Geography 10
Video Earth’s Interior
Ms. Ripley
Oil Wells – Our gateway to the center of the Earth
Each of these oil wells – 100 barrels of oil every day
Oil fuelled engines and economies for over a century
Oil wells – for geologists vital windows to the earth’s interior
These wells are 1.5 km
Deepest wells are about 8km
Deeper oil well – Soviet Union – Scientific oil – 15km, ¼ of 1% to the center of the
earth
Drilling deep wells – rivals space exploration, Deep well Soviet Rocks of 5000psi
pressure and very hot areas
Technical obstacles must be overcome to give geologists a direct view of the
earth’s interior
We can learn about this without going there
Geophysics – indirect techniques (seismitic vibrations, variations in temperature,
magnetic field, gravity, computer models to simulate what is happening deep into
the earth)
This has given us a good understanding of the interior of our planet
San Francisco Earthquake
October 17, 1989 (Just before game 3 of the World Series)
San Francisco bay area
Severe earthquake – shows gyms, shops, stores and the chaos from the
earthquake. Death toll 63, 6 billion dollars
Big quakes
Seismic waves are like sound waves in air. Travel through the body of the earth.
Similar to my talking – waves from my mouth to all points in the room.
Seismic waves pass in 3 dimensions. (Like sound waves) forming spherical wave
fronts. Where they encounter rock layers of differing density– the waves reflect
off these rocks. These waves are recorded on seismic graphs by geophysicist. To
see how long it takes them to reflect off the rock – to the boundary and back. This
will allow them to figure out how far down in the earth this is happening.
Scientists can calculate the depth of the boundary and thus telling us more about
the depth. Geometry of the object. As we illuminate seismic waves – outlining the
shape of the body. Not all seismic waves will bounce off, much will pass through.
Velocity and direction of the waves will change they will bend or refract from one
rock layer to the next.
Studying the travel time from the reflected and refracted seismic waves,
geophysicists have pieced together the general structure of the earth’s interior.
Constantly listen to earthquakes and piece together 3D images to see what the
earth’s interior is actually like.
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Geography 10
Video Earth’s Interior
Ms. Ripley
Earthquakes aren’t the only way for seismic waves. Seismic waves can be
generated artificially by explosions, devices that pound on the earth’s surface.
Waves then transfer through the earth. Very similar to x-rays for people’s body.
We can see the interior of the earth from information from seismic waves.
Earth- 3 Layers
Geophysicists have deduced the earth is made up of 3 layers
Crust – thin skin at the surface
Mantel – thick shell of rock beneath the crust, extends down to the core
Core – A central sphere half the diameter of the planet
Earth’s crust 2 types – continental crust (all the large land masses) oceanic crust
(that underlies all the ocean bases).
They are made up of different types of rocks.
Granitic – continental crust
Basalt – ocean basins
Continents (25-40km thick) are thicker than the oceans (5-10km thick).
Continental crust is made up of materials that were swept together thru dynamic
processes. Igneous (from volcanic events), sedimentary (pushed in through the
oceans), metamorphic (modified by heat and pressures, they have been taken to
depths to the earth’s interior and they have been regurtated again).
Much of the crust is silicates aluminium, potassium, calcium with iron and
magnesium
Oceanic crust – more uniform and primitive – iron and magnesium silicate. Much
less structure then the continents
Direct evidence (touching the rock or sampling it)
Direct evidence – lava might carry rocks – xenoliths (foreign rocks) some will come
from the mantel.
Tectonic activity sea floor rock slices. Ophiolites – sequence of rock geologists
interpret as a cross section of the oceanic crust. Gros Morne national park
outcropping. Sequence of rocks that might be 3-5 km thick. Bottom most layers Rocks of the Earth’s mantle – minerals rich in zinc and magnesium. Moving up Rocks that are called gabbros richer in silica (white rock) (quartz) silica is an a
composition of 2 elements – silicone and oxygen, basic building block of minerals
white mineral – very common white hard mineral) Glass is made of pure silica sand.
Finally to rocks that seem to be the floor of the ocean – rocks that erupted
volcanic. Ophiolites special interest to geologist easiest way to see into the earth’s
mantle. Crust of the oceans is much thinner (Mantle 3-4km down). Ophiolites –
changed for geologists to look down 3-4 km Sequence of rocks that geologists
interpret
Continent crust – 60-70km deep. Mantle – 3-4
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Geography 10
Video Earth’s Interior
Ms. Ripley
Walking across an ovaoliate section is the only way of having a direct view at the
earth’s mantle.
Dark Rock means rich in minerals typical of the earth’s mantle
Rocks – lighter colored – they are rich in the minerals feldspar (mineral that has a
lot of silica in it), coming the quarter mile, I have walked from the earth’s mantle
into the earth’s crust
Ophiolites outcroppings are a rare find and only provide a view of the upper
mantle.
Seismic Studies (indirect – b/c you are not actually seeing anything)
Primary (P Waves) = compressional wave that makes the rock vibrate parallel to
the direction of its movement very quickly travels through rock at 4-7 km per
second. P wave is the first wave that occurs at a recording station.
Secondary (S Waves) = shearing motion that makes the rock vibrate perpendicular
to its path. It travels half the speed as P waves, they arrive later.
Another difference: P Waves can pass through gases and liquids. They can both
pass through rocks.
Properties of the waves give the geophysicists an understanding of what the
image of the earth core must look like. When large earthquakes occur this gives
them lots of data to record arrivals of P and S waves.
P waves are not recorded if the focus of the quake is around the other side of the
world. This area is called the P wave shadow zone. Almost the opposite point on
the earth’s surface. The waves are refracted and are diverted from their original
paths.
S waves leave a big shadow area. They cannot pass through the core. They cannot
traverse liquids. The way that they pass through the core tells us that there is a
solid inner core and a liquid outer core. Mostly of iron and other elements (the
core).
Geophysics
Gravity
Force of gravity force and mass. Closer they are together the stronger.
Tides gravt. Pull of the moon and the sun. The sun is more massive but the moon
is closer to the earth. (And twice as great)
Gravitotor- incredibly sensitive, 1 part in 100 million. Changes in gravity – due to
variations in the mass of the rock in the earth’s interior. Earth’s mantle rock (high
gravity readings b/c of the density of the rock).
Heart of the gravitator – spring with a weight on the end. Higher the pull of
gravity the more the spring is extended. Magnetism – provides lots of information
about the earth
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Geography 10
Video Earth’s Interior
Ms. Ripley
Walk along the earth’s surface with the tool and you measure the gravt pull at
that spot, move 10 m and do it again (survey of data) develop a map – plot individual
points a pattern might develop where there is more gravity and less gravity.
One way to find minerals. More for finding big rocks (deposits) general areas of
high density rocks. You’ll find a huge rock and inside the rock will be gold.
Giant bar magnet is present at the center of the earth. Cover magnet with a
piece of paper – magnetic lines of force become easily visible. Align the needle of a
compass.
Characteristics of the earth’s field.
Magnetic field has been observed since the days of the Greeks.
Curious metal can be.
Always turns to face north. Simple compass – to navigate to every corner of the
globe. Michael Ferriday – illustrate principle – electric current passes through wire
– there is an electrical current – iron filings same principle. Electrical current can
be made, moving a magnet thru a coil. He gives us the basis of our knowledge.
Outer part of the earth’s core –molten iron. Liquid metal – it can conduct
electricity. Heat coming from deeper – convection currents – electrical conductor
through a magnetic field.
A self sustained electrical cycle – active deep inside earth. Lightshow can be seen
occasionally. Surface of the sun – constant flow of electrical particles called the
solar wind enter space– mostly deflected by our magnetic field but by the poles
these particles are drawn magnetically towards earth’s surface. As they hit the
earth’s atmosphere they causes gases to glow with red, white and green lights.
When solar flares occur – especially spectacular – Aurora – northern lights b/c
charged particles bombard atmosphere at higher latitudes.
Strength of the earth’s field – has decreased 5% over the past 150 years. Rocks
can have a magnetic field – iron rich rocks that have been heated, become
magnetized – it gets locked in when the rock cools. Magnetized rocks are records –
Rocks are magnetized- magnetic field is aligned to magnetic north pole. This
alignment is called magnetic polarity permanent in magnetized rocks.
Gravity survey to detect where heavier rocks are b/c where there is a heavier
rock there will be more gravity. The more mass the greater the gravity. Mass and
distance. More mass and less distance more gravt. Pull.
High density rocks are valuable b/c they contain heavier minerals – which are gold
(reflects 95% of light used for insulation in space, insulation on some buildings, that
is why it’s so shiny, it’s very stable, doesn’t’ oxide (losing electrons and combines
with oxygen), silver is second to gold in shininess, lead, zinc, silver, copper.
During reverse polarity lines run from N to S. Magnetic poles switch places.
Rocks can contain this history. Mohave desert California – cores of lava for keys to
our magnetic past. Lava flows preserve this history. Magnetic behaviour.
Paleomagnetism- studying this history of magnetism in rocks.
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Geography 10
Video Earth’s Interior
Ms. Ripley
Magnetic Reversal decreases – as the earth is entering a decrease – decrease in
magnetic intensity. Earth’s magnetic field is decreasing in strength. It might be
entering a reversal right now (only happens every few hundreds/thousands of
years)
How does this affect animals? Who depend on magnetism for their migration?
Magnetism, seismity and gravity – indirect evidence – knowledge for structure and
composition. Geophysics without having actually seen it it’s very complex.
Tectonic forces and the origin of the Earth are questions that can be answered
with the help of geophysics!
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