EESA06H3 Chapter Notes - Chapter 4: Magnetometer, Geothermal Gradient, Geomagnetic Reversal

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12 Aug 2016
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A CAT Scan of the Mantle
New technique of looking at the mantle similar to the medical technique of CAT scanning
Seismic tomography uses earthquake waves and powerful computers to study planar cross sections of
the mantle following large earthquakes
Deeper CAT scans of the mantle indicate that some mantle plumes emanate from the core mantle
boundary and are fed by heat loss from the core
New tomographic images also reveal high velocity areas, interpreted as cold sinking slabs of
subducted plates are also extended all the way to the core-mantle boundary
Other plates stop descending at the 67-km boundary within the mantle; perhaps depth of sinking is
controlled by plate density
The Core
Seismic-wave data provide the primary evidence for the existence of the core of the Earth
Seismic P waves spread out from the quake until 103 degress of arc (11 500km) from the epicentre,
they suddently disappear from seismographs at more than 142 degreees from epicentre, P waves
reappear
oRegion between 103-142 degrees lacks p waves- called the P-waves Shadow Zone
P wave Shadow Zone
oRefraction of P waves when they encounter the core boundary deep within Earth’s interior
oP waves can travel through solids and liquids, S waves can only travel through solids
oS wave shadow zone indicates S waves do not travel through the core at all
Implies the core of the earth is liquid or at least acts like a liquid
oP waves are refracted within the Earth’s core suggests that the core has two parts – a liquid
outer core and a solid inner core
Diamonds- A Window into the Mantle
Mantle materials are found in diamond bearin igneous rocks called kimberlites; forming carrot
shaped bodies
How do we know that Diamonds Form in the Mantle?
Diamonds are made of high-pressure form of crystalline carbon
oProduced experimentally at extreme temperatures and pressures
These conditions found in nature in depths more than 150km below the crust in the mantle
How do Diamonds Form?
Throughout to have originated from carbon-bearing rocks on oceanic plates that were subducted at
collisional plate margins
Carbon transformed into diamonds under extreme heat and pressure trapped in the mantle below
continents
Eruptions of kimberlite magmas through volcano-like cents of kumberlite pipes brought diamonds to
surface of continents
Diamonds are not stable at the earth’s surface and will eventually over geologic time, break down to
form graphite
When Did Diamonds form?
Kimberley diamonds are 3.3 billion years old and found in the oldest part of continents
Kimberlite pipes range in age from Precambrian to cretaceous
oFormation of diamonds was preculiar to the early earth, sitting in the mantle below continents
until major continental rifting events released them, bringing them to the surface
Where can we find diamonds in North America?
Found in ancient continental cratons – southern and central Africa, Siberia, india, Australia and
Canadian shield – first commercial mine = Ekati mine (North east of Yellowknife in NWT- 2 million
carats per year)
Composition of the Core
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