves travel through the
mantle it is classified with three main layers: upper,
transition zone, and lower.
• Upper Mantle (~400km thick) – This is where the least
dense materials rise too; it is also the least viscous which
enables the strongest convection currents.
• Transition Zone (~250km thick) – A complex area which
little is known about. It is suspected to be a main source of
basaltic magmas.
• Lower Mantle (~2000km thick) – This is the most dense
region of the mantle and is possibly made of several
different chemical compositions.
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The Core
The earth’s core is the densest region in the planet and
makes up about 16% of the earth’s volume (and about 32%
of the earth’s mass!). The temperature of the core is
estimated to be in the range of 5000 to 60000C.
The core itself is in two layers: a liquid outer core and a
solid inner core.
• Outer core (~2200km thick and density around 11 g/cm3)
A liquid layer made of iron, nickel, sulphur, and lighter
elements.
• Inner core (~1200km thick and density around 13 g/cm3)
A solid core under extreme pressure made of iron, nickel
and perhaps some other elements like gold and platinum.
It appears to be rotating slightly faster than the earth.
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Inside the Earth again
Inside the Earth again – A little more detail
Using information generated by earthquake waves,
scientists are able to determine even more information
about the earth’s interior than we learned with the “Apple
Analogy”.
Seismology Part 1 – Body Waves
Earthquakes generate two types of waves: Primary (P)
waves and Secondary (S) waves.
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Primary Waves
- These are compression (or “longitudinal”) waves as they are
like the pulse that can travel along a stretched spring when
you quickly compress (push) one end.
- Compression waves can be transmitted, refracted and
reflected much like other waves you studied in previous
grades.
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Secondary Waves
- These are called secondary because they travel slower
than primary (P) waves.
- This type of wave is a transverse wave; it is like the wave
you produce when you wiggle one end of a rope.
- Transverse waves do not travel through liquids.
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Seismology
Seismology is the study of earthquakes
using information like P-waves and Swaves. Seismologist set up devices
(seismographs) around the world to
record earthquakes.
When an earthquake occurs, they can
determine the depth and location of the
earthquake source. They can also
determine the makeup of the earth’s
interior.
Consider the diagram to the right which
traces the waves produced from an
earthquake:
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On the other side of the globe, S-waves are not recorded
because they can’t travel through the liquid outer core.
There is also a “shadow zone” where no P-waves are
recorded because they are refracted by the liquid outer
core. They are also refracted by the varying densities of
the layers of earth’s interior.
There are earthquakes everyday on earth and so
seismologist continually receive more and more data to
be able analyze the interior design of earth.
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Look at the following image from USGS
(http://earthquake.usgs.gov/eqcenter/recenteqsanim/world.php) which shows
earthquake activity (460 earthquakes) in the world for one week:
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Conclusion
The majority of the above earthquakes are smaller in magnitude
(under 7 magnitudes). An earthquake of magnitude 8 or larger on the
Richter scale only occurs a few times a year.
An earthquake occurs when a sudden release of energy occurs in the
earth’s surface. This is usually the result of moving land masses or
volcanic activity.
The location of the source of the earthquake is called the “focus”. The
sources of most earthquakes occur within 70km of the surface and are
called “shallow-focus” earthquakes.
Deeper focus earthquakes cause more damage than shallow focus
earthquakes. The surface of the earth directly above the focus is called
the “epicenter” of the earthquake.
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