lecture 3 earths core and geomagnetism Flashcards
how was the core mantle boundary discovered
s h a d o w z o n e
what is the shadow zone
an area of the earth where waves from an earthquakes are not detected. s waves disappear completely and p waves come back later than expected
how do you interpret the s h a d o w z o n e
waves are refracted away from the surface (towards the normal) at a low velocity layer
low velocity explains why p waves arrive later than expected
s waves don’t reappear so must be liquid
how deep is the core mantle boundary
2900km
how was the inner core discovered
identification of PKIKP phase in shadow zone
what does the main p wave shadow zone contain
some weak arrivals with long travel times
to what did inge lehmann attribute the some weak arrivals with long travel times
refractions and reflections from an inner core
p wave velocities increase in inner core
s waves reappear transformed from s waves so inner core must be solid
how deep is the inner core
5150km
what is the composition of the core
mostly iron with lighter elements like nitrogen, sulphur and oxygen
how do we know the cores composition
geochemical evidence
sun and meteorites have lots of iron
how do we know the cores composition
geophysical evidance 1 seismology
constrains density and elasticity which can be tested in a lab
Seismic waves through the inner core travel about 4
percent faster when they’re moving N-S, (along spin
axis), than when they’re moving E-W which explained by computer modelling could mean the core is one giant Fe crystal!
inner core is growing slowley as it cools
how do we know the cores composition
geophysical evidence 2 electric boogaloo geomagnetism
The Earth has a magnetic field, which must be
generated by convection of an electrically
conducting liquid in the outer core – liquid iron
fits the bill!
how many degrees is the earths magnetic field tilted from earths rotation axis
10 degrees
what is a dipmeter
• A compass whose needle can move in
a vertical plane is called a
what is another name for angle of dip
magnetic inclination
how is angle of dip measured
relative to the horizon
if its horizontal dip =
0 degrees
if its verticle dip =
90 degrees + or -
magnetic field is strongest at
magnetic poles
magnetic field is weakest at
magnetic equator
how is the geomagnetic field generated
• The Earth’s interior is too hot (above the Curie temp) for
permanent magnetism to be possible.
• The geomagnetic field is generated by convection currents
in an electrically conducting (metallic) liquid in the outer
core. The core acts like a self-exciting dynamo. • The pattern of fluid flow is not well understood, but is
probably crucially affected by the presence of the solid inner
core, and the rotation of the Earth.
• This explains why the geomagnetic field is aligned, on
average, with the Earth’s spin axis.
what happens every few thousand years
poles flip 180
